Chapter 3: Dealing with basic shell syntax

This chapter is a more thorough examination of much of what appeared in the chapter 2; to be more specific, I assume you're sitting in front of your terminal about to use the features you just set up in your initialisation files and want to know enough to get them going. Actually, you will probably spend most of the time editing command lines and in particular completing commands --- both of these activities are covered in later chapters. For now I'm going to talk about commands and the syntax that goes along with using them. This will let you write shell functions and scripts to do more of your work for you.

In the following there are often several consecutive paragraphs about quite minor features. If you find you read this all through the first time, maybe you need to get out more. Most people will probably find it better to skim through to find what the subject matter is, then come back if they later find they want to know more about a particular aspect of the shell's commands and syntax.

One aspect of the syntax is left to chapter 5: there's just so much to it, and it can be so useful if you know enough to get it right, that it can't all be squashed in here. The subject is expansion, covering a multitude of things such as parameter expansion, globbing and history expansions. You've already met the basics of these in chapter 2; but if you want to know how to pick a particular file with a globbing expression with pinpoint accuracy, or how to make a single parameter expansion reduce a long expression to the words you need, you should read that chapter; it's more or less self-contained, so you don't necessarily need to know everything in this one.

We start with the most basic issue in any command line interpreter, running commands. As you know, you just type words separated by spaces, where the first word is a command and the remainder are arguments to it. It's important to distinguish between the types of command.

3.1: External commands

External commands are the easiest, because they have the least interaction with the shell --- many of the commands provided by the shell itself, which are described in the next section, are built into the shell especially to avoid this difficulty.

The only major issue is therefore how to find them. This is done through the parameters $path and $PATH, which, as I described in chapter 2, are tied together because although the first one is more useful inside the shell --- being an array, its various parts can be manipulated separately --- the second is the one that is used by other commands called by the shell; in the jargon, $PATH is `exported to the environment', which means exactly that other commands called by the shell can see its value.

So suppose your $path contains

  /home/pws/bin /usr/local/bin /bin /usr/bin
and you try to run `ls'. The shell first looks in /home/pws/bin for a command called ls, then in /usr/local/bin, then in /bin, where it finds it, so it executes /bin/ls. Actually, the operating system itself knows about paths if you execute a command the right way, so the shell doesn't strictly need to.

There is a subtlety here. The shell tries to remember where the commands are, so it can find them again the next time. It keeps them in a so-called `hash table', and you find the word `hash' all over the place in the documentation: all it means is a fast way of finding some value, given a particular key. In this case, given the name of a command, the shell can find the path to it quickly. You can see this table, in the form `key=value', by typing `hash'.

In fact the shell only does this when the option HASH_CMDS is set, as it is by default. As you might expect, it stops searching when it finds the directory with the command it's looking for. There is an extra optimisation in the option HASH_ALL, also set by default: when the shell scans a directory to find a command, it will add all the other commands in that directory to the hash table. This is sensible because on most UNIX-like operating systems reading a whole lot of files in the same directory is quite fast.

The way commands are stored has other consequences. In particular, zsh won't look for a new command if it already knows where to find one. If I put a new ls command in /usr/local/bin in the above example, zsh would continue to use /bin/ls (assuming it had already been found). To fix this, there is the command rehash, which actually empties the command hash table, so that finding commands starts again from scratch. Users of csh may remember having to type rehash quite a lot with new commands: it's not so bad in zsh, because if no command was already hashed, or the existing one disappeared, zsh will automatically scan the path again; furthermore, zsh performs a rehash of its own accord if $path is altered. So adding a new duplicate command somewhere towards the head of $path is the main reason for needing rehash.

One thing that can happen if zsh hasn't filled its command hash table and so doesn't know about all external commands is that the AUTO_CD option, mentioned in the previous chapter and again below, can think you are trying to change to a particular directory with the same name as the command. This is one of the drawbacks of AUTO_CD.

To be a little bit more technical, it's actually not so obvious that command hashing is needed at all; many modern operating systems can find commands quickly without it. The clincher in the case of zsh is that the same hash table is necessary for command completion, a very commonly used feature. If you type `compr<TAB>', the shell completes this to `compress'. It can only do this if it has a list of commands to complete, and this is the hash table. (In this case it didn't need to know where to find the command, just its name, but it's only a little extra work to store that too.) If you were following the previous paragraphs, you'll realise zsh doesn't necessarily know all the possible commands at the time you hit TAB, because it only looks when it needs to. For this purpose, there is another option, HASH_LIST_ALL, again set by default, which will make sure the command hash table is full when you try to complete a command. It only needs to do this once (unless you alter $path), but it does mean the first command completion is slow. If HASH_LIST_ALL is not set, command completion is not available: the shell could be rewritten to search the path laboriously every single time you try to complete a command name, but it just doesn't seem worth it.

The fact that $PATH is passed on from the shell to commands called from it (strictly only if the variable is marked for export, as it usually is --- this is described in more detail with the typeset family of builtin commands below) also has consequences. Some commands call subcommands of their own using $PATH. If you have that set to something unusual, so that some of the standard commands can't be found, it could happen that a command which is found nonetheless doesn't run properly because it's searching for something it can't find in the path passed down to it. That can lead to some strange and confusing error messages.

One important thing to remember about external commands is that the shell continues to exist while they are running; it just hangs around doing nothing, waiting for the job to finish (though you can tell it not to, as we'll see). The command is given a completely new environment in which to run; changes in that don't affect the shell, which simply starts up where it left off after the command has run. So if you need to do something which changes the state of the shell, an external command isn't good enough. This brings us to builtin commands.

3.2: Builtin commands

Builtin commands, or builtins for short, are commands which are part of the shell itself. Since builtins are necessary for controlling the shell's own behaviour, introducing them actually serves as an introduction to quite a lot of what is going on in the shell. So a fair fraction of what would otherwise appear later in the chapter has accumulated here, one way or another. This does make things a little tricksy in places; count how many times I use the word `subtle' and keep it for your grandchildren to see.

I just described one reason for builtins, but there's a simpler one: speed. Going through the process of setting up an entirely new environment for the command at the beginning, swapping between this command and anything else which is being run on the computer, then destroying it again at the end is considerable overkill if all you want to do is, say, print out a message on the screen. So there are builtins for this sort of thing.

3.2.1: Builtins for printing

The commands `echo' and `print' are shell builtins; they just show what you typed, after the shell has removed all the quoting. The difference between the two is really historical: `echo' came first, and only handled a few simple options; ksh provided `print', which had more complex options and so became a different command. The difference remains between the two commands in zsh; if you want wacky effects, you should look to print. Note that there is usually also an external command called echo, which may not be identical to zsh's; there is no standard external command called print, but if someone has installed one on your system, the chances are it sends something to the printer, not the screen.

One special effect is `print -z' puts the arguments onto the editing buffer stack, a list maintained by the shell of things you are about to edit. Try:

  print -z print -z print This is a line
(it may look as if something needs quoting, but it doesn't) and hit return three times. The first time caused everything after the first `print -z' to appear for you to edit, and so on.

For something more useful, you can write functions that give you a line to edit:

  fn() { print -z print The time now is $(date); }
Now when you type `fn', the line with the date appears on the command line for you to edit. The option `-s' is a bit similar; the line appears in the history list, so you will see it if you use up-arrow, but it doesn't reappear automatically.

A few other useful options, some of which you've already seen, are

-r
don't interpret special character sequences like `\n'

-P
use `%' as in prompts

-n
don't put a newline at the end in case there's more output to follow

-c
print the output in columns --- this means that `print -c *' has the effect of a sort of poor person's `ls', only faster

-l
use one line per argument instead of one column, which is sometimes useful for sticking lists into files, and for working out what part of an array parameter is in each element.

If you don't use the -r option, there are a whole lot of special character sequences. Many of these may be familiar to you from C.

\n
newline

\t
tab

\e or \E
escape character

\a
ring the bell (alarm), usually a euphemism for a hideous beep

\b
move back one character.

\c
don't print a newline --- like the -n option, but embedded in the string. This alternative comes from Berkeley UNIX.

\f
form feed, the phrase for `advance to next page' from the days when terminals were called teletypes, maybe more familiar to you as ^L

\r
carriage return --- when printed, the annoying ^M's you get in DOS files, but actually rather useful with `print', since it will erase everything to the start of the line. The combination of the -n option and a \r at the start of the print string can give the illusion of a continuously changing status line.

\v
vertical tab, which I for one have never used (I just tried it now and it behaved like a newline, only without assuming a carriage return, but that's up to your terminal).
In fact, you can get any of the 255 characters possible, although your terminal may not like some or all of the ones above 127, by specifying a number after the backslash. Normally this consists of three octal characters, but you can use two hexadecimal characters after \x instead --- so `\n', `\012' and `\x0a' are all newlines. `\' itself escapes any other character, i.e. they appear as themselves even if they normally wouldn't.

Two notes: first, don't get confused because `n' is the fourteenth letter of the alphabet; printing `\016' (fourteen in octal) won't do you any good. The remedy, after you discover your text is unreadable (for VT100-like terminals including xterm), is to print `\017'.

Secondly, those backslashes can land you in real quoting difficulties. Normally a backslash on the command line escapes the next character --- this is a different form of escaping to print's --- so

  print \n
doesn't produce a newline, it just prints out an `n'. So you need to quote that. This means
  print \\
passes a single backslash to quote, and
  print \\n
or
  print '\n'
prints a newline (followed by the extra one that's usually there). To print a real backslash, you would thus need
  print \\\\
Actually, you can get away with the two if there's nothing else after --- print just shrugs its shoulders and outputs what it's been given --- but that's not a good habit to get into. There are other ways of doing this: since single quotes quote anything, including backslashes (they are the only way of making backslashes behave like normal characters), and since the `-r' option makes print treat characters normally,
  print -r '\'
has the same effect. But you need to remember the two levels of quoting for backslashes. Quotes aren't special to print, so
  print \'
is good enough for printing a quote.

echotc

There's an oddity called `echotc', which takes as its argument `termcap' capabilities. This now lives in its own module, zsh/termcap.

Termcap is a now rather old-fashioned way of giving the commands necessary for performing various standard operations on terminals: moving the cursor, clearing to the end of the line, turning on standout mode, and so on. It has now been replaced almost everywhere by `terminfo', a completely different way of specifying capabilities, and by `curses', a more advanced system for manipulating objects on a character terminal. This means that the arguments you need to give to echotc can be rather hard to come by; try the termcap manual page; if there are two, it's probably the one in section five which gives the codes, i.e. `man 5 zsh' or `man -s 5 zsh' on Solaris. Otherwise you'll have to search the web. The reason the zsh manual doesn't give a list is that the shell only uses a few well-known sequences, and there are very many others which will work with echotc, because the sequences are interpreted by a the terminal, not the shell.

This chunk gives you a flavour:

  zmodload -i zsh/termcap
  echotc md
  echo -n bold
  echotc mr
  echo -n reverse
  echotc me
  echo
First we make sure the module is loaded into the shell; on some older operating systems, this only works if it was compiled in when zsh was installed. The option -i to zmodload stops the shell from complaining if the module was already loaded. This is a sensible way of ensuring you have the right facilities available in a shell function, since loading a module makes it available until it is explicitly unloaded.

You should see `bold' in bold characters, and `reverse' in bold reverse video. The `md' capability turns on bold mode; `mr' turns on reverse video; `me' turns off both modes. A more typical zsh way of doing this is:

  print -P '%Bbold%Sreverse%b%s'
which should show the same thing, but using prompt escapes --- prompts are the most common use of special fonts. The `%S' is because zsh calls reverse `standout' mode, because it does. (On a colour xterm, you may find `bold' is interpreted as `blue'.)

There's a lot more you can do with echotc if you really try. The shell has just acquired a way of printing terminfo sequences, predictably called echoti, although it's only available on systems where zsh needs terminfo to compile --- this happens when the termcap code is actually a part of terminfo. The good news about this is that terminfo tends to be better documented, so you have a good chance of finding out the capabilities you want from the terminfo manual page. The echoti command lives in another predictably named module, zsh/terminfo.

3.2.2: Other builtins just for speed

There are only a few other builtins which are there just to make things go faster. Strictly, tests could go into this category, but as I explained in the last chapter it's useful to have tests in the form

  if [[ $var1 = $var2 ]]; then
    print doing something
  fi
be treated as a special syntax by the shell, in case $var1 or $var2 expands to nothing which would otherwise confuse it. This example consists of two features described below: the test itself, between the double square brackets, which is true if the two substituted values are the same string, and the `if' construct which runs the commands in the middle (here just the print) if that test was true.

The builtins `true' and `false' do nothing at all, except return a command status zero or one, respectively. They're just used as placeholders: to run a loop forever --- while will also be explained in more detail later --- you use

  while true; do
    print doing something over and over
  done
since the test always succeeds.

A synonym for `true' is `:'; it's often used in this form to give arguments which have side effects but which shouldn't be used --- something like

  : ${param:=value}
which is a common idiom in all Bourne shell derivatives. In the parameter expansion, $param is given the value value if it was empty before, and left alone otherwise. Since that was the only reason for the parameter expansion, you use : to ignore the argument. Actually, the shell blithely builds the command line --- the colon, followed by whatever the value of $param is, whether or not the assignment happened --- then executes the command; it just so happens that `:' takes no notice of the arguments it was given. If you're switching from ksh, you may expect certain synonyms like this to be aliases, rather than builtins themselves, but in zsh they are actually builtins; there are no aliases predefined by the shell. (You can still get rid of them using `disable', as described below.)

3.2.3: Builtins which change the shell's state

A more common use for builtins is that they change something inside the shell, or report information about what's going on in the shell. There is one vital thing to remember about external commands. It applies, too, to other cases we'll meet where the shell `forks', literally splitting itself into two parts, where the forked-off part behaves just like an external command. In both of these cases, the command is in a different process, UNIX's basic unit of things that run. (In fact, even Windows knows about processes nowadays, although they interact a little bit differently with one another.)

The vital thing is that no change in a separate process started by the shell affects the shell itself. The most common case of this is the current directory --- every process has its own current directory. You can see this by starting a new zsh:

  % pwd               # show the current directory
  ~
  % zsh               # start a new shell, which 
                      # is a separate process
  % cd tmp
  % pwd               # now I'm in a different
                      # directory...
  ~/tmp
  % exit              # leave the new shell...
  % pwd               # now I'm back where I was...
  ~
Hence the cd command must be a shell builtin, or this would happen every time you ran it.

Here's a more useful example. Putting parentheses around a command asks the shell to start a different process for it. That's useful when you specifically don't want the effects propagating back:

  (cd some-other-dir; run-some-command)
runs the command, but doesn't change the directory the `real' shell is in, only its forked-off `subshell'. Hence,
  % pwd
  ~
  % (cd /; pwd)
  /
  % pwd
  ~

There's a more subtle case:

  cd some-other-dir | print Hello
Remember, the `|' (`pipe') connects the output of the first command to the input of the next --- though actually no information is passed that way in this example. In zsh, all but the last portion of the `pipeline' thus created is run in different processes. Hence the cd doesn't affect the main shell. I'll refer to it as the `parent' shell, which is the standard UNIX language for processes; when you start another command or fork off a subshell, you are creating `children' (without meaning to be morbid, the children usually die first in this case). Thus, as you would guess,
  print Hello | cd some-other-dir
does have the effect of changing the directory. Note that other shells do this differently; it is always guaranteed to work this way in zsh, because many people rely on it for setting parameters, but many shells have the left hand of the pipeline being the bit that runs in the parent shell. If both sides of the pipe symbol are external commands of some sort, both will of course run in subprocesses.

There are other ways you change the state of the shell, for example by declaring parameters of a particular type, or by telling it how to interpret certain commands, or, of course, by changing options. Here are the most useful, grouped in a vaguely logical fashion.

3.2.4: cd and friends

You will not by now be surprised to learn that the `cd' command changes directory. There is a synonym, `chdir', which as far as I know no-one ever uses. (It's the same name as the system call, so if you had been programming in C or Perl and forgot that you were now using the shell, you might use `chdir'. But that seems a bit far-fetched.)

There are various extra features built into cd and chdir. First, if you miss out the directory to which you want to change, you will be taken to your home directory, although it's not as if `cd ~' is all that hard to type.

Next, the command `cd -' is special: it takes you to the last directory you were in. If you do a sequence of cd commands, only the immediately preceding directory is remembered; they are not stacked up.

Thirdly, there is a shortcut for changing between similarly named directories. If you type `cd <old> <new>', then the shell will look for the first occurrence of the string `<old>' in the current directory, and try to replace it with `<new>'. For example,

  % pwd
  ~/src/zsh-3.0.8/Src
  % cd 0.8 1.9
  ~/src/zsh-3.1.9/Src
The cd command actually reported the new directory, as it usually does if it's not entirely obvious where it's taken you.

Note that only the first match of <old> is taken. It's an easy mistake to think you can change from /home/export1/pws/mydir1/something to /home/export1/pws/mydir2/something with `cd 1 2', but that first `1' messes it up. Arguably the shell could be smarter here. Of course, `cd r1 r2' will work in this case.

cd's friend `pwd' (print working directory) tells you what the current working directory is; this information is also available in the shell parameter $PWD, which is special and automatically updated when the directory changes. Later, when you know all about expansion, you will find that you can do tricks with this to refer to other directories. For example, ${PWD/old/new} uses the parameter substitution mechanism to refer to a different directory with old replaced by new --- and this time old can be a pattern, i.e. something with wildcard matches in it. So if you are in the zsh-3.0.8/Src directory as above and want to copy a file from the zsh-3.1.9/Src directory, you have a shorthand:

  cp ${PWD/0.8/1.9}/myfile.c .

Symbolic links

Zsh tries to track directories across symbolic links. If you're not familiar with these, you can think of them as a filename which behaves like a pointer to another file (a little like Windows' shortcuts, though UNIX has had them for much longer and they work better). You create them like this (ln is not a builtin command, but its use to make symbolic links is very standard these days):

  ln -s existing-file-name name-of-link
for example
  ln -s /usr/bin/ln ln
creates a file called ln in the current directory which does nothing but point to the file /usr/bin/ln. Symbolic links are very good at behaving as much like the original file as you usually want; for example, you can run the ln link you've just created as if it were /usr/bin/ln. They show up differently in a long file listing with `ls -l', the last column showing the file they point to.

You can make them point to any sort of file at all, including directories, and that is why they are mentioned here. Suppose you create a symbolic link from your home directory to the root directory and change into it:

  ln -s / ~/mylink
  cd ~/mylink
If you don't know it's a link, you expect to be able to change to the parent directory by doing `cd ..'. However, the operating system --- which just has one set of directories starting from / and going down, and ignores symbolic links after it has followed them, they really are just pointers --- thinks you are in the root directory /. This can be confusing. Hence zsh tries to keep track of where you probably think you are, rather than where the system does. If you type `pwd', you will see `/home/you/mylink' (wherever your home directory is), not `/'; if you type `cd ..', you will find yourself back in your home directory.

You can turn all this second-guessing off by setting the option CHASE_LINKS; then `cd ~/mydir; pwd' will show you to be in /, where changing to the parent directory has no effect; the parent of the root directory is the root directory, except on certain slightly psychedelic networked file systems. This does have advantages: for example, `cd ~/mydir; ls ..' always lists the root directory, not your home directory, regardless of the option setting, because ls doesn't know about the links you followed, only zsh does, and it treats the .. as referring to the root directory. Having CHASE_LINKS set allows `pwd' to warn you about where the system thinks you are.

An aside for non-UNIX-experts (over 99.9% of the population of the world at the last count): I said `symbolic links' instead of just `links' because there are others called `hard links'. This is what `ln' creates if you don't use the -s option. A hard link is not so much a pointer to a file as an alternative name for a file. If you do

  ln myfile othername
  ls -l
where myfile already exists you can't tell which of myfile and othername is the original --- and in fact the system doesn't care. You can remove either, and the other will be perfectly happy as the name for the file. This is pretty much how renaming files works, except that creating the hard link is done for you in that case. Hard links have limitations --- you can't link to directories, or to a file on another disk partition (and if you don't know what a disk partition is, you'll see what a limitation that can be). Furthermore, you usually want to know which is the original and which is the link --- so for most users, creating symbolic links is more useful. The only drawback is that following the pointers is a tiny bit slower; if you think you can notice the difference, you definitely ought to slow down a bit.

The target of a symbolic link, unlike a hard link, doesn't actually have to exist and no checking is performed until you try to use the link. The best thing to do is to run `ls -lL' when you create the link; the -L part tells ls to follow links, and if it worked you should see that your link is shown as having exactly the same characteristics as the file it points to. If it is still shown as a link, there was no such file.

While I'm at it, I should point out one slight oddity with symbolic links: the name of the file linked to (the first name), if it is not an absolute path (beginning with / after any ~ expansion), is treated relative to the directory where the link is created --- not the current directory when you run ln. Here:

  ln -s ../mydir ~/links/otherdir
the link otherdir will refer to mydir in its own parent directory, i.e. ~/links --- not, as you might think, the parent of the directory where you were when you ran the command. What makes it worse is that the second word, if is not an absolute path, is interpreted relative to the directory where you ran the command.

$cdpath and AUTO_CD

We're nowhere near the end of the magic you can do with directories yet (and, in fact, I haven't even got to the zsh-specific parts). The next trick is $cdpath and $CDPATH. They look a lot like $path and $PATH which you met in the last chapter, and I mentioned them briefly back in the last chapter in that context: $cdpath is an array of directories, while $CDPATH is colon-separated list behaving otherwise like a scalar variable. They give a list of directories whose subdirectories you may want to change into. If you use a normal cd command (i.e. in the form `cd dirname', and dirname does not begin with a / or ~, the shell will look through the directories in $cdpath to find one which contains the subdirectory dirname. If $cdpath isn't set, as you'd guess, it just uses the current directory.

Note that $cdpath is always searched in order, and you can put a . in it to represent the current directory. If you do, the current directory will always be searched at that point, not necessarily first, which may not be what you expect. For example, let's set up some directories:

  mkdir ~/crick ~/crick/dna
  mkdir ~/watson ~/watson/dna
  cdpath=(~/crick .)
  cd ~/watson
  cd dna
So I've moved to the directory ~/watson, which contains the subdirectory dna, and done `cd dna'. But because of $cdpath, the shell will look first in ~/crick, and find the dna there, and take you to that copy of the self-reproducing directory, not the one in ~/watson. Most people have . at the start of their cdpath for that reason. However, at least cd warns you --- if you tried it, you will see that it prints the name of the directory it's picked in cases like this.

In fact, if you don't have . in your directory at all, the shell will always look there first; there's no way of making cd never change to a subdirectory of the current one, short of turning cd into a function. Some shells don't do this; they use the directories in $cdpath, and only those.

There's yet another shorthand, this time specific to zsh: the option AUTO_CD which I mentioned in the last chapter. That way a command without any arguments which is really a directory will take you to that directory. Normally that's perfect --- you would just get a `command not found' message otherwise, and you might as well make use of the option. Just occasionally, however, the name of a directory clashes with the name of a command, builtin or external, or a shell function, and then there can be some confusion: zsh will always pick the command as long as it knows about it, but there are cases where it doesn't, as I described above.

What I didn't say in the last chapter is that AUTO_CD respects $cdpath; in fact, it really is implemented so that `dirname' on its own behaves as much like `cd dirname' as is possible without tying the shell's insides into knots.

The directory stack

One very useful facility that zsh inherited from the C-shell family (traditional Korn shell doesn't have it) is the directory stack. This is a list of directories you have recently been in. If you use the command `pushd' instead of `cd', e.g. `pushd dirname', then the directory you are in is saved in this list, and you are taken to dirname, using $CDPATH just as cd does. Then when you type `popd', you are taken back to where you were. The list can be as long as you like; you can pushd any number of directories, and each popd will take you back through the list (this is how a `stack', or more precisely a `last-in-first-out' stack usually operates in computer jargon, hence the name `directory stack').

You can see the list --- which always starts with the current directory --- with the dirs command. So, for example:

  cd ~
  pushd ~/src
  pushd ~/zsh
  dirs
displays
  ~/zsh ~/src ~
and the next popd will take you back to ~/src. If you do it, you will see that pushd reports the list given by dirs automatically as it goes along; you can turn this off with the option PUSHD_SILENT, when you will have to rely on typing dirs explicitly.

In fact, a lot of the use of this comes not from using simple pushd and popd combinations, but from two other features. First, `pushd' on its own swaps the top two directories on the stack. Second, pushd with a numeric argument preceded by a `+' or `-' can take you to one of the other directories in the list. The command `dirs -v' tells you the numbers you need; 0 is the current directory. So if you get,

  0       ~/zsh
  1       ~/src
  2       ~
then `pushd +2' takes you to ~. (A little suspension of disbelief that I didn't just use AUTO_CD and type `..' is required here.) If you use a -, it counts from the other end of the list; -0 (with apologies to the numerate) is the last item, i.e. the same as ~ in this case. Some people are used to having the `-' and `+' arguments behave the other way around; the option PUSHD_MINUS exists for this.

Apart from PUSHD_SILENT and PUSHD_MINUS, there are a few other relevant options. Setting PUSHD_IGNORE_DUPS means that if you pushd to a directory which is already somewhere in the list, the duplicate entry will be silently removed. This is useful for most human operations --- however, if you are using pushd in a function or script to remember previous directories for a future matching popd, this can be dangerous and you probably want to turn it off locally inside the function.

AUTO_PUSHD means that any directory-changing command, including an auto-cd, is treated as a pushd command with the target directory as argument. Using this can make the directory stack get very long, and there is a parameter $DIRSTACKSIZE which you can set to specify a maximum length. The oldest entry (the highest number in the `dirs -v' listing) is automatically removed when this length is exceeded. There is no limit unless this is explicitly set.

The final pushd option is PUSHD_TO_HOME. This makes pushd on its own behave like cd on its own in that it takes you to your home directory, instead of swapping the top two directories. Normally a series of `pushd' commands works pretty much like a series of `cd -' commands, always taking you the directory you were in before, with the obvious difference that `cd -' doesn't consult the directory stack, it just remembers the previous directory automatically, and hence it can confuse pushd if you just use `cd -' instead.

There's one remaining subtlety with pushd, and that is what happens to the rest of the list when you bring a particular directory to the front with something like `pushd +2'. Normally the list is simply cycled, so the directories which were +3, and +4 are now right behind the new head of the list, while the two directories which were ahead of it get moved to the end. If the list before was:

  dir1  dir2  dir3  dir4
then after pushd +2 you get
  dir3  dir4  dir1 dir2
That behaviour changed during the lifetime of zsh, and some of us preferred the old behaviour, where that one directory was yanked to the front and the rest just closed the gap:
  # Old behaviour
  dir3  dir1  dir2  dir4
so that after a while you get a `greatest hits' group at the front of the list. If you like this behaviour too (I feel as if I'd need to have written papers on group theory to like the new behaviour) there is a function pushd supplied with the source code, although it's short enough to repeat here --- this is in the form for autoloading in the zsh fashion:
  # pushd function to emulate the old zsh behaviour.
  # With this, pushd +/-n lifts the selected element
  # to the top of the stack instead of cycling
  # the stack.

  emulate -R zsh
  setopt localoptions

  if [[ ARGC -eq 1 && "$1" == [+-]<-> ]] then
          setopt pushdignoredups
          builtin pushd ~$1
  else
          builtin pushd "$@"
  fi
The `&&' is a logical `and', requiring both tests to be true. The tests are that there is exactly one argument to the function, and that it has the form of a `+' or a `-' followed by any number (`<->' is a special zsh pattern to match any number, an extension of forms like `<1-100>' which matches any number in the range 1 to 100 inclusive).

Referring to other directories

Zsh has two ways of allowing you to refer to particular directories. They have in common that they begin with a ~ (in very old versions of zsh, the second form actually used an `=', but the current way is much more logical).

You will certainly be aware, because I've made a lot of use of it, that a `~' on its own or followed by a / refers to your own home directory. An extension of this --- again from the C-shell, although the Korn shell has it too in this case --- is that ~name can refer to the home directory of any user on the system. So if your user name is pws, then ~ and ~pws are the same directory.

Zsh has an extension to this; you can actually name your own directories. This was described in chapter 2, à propos of prompts, since that is the major use:

  host% PS1='%~? '
  ~? cd zsh/Src
  ~/zsh/Src? zsrc=$PWD
  ~/zsh/Src? echo ~zsrc
  /home/pws/zsh/Src
  ~zsrc?
Consult that chapter for the ways of forcing a parameter to be recognised as a named directory.

There's a slightly more sophisticated way of doing this directly:

  hash -d zsrc=~/zsh/Src
makes ~zsrc appear in prompts as before, and in this case there is no parameter $zsrc. This is the purist's way (although very few zsh users are purists). You can guess what `unhash -d zsrc' does; this works with directories named via parameters, too, but leaves the parameter itself alone.

It's possible to have a named directory with the same name as a user. In that case `~name' refers to the directory you named explicitly, and there is no easy way of getting name's home directory without removing the name you defined.

If you're using named directories with one of the cd-like commands or AUTO_CD, you can set the option CDABLEVARS which allows you to omit the leading ~; `cd zsrc' with this option would take you to ~zsrc. The name is a historical artifact and now a misnomer; it really is named directories, not parameters (i.e. variables), which are used.

The second way of referring to directories with ~'s is to use numbers instead of names: the numbers refer to directories in the directory stack. So if dirs -v gives you

  0       ~zsf
  1       ~src
then ~+1 and ~-0 (not very mathematical, but quite logical if you think about it) refer to ~src. In this case, unlike pushd arguments, you can omit the + and use ~1. The option PUSHD_MINUS is respected. You'll see this was used in the pushd function above: the trick was that ~+3, for example, refers to the same element as pushd +3, hence pushd ~+3 pushed that directory onto the front of the list. However, we set PUSHD_IGNORE_DUPS, so that the value in the old position was removed as well, giving us the effect we wanted of simply yanking the directory to the front with no trick cycling.

3.2.5: Command control and information commands

Various builtins exist which control how you access commands, and which show you information about the commands which can be run.

The first two are strictly speaking `precommand modifiers' rather than commands: that means that they go before a command line and modify its behaviour, rather than being commands in their own right. If you put `command' in front of a command line, the command word (the next one along) will be taken as the name of an external command, however it would normally be interpreted; likewise, if you put `builtin' in front, the shell will try to run the command as a builtin command. Normally, shell functions take precedence over builtins which take precedence over external commands. So, for example, if your printer control system has the command `enable' (as many System V versions do), which clashes with a builtin I am about to talk about, you can run `command enable lp' to enable a printer; otherwise, the builtin enable would have been run. Likewise, if you have defined cd to be a function, but this time want to call the normal builtin cd, you can say `builtin cd mydir'.

A common use for command is inside a shell function of the same name. Sometimes you want to enhance an ordinary command by sticking some extra stuff around it, then calling that command, so you write a shell function of the same name. To call the command itself inside the shell function, you use `command'. The following works, although it's obviously not all that useful as it stands:

  ls() {
    command ls "$[@]"
  }
so when you run `ls', it calls the function, which calls the real ls command, passing on the arguments you gave it.

You can gain longer lasting control over the commands which the shell will run with the `disable' and `enable' commands. The first normally takes builtin arguments; each such builtin will not be recognised by the shell until you give an `enable' command for it. So if you want to be able to run the external enable command and don't particularly care about the builtin version, `disable enable' (sorry if that's confusing) will do the trick. Ha, you're thinking, you can't run `enable enable'. That's correct: some time in the dim and distant past, builtin enable enable' would have worked, but currently it doesn't; this may change, if I remember to change it. You can list all disabled builtins with just `disable' on its own --- most of the builtins that do this sort of manipulation work like that.

You can manipulate other sets of commands with disable and enable by giving different options: aliases with the option -a, functions with -f, and reserved words with -r. The first two you probably know about, and I'll come to them anyway, but `reserved words' need describing. They are essentially builtin commands which have some special syntactic meaning to the shell, including some symbols such as `{' and `[['. They take precedence over everything else except aliases --- in fact, since they're syntactically special, the shell needs to know very early on that it has found a reserved word, it's no use just waiting until it tries to execute a command. For example, if the shell finds `[[' it needs to know that everything until `]]' must be treated as a test rather than as ordinary command arguments. Consequently, you wouldn't often want to disable a reserved word, since the shell wouldn't work properly. The most obvious reason why you might would be for compatibility with some other shell which didn't have one. You can get a complete list with:

  whence -wm '*' | grep reserved
which I'll explain below, since I'm coming to `whence'.

Furthermore, I tend to find that if I want to get rid of aliases or functions I use the commands `unalias' and `unfunction' to get rid of them permanently, since I always have the original definitions stored somewhere, so these two options may not be that useful either. Disabling builtins is definitely the most useful of the four possibilities for disable.

External commands have to be manipulated differently. The types given above are handled internally by the shell, so all it needs to do is remember what code to call. With external commands, the issue instead is how to find them. I mentioned rehash above, but didn't tell you that the hash command, which you've already seen with the -d option, can be used to tell the shell how to find an external command:

  hash foo=/path/to/foo
makes foo execute the command using the path shown (which doesn't even have to end in `foo'). This is rather like an alias --- most people would probably do this with an alias, in fact --- although a little faster, though you're unlikely to notice the difference. You can remove this with unhash. One gotcha here is that if the path is rehashed, either by calling rehash or when you alter $path, the entire hash table is emptied, including anything you put in in this way; so it's not particularly useful.

In the midst of all this, it's useful to be able to find out what the shell thinks a particular command name does. The command `whence' tells you this; it also exists, with slightly different options, under the names where, which and type, largely to provide compatibility with other shells. I'll just stick to whence.

Its standard output isn't actually sparklingly interesting. If it's a command somehow known to the shell internally, it gets echoed back, with the alias expanded if it was an alias; if it's an external command it's printed with the full path, showing where it came from; and if it's not known the command returns status 1 and prints nothing.

You can make it more useful with the -v or -c options, which are more verbose; the first prints out an information message, while the second prints out the definitions of any functions it was asked about (this is also the effect of using `which' instead of `whence). A very useful option is -m, which takes any arguments as patterns using the usual zsh pattern format, in other words the same one used for matching files. Thus

  whence -vm "*"
prints out every command the shell knows about, together with what it thinks of it.

Note the quotes around the `*' --- you have to remember these anywhere where the pattern is not to be used to generate filenames on the command line, but instead needs to be passed to the command to be interpreted. If this seems a rather subtle distinction, think about what would happen if you ran

  # Oops.  Better not try this at home.
  # (Even better, don't do it at work either.)
  whence -vm *
in a directory with the files `foo' and (guess what) `bar' in it. The shell hasn't decided what command it's going to run when it first looks at the command line; it just sees the `*' and expands the line to
  whence -vm foo bar
which isn't what you meant.

There are a couple of other tricks worth mentioning: -p makes the shell search your path for them, even if the name is matched as something else (say, a shell function). So if you have ls defined as a function,

  which -p ls
will still tell what `command ls' would find. Also, the option -a searches for all commands; in the same example, this would show you both the ls command and the ls function, whereas whence would normally only show the function because that's the one that would be run. The -a option also shows if it finds more than one external command in your path.

Finally, the option -w is useful because it identifies the type of a command with a single word: alias, builtin, command, function, hashed, reserved or none. Most of those are obvious, with command being an ordinary external command; hashed is an external command which has been explicitly given a path with the hash builtin, and none means it wasn't recognised as a command at all. Now you know how we extracted the reserved words above.

A close relative of whence is functions, which applies, of course, to shell functions; it usually lists the definitions of all functions given as arguments, but its relatives (of which autoload is one) perform various other tricks, to be described in the section on shell functions below. Be careful with function, without the `s', which is completely different and not like command or builtin --- it is actually a keyword used to define a function.

3.2.6: Parameter control

There are various builtins for controlling the shells parameters. You already know how to set and use parameters, but it's a good deal more complicated than that when you look at the details.

Local parameters

The principal command for manipulating the behaviour of parameters is `typeset'. Its easiest usage is to declare a parameter; you just give it a list of parameter names, which are created as scalar parameters. You can create parameters just by assigning to them, but the major point of `typeset' is that if a parameter is created that way inside a function, the parameter is restored to its original value, or removed if it didn't previously exist, at the end of the function --- in other words, it has `local scope' like the variables which you declare in most ordinary programming languages. In fact, to use the jargon it has `dynamical' rather than `syntactic' scope, which means that the same parameter is visible in any function called within the current one; this is different from, say, C or FORTRAN where any function or subroutine called wouldn't see any variable declared in the parent function.

The following makes this more concrete.

  var='Original value'
  subfn() {
    print $var
  }
  fn() {
    print $var
    typeset var='Value in function'
    print $var
    subfn
  }
  fn
  print $var
This chunk of code prints out
  Original value
  Value in function
  Value in function
  Original value
The first three chunks of the code just define the parameter $var, and two functions, subfn and fn. Then we call fn. The first thing this does is print out $var, which gives `Original value' since we haven't changed the original definition. However, the typeset next does that; as you see, we can assign to the parameter during the typeset. Thus when we print $var out again, we get `Value in function'. Then subfn is called, which prints out the same value as in fn, because we haven't changed it --- this is where C or FORTRAN would differ, and wouldn't recognise the variable because it hadn't been declared in that function. Finally, fn exits and the original value is restored, and is printed out by the final `print'.

Note the value changes twice: first at the typeset, then again at the end of fn. The value of $var at any point will be one of those two values.

Although you can do assignments in a typeset statement, you can't assign to arrays (I already said this in the last chapter):

  typeset var=(Doesn\'t work\!)
because the syntax with the parentheses is special; it only works when the line consists of nothing but assignments. However, the shell doesn't complain if you try to assign an array to a scalar, or vice versa; it just silently converts the type:
  typeset var='scalar value'
  var=(array value)
I put in the assignment in the typeset statement to rub the point in that it creates scalars, but actually the usual way of setting up an array in a function is
  typeset var
  var=()
which creates an empty scalar, then converts that to an empty array. Recent versions of the shell have `typeset -a var' to do that in one go --- but you still can't assign to it in the same statement.

There are other catches associated with the fact that typeset and its relatives are just ordinary commands with ordinary sets of arguments. Consider this:

  % typeset var=`echo two words`
  % print $var
  two
What has happened to the `words'? The answer is that backquote substitution, to be discussed below, splits words when not quoted. So the typeset statement is equivalent to
  % typeset var=two words
There are two ways to get round this; first, use an ordinary assignment:
  % typeset var
  % var=`echo two words`
which can tell a scalar assignment, and hence knows not to split words, or quote the backquotes,
  % typeset var="`echo two words`"

There are three important types we haven't talked about; both of these can only be created with typeset or one of the similar builtins I'll list in a moment. They are integer types, floating point types, and associative array types.

Numeric parameters

Integers are created with `typeset -i', or `integer' which is another way of saying the same thing. They are used for arithmetic, which the shell can do as follows:

  integer i
  (( i = 3 * 2 + 1 ))
The double parentheses surround a complete arithmetic expression: it behaves as if it's quoted. The expression inside can be pretty much anything you might be used to from arithmetic in other programming languages. One important point to note is that parameters don't need to have the $ in front, even when their value is being taken:
  integer i j=12
  (( i = 3 * ( j + 4 ) ** 2 ))
Here, j will be replaced by 12 and $i gets the value 768 (sixteen squared times three). One thing you might not recognise is the **, which is the `to the power of' operator which occurs in FORTRAN and Perl. Note that it's fine to have parentheses inside the double parentheses --- indeed, you can even do
  (( i = (3 * ( j + 4 )) ** 2 ))
and the shell won't get confused because it knows that any parentheses inside must be in balanced pairs (until you deliberately confuse it with your buggy code).

You would normally use `print $i' to see what value had been given to $i, of course, and as you would expect it gets printed out as a decimal number. However, typeset allows you to specify another base for printing out. If you do

  typeset -i 16 i
  print $i
after the last calculation, you should see 16#900, which means 900 in base 16 (hexadecimal). That's the only effect the option `-i 16' has on $i --- you can assign to it and use it in arithmetical expressions just as normal, but when you print it out it appears in this form. You can use this base notation for inputting numbers, too:
  (( i = 16#ff * 2#10 ))
which means 255 (ff in hexadecimal) times 2 (10 in binary). The shell understands C notation too, so `16#ff' could have been expressed `0xff'.

Floating point variables are very similar. You can declare them with `typeset -F' or `typeset -E'. The only difference between the two is, again, on output; -F uses a fixed point notation, while -E uses scientific (mnemonic: exponential) notation. The builtin `float' is equivalent to `typeset -E' (because Korn shell does it, that's why). Floating point expressions also work the way you are probably used to:

  typeset -E e
  typeset -F f
  (( e = 32/3, f = 32.0/3.0 ))
  print $e $f
prints
  1.000000000e+01 10.6666666667
Various points: the `,' can separate different expressions, just like in C, so the e and f assignments are performed separately. The e assignment was actually an integer division, because neither 32 nor 3 is a floating point number, which must contain a dot. That means an integer division was done, producing 10, which was then converted to a floating point number only at the end. Again, this is just how grown-up languages work, so it's no use cursing. The f assignment was a full floating point performance. Floating point parameters weren't available before version 3.1.7.

Although this is really a matter for a later chapter, there is a library of floating point functions you can load (actually it's just a way of linking in the system mathematical library). The usual incantation is `zmodload zsh/mathfunc'; you may not have `dynamic loading' of libraries on your system, which may mean that doesn't work. If it does, you can do things like

  (( pi = 4.0 * atan(1.0) ))
Broadly, all the functions which appear in most system mathematical libraries (see the manual page for math) are available in zsh.

Like all other parameters created with typeset or one of its cousins, integer and floating point parameters are local to functions. You may wonder how to create a global parameter (i.e. one which is valid outside as well as inside the function) which has an integer or floating point value. There's a recent addition to the shell (in version 3.1.6) which allows this: use the flag -g to typeset along with any others. For example,

  fn() {
    typeset -Fg f
    (( f = 42.75 ))
  }
  fn
  print $f
If you try it, you will see the value of $f has survived beyond the function. The g stands for global, obviously, although it's not quite that simple:
  fn() {
    typeset -Fg f
  }
  outerfn() {
    typeset f='scalar value'
    fn
    print $f
  }
  outerfn
The function outerfn creates a local scalar value for f; that's what fn sees. So it was not really operating on a `global' value, it just didn't create a new one for the scope of fn. The error message comes because it tried to preserve the value of $f while changing its type, and the value wasn't a proper floating point expression. The error message,
  fn: bad math expression: operator expected at `value'
comes about because assigning to numeric parameters always does an arithmetic evaluation. Operating on `scalar value' it found `scalar' and assumed this was a parameter, then looked for an operator like `+' to come next; instead it found `value'. If you want to experiment, change the string to `scalar + value' and set `value=42', or whatever, then try again. This is a little confusing (which is a roundabout way of saying it confused me), but consistent with how zsh usually treats parameters.

Actually, to a certain extent you don't need to use the integer and floating point parameters. Any time zsh needs a numeric expression it will force a scalar to the right value, and any time it produces a numeric expression and assigns it to a scalar, it will convert the result to a string. So

  typeset num=3            # This is the *string* `3'.
  (( num = num + 1 ))      # But this works anyway
                           # ($num is still a string).
This can be useful if you have a parameter which is sometimes a number, sometimes a string, since zsh does all the conversion work for you. However, it can also be confusing if you always want a number, because zsh can't guess that for you; plus it's a little more efficient not to have to convert back and forth; plus you lose accuracy when you do, because if the number is stored as a string rather than in the internal numeric representation, what you say is what you get (although zsh tends to give you quite a lot of decimal places when converting implicitly to strings). Anyway, I'd recommend that if you know a parameter has to be an integer or floating point value you should declare it as such.

There is a builtin called let to handle mathematical expressions, but since

  let "num = num + 1"
is equivalent to
  (( num = num + 1 ))
and the second form is easier and more memorable, you probably won't need to use it. If you do, remember that (unlike BASIC) each mathematical expression should appear as one argument in quotes.

Associative arrays

The one remaining major type of parameter is the associative array; if you use Perl, you may call it a `hash', but we tend not to since that's really a description of how it's implemented rather than what it does. (All right, what it does is hash things. Now shut up.)

These have to be declared by a typeset statement --- there's no getting round it. There are some quite eclectic builtins that produce a filled-in associative array for you, but the only way to tell zsh you want your very own associative array is

  typeset -A assoc
to create $assoc. As to what it does, that's best shown by example:
  typeset -A assoc
  assoc=(one eins two zwei three drei)
  print ${assoc[two]}
which prints `zwei'. So it works a bit like an ordinary array, but the numeric subscript of an ordinary array which would have appeared inside the square bracket is replaced by the string key, in this case two. The array assignment was a bit deceptive; the `values' were actually pairs, with `one' being the key for the value `eins', and so on. The shell will complain if there are an odd number of elements in such a list. This may also be familiar from Perl. You can assign values one at a time:
  assoc[four]=vier
and also unset one key/value pair:
  unset 'assoc[one]'
where the quotes stop the square brackets from being interpreted as a pattern on the command line.

Expansion has been held over, but you might like to know about the ways of getting back what you put in. If you do

  print $assoc
you just see the values --- that's exactly the same as with an ordinary array, where the subscripts 1, 2, 3, etc. aren't shown. Note they are in random order --- that's the other main difference from ordinary arrays; associative arrays have no notion of an order unless you explicitly sort them.

But here the keys may be just as interesting. So there is:

  print ${(k)assoc}
  print ${(kv)assoc}
giving (if you've followed through all the commands above):
  four two three
  four vier two zwei three drei
which print out the keys instead of the values, and the key and value pairs much as you entered them. You can see that, although the order of the pairs isn't obvious, it's the same each time. From this example you can work out how to copy an associative array into another one:
  typeset -A newass
  newass=(${(kv)assoc})
where the `(kv)' is important --- as is the typeset just before the assignment, otherwise $newass would be a badass ordinary array. You can also prove that ${(v)assoc} does what you would probably expect. There are lots of other tricks, but they are mostly associated with clever types of parameter expansion, to be described in chapter 5.

Other typeset and type tricks

There are variants of typeset, some mentioned sporadically above. There is nothing you can do with any of them that you can't do with typeset --- that wasn't always the case; we've tried to improve the orthogonality of the options. They differ in the options which are set by default, and the additional options which are allowed. Here's a list: declare, export, float, integer, local, readonly. I won't confuse you by describing all in detail; see the manual.

If there is an odd one out, it's export, which not only marks a parameter for export but has the -g flag turned on by default, so that that parameter is not local to the function; in other words, it's equivalent to typeset -gx. However, one holdover from the days when the options weren't quite so logical is that typeset -x behaves like export, in other words the -g flag is turned on by default. You can fix this by unsetting the option GLOBAL_EXPORT --- the option only exists for compatibility; logically it should always be unset. This is partly because in the old days you couldn't export local parameters, so typeset -x either had to turn on -g or turn off -x; that was fixed for the 3.1.9 release, and (for example) `local -x' creates a local parameter which is exported to the environment; both the parameter itself, and the value in the environment, will be restored when the function exits. The builtin local is essentially a form of typeset which renounces the -g flag and all its works.

Another old restriction which has gone is that you couldn't make special parameters, in particular $PATH, local to a function; you just modified the original parameter. Now if you say `typeset PATH', things happen the way you probably expect, with $PATH having its usual effect, and being restored to its old value when the function exits. Since $PATH is still special, though, you should make sure you assign something to it in the function before calling external commands, else it will be empty and no commands will be found. It's possible that you specifically don't want some parameter you make local to have the special property; 3.1.7 and after allow the typeset flag -h to hide the specialness for that parameter, so in `typeset -h PATH', PATH would be an ordinary variable for the duration of the enclosing function. Internally, the same value as was previously set would continue to be used for finding commands, but it wouldn't be exported.

The second main use of typeset is to set attributes for the parameters. In this case it can operate on an existing parameter, as well as creating a new one. For example,

  typeset -r msg='This is an important message.'
sets the readonly flag (-r) for the parameter msg. If the parameter didn't exist, it would be created with the usual scoping rules; but if it did exist at the current level of scoping, it would be made readonly with the value assigned to it, meaning you can't set that particular copy of the parameter. For obvious reasons, it's normal to assign a value to a readonly parameter when you first declare it. Here's a reality check on how this affects scoping:
   msg='This is an ordinary parameter'
   fn() {
     typeset msg='This is a local ordinary parameter'
     print $msg
     typeset -r msg='This is a local readonly parameter'
     print $msg
     msg='Watch me cause an error.'
   }
   fn
   print $msg
   msg='This version of the parameter'\ 
   ' can still be overwritten'
   print $msg
outputs
  This is a local ordinary parameter
  This is a local readonly parameter
  fn:5: read-only variable: msg
  This is an ordinary parameter
  This version of the parameter can still be overwritten
Unfortunately there was a bug with this code until recently --- thirty seconds ago, actually: the second typeset in fn incorrectly added the readonly flag to the existing msg before attempting to set the new value, which was wrong and inconsistent with what happens if you create a new local parameter. Maybe it's reassuring that the shell can get confused about local parameters, too. (I don't find it reassuring in the slightest, since typeset is one of the parts of the code where I tend to fix the bugs, but maybe you do.)

Anyway, when the bug is fixed, you should get the output shown, because the first typeset created a local variable which the second typeset made readonly, so that the final assignment caused an error. Then the $msg in the function went out of scope, and the ordinary parameter, with no readonly restriction, was visible again.

I mentioned another special typeset option in the previous chapter:

  typeset -T TEXINPUTS texinputs
to tie together the scalar $TEXINPUTS and the array $texinputs in the same way that $PATH and $path work. This is a one-off; it's the only time typeset takes exactly two parameter names on the command line. All other uses of typeset take a list of parameters to which any flags given are applied. See the manual for the remaining flags, although most of the more interesting ones have been discussed.

The other thing you need to know about flags is that you use them with a `+' sign to turn off the corresponding attribute. So

  typeset +r msg
allows you to set $msg again. From version 4.1, you won't be able to turn off the readonly attribute for a special parameter; that's because there's too much scope for confusion, including attempting to set constant strings in the code. For example, `$ZSH_VERSION' always prints a fixed string; attempting to change that is futile.

The final use of typeset is to list parameters. If you type `typeset' on its own, you get a complete list of parameters and their values. From 3.1.7, you can turn on the flag -H for a parameter, which means to hide its value while you're doing this. This can be useful for some of the more enormous parameters, particularly special parameters which I'll talk about in the section in chapter 7 on modules, which tend to swamp the display typeset produces.

You can also list parameters of a particular type, by listing the flags you want to know about. For example,

  typeset -r
lists all readonly parameters. You might expect `typeset +r' to list parameters which don't have that attribute, but actually it lists the same parameters but without showing their value. `typeset +' lists all parameters in this way.

Another good way of finding out about parameters is to use the special expansion `${(t)param}', for example

  print ${(t)PATH}
prints `scalar-export-special': $PATH is a scalar parameter, with the -x flag set, and has a special meaning to the shell. Actually, `special' means something a bit more than that: it means the internal code to get and set the parameter behaves in a way which has side effects, either to the parameter itself or elsewhere in the shell. There are other parameters, like $HISTFILE, which are used by the shell, but which are get and set in a normal way --- they are only special in that the value is looked at by the shell; and, after all, any old shell function can do that, too. Contrast this with $PATH which has all that paraphernalia to do with hashing commands to take care of when it's set, as I discussed above, and I hope you'll see the difference.

Reading into parameters

The `read' builtin, as its name suggests, is the opposite to `print' (there's no `write' command in the shell, though there is often an external command of that name to send a message to another user), but reading, unlike printing, requires something in the shell to change to take the value, so unlike print, read is forced to be a builtin. Inevitably, the values are read into a parameter. Normally they are taken from standard input, very often the terminal (even if you're running a script, unless you redirected the input). So the simplest case is just

  read param
and if you type a line, and hit return, it will be put into $param, without the final newline.

The read builtin actually does a bit of processing on the input. It will usually strip any initial or final whitespace (spaces or tabs) from the line read in, though any in the middle are kept. You can read a set of values separated by whitespace just by listing the parameters to assign them to; the last parameter gets all the remainder of the line without it being split. Very often it's easiest just to read into an array:

  % read -A array
        this is a line typed in now, \ 
      by me,    in this   space
  % print ${array[1]} ${array[12]}
  this space
(I'm assuming you're using the native zsh array format, rather than the one set with KSH_ARRAYS, and shall continue to assume this.)

It's useful to be able to print a prompt when you want to read something. You can do this with `print -n', but there's a shorthand:

  % read line'?Please enter a line: '
  Please enter a line: some words
  % print $line
  some words
Note the quotes surround the `?' to prevent it being taken as part of a pattern on the command line. You can quote the whole expression from the beginning of `line', if you like; I just write it like that because I know parameter names don't need quoting, because they can't have funny characters in. It's almost logical.

Another useful trick with read is to read a single character; the `-k' option does this, and in fact you can stick a number immediately after the `k' which specifies a number to read. Even easier, the `-q' option reads a single character and returns status 0 if it was y or Y, and status 1 otherwise; thus you can read the answer to yes/no questions without using a parameter at all. Note, however, that if you don't supply a parameter, the reply gets assigned in any case to $REPLY if it's a scalar --- as it is with -q --- or $reply if it's an array --- i.e. if you specify -A, but no parameter name. These are more examples of the non-special parameters which the shell uses --- it sets $REPLY or $reply, but only in the same way you would set them; there are no side-effects.

Like print, read has a -r flag for raw mode. However, this just has one effect for read: without it, a \ at the end of the line specifies that the next line is a continuation of the current one (you can do this when you're typing at the terminal). With it, \ is not treated specially.

Finally, a more sophisticated note about word-splitting. I said that, when you are reading to many parameters or an array, the word is split on whitespace. In fact the shell splits words on any of the characters found in the (genuinely special, because it affects the shell's guts) parameter $IFS, which stands for `input field separator'. By default --- and in the vast majority of uses --- it contains space, tab, newline and a null character (character zero: if you know that these are usually used to mark the end of strings, you might be surprised the shell handles these as ordinary characters, but it does, although printing them out usually doesn't show anything). However, you can set it to any string: enter

  fn() {
    local IFS=:
    read -A array
    print -l $array
  }
  fn
and type
one word:two words:three words:four
The shell will show you what's in the array it's read, one `word' per line:
  one word
  two words
  three words
  four
You'll see the bananas, er, words (joke for the over-thirties) have been treated as separated by a colon, not by whitespace. Making $IFS local didn't work in old versions of zsh, as with other specials; you had to save it and restore it.

The read command in zsh doesn't let you do line editing, which some shells do. For that, you should use the vared command, which runs the line editor to edit a parameter, with the -c option, which allows vared to create a new parameter. It also takes the option -p to specify a prompt, so one of the examples above can be rewritten

  vared -c -p 'Please enter a line: ' line
which works rather like read but with full editing support. If you give the option -h (history), you can even retrieve values from previous command lines. It doesn't have all the formatting options of read, however, although when reading an array (use the option -a with -c if creating a new array) it will perform splitting.

Other builtins to control parameters

The remaining builtins which handle parameters can be dealt with more swiftly.

The builtin set simply sets the special parameter which is passed as an argument to functions or scripts, and which you access as $* or $@, or $<number> (Bourne-like format), or via $argv (csh-like format), known however you set them as the `positional parameters':

  % set a whole load of words
  % print $1
  a
  % print $*
  a whole load of words
  % print $argv[2,-2]
  whole load of
It's exactly as if you were in a function and had called the function with the arguments `a whole load of words'. Actually, set can also be used to set shell options, either as flags, e.g. `set -x', or as words after `-o' , e.g. `set -o xtrace' does the same as the previous example. It's generally easier to use setopt, and the upshot is that you need to be careful when setting arguments this way in case they begin with a `-'. Putting `--' before the real arguments fixes this.

One other use of set is to set any array, via

  set -A any_array words to assign to any_array
which is equivalent to (and the standard Korn shell version of)
  any_array=(words to assign to any_array)
One case where the set version is more useful is if the name of an array itself comes from a parameter:
  arrname=myarray
  set -A $arrname words to assign
has no easy equivalent in the other form; the left hand side of an ordinary assignment won't expand a parameter:
  # Doesn't work; syntax error
  $arrname=(words to assign)
This worked in old versions of zsh, but that was on the non-standard side. The eval command, described below, gives another way around this.

Next comes `shift', which simply moves an array up one element, deleting the original first one. Without an array name, it operates on the positional parameters. You can also give it a number to shift other than one, before the array name.

  shift array
is equivalent to
  array=(${array[2,-1]})
(almost --- I'll leave the subtleties here for the chapter on expansion) which picks the second to last elements of the array and assigns them back to the original array. Note, yet again, that shift operates using the name, not the value of the array, so no `$' should appear in front, otherwise you get something similar to the trick I showed for `set -A'.

Finally, unset unsets a parameter, and I already showed you could unset a key/value pair of an associative array. There is one subtlety to be mentioned here. Normally, unset just makes the parameter named disappear off the face of the earth. However, if you call unset in a function, its ghost lives on in the sense that any parameter you create in the same name will be scoped as the original parameter was. Hence:

  var='global value'
  fn() {
    typeset var='local value'
    unset var
    var='what about this?'
  }
  fn
  print $var
The final statement prints `global value': even though the local copy of $var was unset, the shell remembers that it was local, so the second $var in the function is also local and its value disappears at the end of the function.

3.2.7: History control commands

The easiest way to access the shell's command history is by editing it directly. The second easiest way is to use the `!'-history mechanism. Other ways of manipulating it are based around the fc builtin, which probably once stood for something (according to Oliver Kiddle, `fix command', which is as good as anything). I talked quite a bit about it in the last chapter, and don't really have anything to add. Just note that the two other commands based around it are history and r.

3.2.8: Job control and process control

One of the major contributions of the C-shell was job control. You need to know about foreground and background tasks, and again I introduced these in the last chapter along with the options that control them. Here is an introduction to the relevant builtins.

You start a background job in two ways. First, directly, by putting an `&' after it:

  sleep 10 &
and secondly by starting it in the normal way (i.e. in the foreground), then typing ^Z, and using the bg command to put it in the background. Between typing ^Z and bg, the job is still there, but is not running; it is `suspended' or `stopped' (systems use different descriptions for the same thing), waiting for you to decide what to do with it. In either case, the job then continues without the shell waiting for it. It will still try and read from or write to the terminal if that's how you started it; you need to use the shell's redirection facilities right at the start if you want to change that, there's nothing you can do after the job has already started.

By the way, `sleep' isn't a builtin. Oddly enough, you can suspend a builtin command or sequence of commands (such as shell function) with ^Z, although since the shell has to continue executing your commands as well as being suspended, it does the only thing it can do --- fork, so that the commands you suspend are put into the background. Probably you will only rarely do this with builtins. No other shell, so far as I know, has this feature.

A job will stop if it needs to read from the terminal. You see a message like:

  [1]  + 1348 suspended (tty input)  jobname and arguments
which means the job is suspended very much like you had just typed ^Z. You need to bring the job into the forground, as described below, so that you can type something to it.

By the way, the key to type to suspend a command may not be ^Z; it usually is, but that can be changed. Run `stty -a' and look for what is listed after `susp =' --- probably, but not necessarily, ^Z. So if you want to use another character --- it must be a single character; this is handled deep in the terminal interface, not in the shell --- you can run

  stty susp '^]'
or whatever. You will note from the stty output that various other job control characters can be changed similarly. The stty command is external and its format for both output and input can vary quite a bit from system to system.

Instead of putting the command into the background, you can bring it back to the foreground again with fg. This is useful for temporarily stopping what you are doing so you can do something else. These days you would probably do it in another window; in the old days when people logged in from simple terminals this was even more useful. A typical example of this is

  more file                        # look at file
  ^Z                               # suspend
  [1] + 8592 suspended  more file  # message printed
  ...                              # do something else
  fg %1                            # resume the `more'
The `%' is the usual way of referring to jobs. The number after it is what appeared in square brackets with the suspended message; I don't know why the shell doesn't use the `%' notation there, too. You also see that with the `continued' message when you put something into the background, and again at the end with the `done' message which tells you a background job is finished. The `%' can take other forms; the most common is to follow it by the name of a command, such as `%more' in this case. The forms %+ and %- refer to the most recent and second most recent jobs --- the `+' in the `suspended' message is telling you that the more job could be referred to like that.

Most of the job control commands will actually assume you are talking about `%+' if you don't give an argument, so assuming I hadn't started any other commands in the background, I could just have put `fg' at the end of the sequence of commands above. This actually cuts both ways: fg is the default operation on jobs referred to with the `%' notation, so just typing `%1' with no command name would have worked, too.

You can jog your memory about what's going on with the `jobs' command. It looks like a series of messages of the form beginning with the number in square brackets; usually the jobs will either be `running' or `suspended'. This will tell you the numbers you need.

One other useful thing you can do with a job is to tell the shell to forget about it. This is only really useful if it is already running in the background; then you can run `disown' with the job identifier. It's useful for jobs you want to continue after you've logged out, as well as jobs that have their own windows which you can therefore control directly. With disowned jobs, the shell doesn't warn you that they are still there when you log out. You can actually disown a background job when you start it by putting `&|' or `&!' at the end of the line instead of simply `&'. Note that if the job was suspended when you disowned it, it will stay disowned; this is pretty pointless, so you probably should run `bg' on it first.

The next most likely thing you want to do with a job is kill it, or maybe suspend it when it's already in the background and you can't just type ^Z. This is where the kill builtin comes in. There's more to this than there is to the builtins mentioned above. First, you can use kill with other processes that weren't started from the current shell. In that case, you would use a number to identify it, with no % --- that's why the %'s were there in the other cases. Of course, you need to find out the number; the usual way is with the ps command, which is not a builtin but which appears on all UNIX-like systems. As a stupid example, here I start a disowned process which does very little, look for it, then kill it:

  % sleep 60 &|
  % ps -f
  UID        PID  PPID  C STIME TTY          TIME CMD
  pws        623   614  0 22:12 pts/0    00:00:00 zsh
  pws       8613   623  0 23:12 pts/0    00:00:00 sleep 60
  pws       8615   623  0 23:12 pts/0    00:00:00 ps -f
  % kill 8613
  % ps -f
  UID        PID  PPID  C STIME TTY          TIME CMD
  pws        623   614  0 22:12 pts/0    00:00:00 zsh
  pws       8616   623  0 23:12 pts/0    00:00:00 ps -f
The process has disappeared the second time I look. Notice that in the usual lugubrious UNIX way the shell didn't bother to tell you the process had been killed; however, it will report an error if it failed to send it the signal. Sending it the signal is all the shell cares about; the shell won't warn if you if the process decided it didn't want to die when told to, so it's still a good idea to check.

Sometimes you want to wait for a process to exit; the wait builtin can do this, and like kill can take a process number as well as a job number. However, that's a bit deceptive --- you can't actually wait for a process which wasn't started directly from the shell. Indeed, the mechanism for waiting is all bound up with the way UNIX handles processes; unless its parent waits for it, a process becomes a `zombie' and hangs around until the system's foster parent, the `init' process (always process number 1) waits for it instead. It's all a little bit baroque, but for the shell user, wait just means you can hang on until something you started has finished. Indeed, that's how foreground processes work: the shell in effect uses the internal version of wait to hang around until the job exits. (Well, actually that's a lie; the system wakes it up from whatever it's doing to tell it a child has finished, so all it has to do is doze off to wait.)

Furthermore, you can wait for a process even if job control isn't running. Job control, basically anything involving those %'s, is only useful when you are sitting at a terminal fiddling with commands; it doesn't operate when you run scripts, say. Then the shell has much less freedom in how to control its jobs, but it can still wait for a background process, and it can still use kill on a process if it knows its number. For this purpose, the shell stores the ID of the last process started in the background in the parameter $!; there's probably a good reason for the `!', but I don't know what it is. This happens regardless of job control.

Signals

The kill command can do a good deal more than just kill a process. That is the default action, which is why the command has that name. But what it's really doing is sending a `signal' to a process. Signals are the simplest way of communicating to another process; in fact, they are about the only simple way if you haven't made special arrangements for the process to read messages from you. Signal names are written like SIGINT, SIGTSTP, SIGKILL; to send a particular signal to a process, you remove the SIG, stick a hyphen in front, and use that as the first argument to kill, e.g.:

  kill -KILL 8613
Some of the things you already know about are actually doing just that. When you type ^C to stop a process, you are actually sending it a SIGINT for `interrupt', as if you had done
  kill -INT 8613
The usual signal sent by kill is not, as you might have guessed, SIGKILL, but actually SIGTERM for `terminate'; SIGKILL is stronger as the process can't block that signal, as it can with many (we'll see how the shell can do that in a moment). It's familiar to UNIX hackers as `kill -9', because all the signals also have numbers. You can see the list of signals in zsh by doing:
  % print $signals
  EXIT HUP INT QUIT ILL TRAP ABRT BUS FPE KILL USR1
  SEGV USR2 PIPE ALRM TERM STKFLT CLD CONT STOP TSTP
  TTIN TTOU URG XCPU XFSZ VTALRM PROF WINCH POLL PWR
  UNUSED ZERR DEBUG
Your list will probably be different from mine; this is for Linux, and the list is very system-specific, even though the first nine are generally the same, and many of the others are virtually always present. Actually, SIGEXIT is an invention by the shell for you to allow the shell to do something when a function exits (see the section on `traps' below); you can't actually use `kill -EXIT'. Thus SIGHUP is the first real signal, and indeed that's number one, so you have to shift the contents of $signals along one to get the right numbers. SIGTERM and SIGINT usually have the same effect, stopping the process, unless that has decided to handle the signal some other way.

The last two signals are bogus, too: SIGZERR is to allow the shell to do something on an error (non-zero exit status), while with SIGDEBUG you can do it on every command. Again, the `something' to be executed is a `trap', as I'll discuss in a short while.

Typing ^Z to suspend a process actually sends the process a SIGTSTP (terminal stop, since it usually comes from the terminal), while SIGSTOP is similar but usually doesn't come from a terminal. Even restarting a process as with bg sends it a signal, in this case SIGCONT. It seems a bit odd to signal a process to restart; why can't the operating system just restart it when you ask? The real answer is probably that signals provide an easy way for you to talk to the operating system without grovelling around in the dirt too much.

Before I talk about how you make the shell handle signals it receives, there is one extra oddment: the suspend builtin effectively sends the shell a signal to suspend it, as if you'd typed ^Z, though as you've probably found by now that doesn't suspend the shell itself. It's only useful to do this if the shell is running under some other programme, else there's no way of restoring it and suspending is effectively the same as exiting the shell. For this reason, the shell won't let you call suspend in a login shell, because it assumes that is running as the top level (though in the previous chapter you learnt there's actually nothing that special about login shells; you can start one just with `zsh -l'). If you're logged in remotely via rsh or ssh, it's usually more convenient to use the keystrokes `~^Z' which those define, rather than zsh's mechanism; they have to be at the beginning of a line, so hit return first if necessary. This returns you to your local terminal; you can resume the remote login with `fg' just like any other programme.

Traps

The way of making the shell handle signals is called `traps'. There are actually two mechanisms for this. I'll present the more standard one and then talk about the advantages and drawbacks of the other one at the end.

The standard version (shared with other shells) is via the `trap' builtin. The first argument is a chunk of shell code to execute, which obviously needs to be quoted when you pass it as an argument, and the remaining arguments are a list of signals to handle, minus the SIG prefix. So:

  trap "echo I\\'m trapped." INT
tells the shell what to do on SIGINT, i.e. ^C. Note the extra layer of quoting: the double quotes surround the code, so that when they are stripped trap sees the chunk
  echo I\'m trapped
Usually the shell would abort what it was doing and return to the main prompt when you hit ^C. Now, however, it will simply print the message and carry on. You can try this, for example, with
  read line
If you hit ^C while it's waiting for input, you'll see the message go up, but the shell will still wait for you to type a line.

A warning about this: ^C is only trapped within the shell itself. If you start up an external programme, it will have its own mechanism for handling signals, and if it usually aborts on ^C it still will. But there's a sting in the tail: do

  cat
which waits for input to output again (you need to use ^D to exit normally). If you type ^C here, the command will be aborted, as I said --- but you still get the message `I'm trapped'. That's because the shell is able to tell that the command got that particular signal, and calls the trap when the cat exits. Not all shells do this; furthermore, some commands which handle signals themselves won't give the shell enough information to know that a signal arrived, and in that case the trap won't be called. Such commands are usually the more sophisticated things like editors or screen managers or whatever; you just have to find out by trial and error.

You can also make the shell ignore the signal completely. To do this, the first argument should be an empty string:

  trap '' INT
Now ^C will have no effect, and this time the effect is passed on directly to commands called from the shell --- try the cat example and you won't be able to interrupt it; type ^D or use the lesser known but more powerful ^\ (control with backslash), which sends SIGQUIT. If it hasn't been disabled, this will also produce a file core, which contains debugging information about what the programme was doing when it exited --- never call your own files core. You can trap SIGQUIT too, if you want. (The shell itself usually ignores SIGQUIT; it's only useful for external commands.)

Now the other sort of trap. I could have written for the first example:

  TRAPINT() {
    print I\'m trapped.
  }
As you can see, this is just a function: functions beginning TRAP are special. However, it's a real function too; you can call it by hand with the command `TRAPINT', and it will run perfectly happily with no funny side effects.

There is a difference between the way the two types work. In the `trap' sort of trap, the code is just evaluated just as if it appeared as instructions to the shell at the point where the trap happened. So if you were in a function, you would see the environment of that function with its local variables; if you set a local variable with typeset, it would be visible in the function just as if it were created there.

However, in the function type of trap, the code is provided with its own function environment. Now if you use typeset the parameter created is local only to the trap. In most cases, that's all the difference there is; it's up to you to decide which is more convenient. As you can see, the function type of trap doesn't require the extra layer of quoting, so looks a little smarter. Conveniently, the `trap' command on its own lists all traps in the form of the shell code you'd need to recreate them, and you can see which sort is which.

There are two cases where the difference sticks out. One is that the function type has some extra wiring to allow you both to trap a signal, and pretend to anyone watching that the shell didn't handle it. An example will show this:

  TRAPINT() {
    print "Signal caught, stopping anyway."
    return $(( 128 + $1 ))
  }
That second line may look as rococo as the Amalienburg, but it's meaning is this: $1, the first argument to the function, is set to the number of the signal. In this case it will be 2 because that's the standard number for SIGINT. That means the arithmetic substitution $((...)) returns 130, the command `return 130' is executed, and the function returns with status 130. Returning with non-zero status is special in function traps: it tells the shell you want to abort the surrounding command even though the trap was handled, and that you want the status associated with that to be 130. It so happens that this is how UNIX handles returns from normal traps. Without setting a trap, do
  % cat
  ^C
  % print $?
and you'll see that this, too, has given the status 130, 128 plus the value of SIGINT. So if you do have the trap set, you'll see the message, but the command will abort --- even if it was running inside the shell.

Try

  % read line
  ^C
to see that happening. If you look at the status in $? you'll find it's actually 1, not 130; that's because the read command, when it aborted, overrode the return value from the trap. But it does that with an untrapped ^C, too, so that's not really an exception to what I've just said.

If you've been paying attention, you'll realise that traps set with the trap builtin can't do it in quite this way, because the function they return from would be whatever function you were in. You can see that:

  trap 'echo Returning...; return;' INT
  fn() {
    print In fn...
    read param
    print Leaving fn..
  }
If you run fn and hit ^C, the signal is trapped and the message printed, but because of the return, the shell quits fn immediately and you don't see the final message. If you missed out the `return;' (try it), the shell would carry on with the rest of fn after you typed something to read. Of course you can use this mechanism to leave functions after trapping a signal; it just so happens that in this case the mechanism with TRAPINT is a little closer to what untrapped signals do and hence a little neater.

One final flourish of late Baroque splendour: the trap for SIGEXIT, the one called when a function (or the shell itself, in fact) exits is a bit special because in the case of exiting a function it will be called in the environment of the calling function. So if you need to do something like set a local variable for an enclosing function you can have

  trap 'typeset param_in_enclosing_func=value' EXIT
do it for you; you couldn't do that with TRAPEXIT because the code would have its own function, so that even though it would be called after the first function exited, it wouldn't run directly in the enclosing one but in a separate TRAPEXIT function. You can even set an EXIT trap for the enclosing function by defining a nested `trap .. EXIT' inside that trap itself.

I lied, because there is one more special thing about TRAPEXIT: it's always reset after you exit a function and the trap itself has been called. Most traps just hang around until you explicitly unset them. There is an option, LOCAL_TRAPS, which makes traps set inside functions as well insulated as possible from those outside, or inside deeper functions. In other words, the old trap is saved and then restored when you exit the function; the scoping works pretty much like that for typeset, and in the same way traps for the enclosing scope, apart from any for EXIT, remain in effect inside a function unless you explicitly override them; and, again in the same way, if you unset it inside the function it will still be restored on exit.

LOCAL_TRAPS is the fixed behaviour of some other shells. In zsh, without the option set:

  trap 'echo Hi.' INT
  fn() {
     trap 'echo Bye.' INT
  }
Calling fn simply replaces the trap defined outside the function with the one defined inside while:
  trap 'echo Hi.' INT
  fn() {
     setopt localtraps
     trap 'echo Bye.' INT
  }
puts the original `Hi' trap back after the function exits.

I haven't told you how to unset a trap for good: the answer is

 trap - INT
As you would guess, you can use unfunction with function-type traps; that will correctly remove the trap as well as deleting the function. However, `trap -' works with both, so that's the recommended way.

Limits on processes

One other way that jobs started by the shell can be controlled is by using limits. These are actually limits set by the operating system, but the shell gives you a way of controlling them: the limit and unlimit commands. Type `limit' on its own to see a summary. I get:

  cputime         unlimited
  filesize        unlimited
  datasize        unlimited
  stacksize       8MB
  coredumpsize    0kB
  memoryuse       unlimited
  maxproc         2048
  descriptors     1024
  memorylocked    unlimited
  addressspace    unlimited
where the item on the left of each line is what is being limited, and on the right is the value. The manual page to look at, at least on Linux is for the function getrusage; that's the function the shell is calling when you run limit or unlimit.

In this case, the items are:

cputime
the total CPU time used by a process

filesize
maximum size of a file

datasize
the maximum size of data in use by a programme

stacksize
the maximum size of the stack, which is the area of memory used to store information during function calls

coredumpsize
the maximum size of a core file, which is an image of memory left by a programme that crashes, allowing you to debug it with gdb, dbx, ddd or some other debugger

memoryuse
the maximum main memory, i.e. programme memory which is in active use and hasn't been `swapped out' to disk

maxproc
the maximum number of simultaneous processes

descriptors
the maximum number of simultaneously open files (`descriptors' are the internal mechanism for referring to an open file on UNIX-like systems)

memorylocked
the maximum amount of memory locked in (I don't know what that is, either)

addressspace
the total amount of virtual memory, i.e. any memory whether it is main memory, or refers to somewhere on a disk, or indeed anything else.
You may well see other names; the shell decides when it is compiled what limits are supported by the system.

Of those, the one I use most commonly is coredumpsize: sometimes when I'm debugging a file I want a crashed programme to produce a `core' files so I can run gdb or dbx on it (`unlimit coredumpsize'), while other times they are just untidy (`limit coredumpsize 0'). Probably you would only alter any of the others if you knew there was a problem, for example a number-crunching programme used so much memory that the rest of the system was badly affected and you wanted to limit datasize to 64 megabyte or whatever. You could write this as:

  limit datasize 64m

There is a distinction made between `hard' and `soft' limits. Both have the same effect on programmes, but you can remove or reduce `soft' limits, while only the superuser (the system administrator's login, root) can do that to `hard' limits. Usually, therefore, limit and unlimit manipulate soft limits; to show or set hard limits, give the option -h. If I do `limit -h', I get the same list of limits as above, but with stacksize and coredumpsize unlimited --- that means I can reduce or remove the limits on those if I want, they're just set for my own convenience.

Why is stacksize set in this way? As I said, it refers to the memory in which the functions in programmes store variables and any other local information. If one function calls another, it uses more memory. You can get into a situation where functions call themselves recursively and there is no way out until the machine runs out of memory; limiting stacksize prevents this. You can actually see this with zsh itself (probably better not to try this if you'd rather the shell you're running didn't crash):

  % fn() { fn; }
  % fn
defines a function which keeps calling itself. To do this, all the functions inside zsh are calling themselves as well, using more and more stack memory. Actually, zsh uses other forms of memory inside each function and my version of zsh crashes due to exhaustion of that memory instead. However, it depends on the system how this works out.

Times

One way of returning information on process resources is with the `times' command. It simply shows the total CPU time used by the shell and by the programmes called for it --- in that order, and without description, so you need to remember. On each line, the first number is the time spent in user space and the second is the time spent in system space. If you're not concerned about the details of programmes the difference is pretty irrelevant, but if you are, then the difference is very roughly that between the time spent in the code you actually see before you compile a programme, and the time spent in `hidden' code where the system is doing something for you. It's not such an obvious distinction, because many library routines, such as mathematical functions, are run in user mode as no privileged access to internal bits of the system is required. Typically, system time is concerned with the details of input and output --- though even there it's not so simple, because the C output routines printf, puts, fread and others have user mode code which then calls the system routines read, write and so on.

You can measure the time taken by a particular external command by putting `time', in the singular this time, in front of it; this is essentially another precommand modifier, and is a shell reserved word rather than a builtin. This gives fairly obvious information. You can specify the information using the $TIMEFMT parameter, which has its own percent escapes, different from the ones used in prompts. It exists partly because the shell allowed you to access all sorts of other information about the process which ran, such as `page faults' --- occasions when the system had to fetch a part of the programme or data from disk because it wasn't in the main memory. However, that disappeared because it was too much work to convert the feature to configure itself automatically for different operating systems. It may be time to resurrect it.

You can also force the time to be shown automatically by setting the parameter $REPORTTIME; if a command runs for more than this many seconds, the $TIMEFMT output will be shown automatically.

3.2.9: Terminals, users, etc.

Watching for other users

Although this is more associated with parameters than builtins, the `log' command will tell you whether any of a group of people you want to watch out for have logged in or out. To use this, you set the $watch array parameter to a list of user names, or `all' for everyone, or `notme' for everyone except yourself. Even if you don't use log, any changes will be reported just before the shell prints a prompt. It will be printed using the $WATCHFMT parameter: once again, this takes its own set of percent escapes, listed in the zshparam manual.

ttyctl

There is a command ttyctl which is designed to keep badly behaved external commands from messing up the terminal settings. Most programmes are careful to restore any settings they change, but there are exceptions. After `ttyctl -f', the terminal is frozen; zsh will restore the settings, no matter what an external programme does with it. This includes deliberate attempts to change the terminal settings with the `stty' command, so the default is unfrozen, `ttyctl -u'.

3.2.10: Syntactic oddments

This section collects together a few builtins which, rather than controlling the behaviour of some feature of the shell, have some other special effect.

Controlling programme flow

The four functions here are exit, return, break, continue and source or .: they determine what the shell does next. You've met exit --- leave the shell altogether --- and return --- leave the current function. Be very careful not to confuse them. Calling exit in a shell function is usually bad:

  % fn() { exit; }
  % fn
This makes you entire shell session go away, not just the function. If you write C programmes, you should be very familiar with both, although there is one difference in this case: return at the top level in an interactive shell actually does nothing, rather than leaving the shell as you might expect. However, in a script, return outside a function does cause the entire script to stop. The reason for this is that zsh allows you to write autoloaded functions in the same form as scripts, so that they can be used as either; this wouldn't work if return did nothing when the file was run as a script. Other shells don't do this: return does nothing at the top level of a script, as well as interactively. However, other shells don't have the feature that function definition files can be run as scripts, either.

The next two commands, break and continue, are to do with constructs like `if'-blocks and loops, and it will be much easier if I introduce them when I talk about those below. They will also already be familiar to C programmers. (If you are a FORTRAN programmer, however, continue is not the statement you are familiar with; it is instead equivalent to CYCLE in FORTRAN90.)

The final pair of commands are . and source. They are similar to one another and cause another file to be read as a stream of commands in the current shell --- not as a script, for which a new shell would be started which would finish at the end of the script. The two are intended for running a series of commands which have some effect on the current shell, exactly like the startup files. Indeed, it's a very common use to have a call to one or other in a startup file; I have in my ~/.zshrc

  [[ -f ~/.aliasrc ]] && . ~/.aliasrc
which tests if the file ~/.aliasrc exists, and if so runs the commands in it; they are treated exactly as if they had appeared directly at that point in .zshrc.

Note that your $path is used to find the file to read from; this is a little surprising if you think of this as like a script being run, since zsh doesn't search for a script, it uses the name exactly as you gave it. In particular, if you don't have `.' in your $path and you use the form `.' rather than `source' you will need to say explicitly when you want to source a file in the current directory:

  . ./file
otherwise it won't be found.

It's a little bit like running a function, with the file as the function body. Indeed, the shell will set the positional parameters $* in just the same way. However, there's a crucial difference: there is no local parameter scope. Any variables in a sourced file, as in one of the startup files, are in the same scope as the point from which it was started. You can, therefore, source a file from inside a function and have the parameters in the sourced file local, but normally the only way of having parameters only for use in a sourced file is to unset them when you are finished.

The fact that both . and source exist is historical: the former comes from the Bourne shell, and the latter from the C shell, which seems deliberately to have done everything differently. The point noted above, that source always searches the current directory (and searches it first), is the only difference.

Re-evaluating an expression

Sometimes it's very useful to take a string and run it as if it were a set of shell commands. This is what eval does. More precisely, it sticks the arguments together with spaces and calls them. In the case of something like

  eval print Hello.
this isn't very useful; that's no different from a simple
  print Hello.
The difference comes when what's on the command line has something to be expanded, like a parameter:
  param='print Hello.'
  eval $param
Here, the $param is expanded just as it would be for a normal command. Then eval gets the string `print Hello.' and executes it as shell command line. Everything --- really everything --- that the shell would normally do to execute a command line is done again; in effect, it's run as a little function, except that no local context for parameters is created. If this sounds familiar, that's because it's exactly the way traps defined in the form
  trap 'print Hello.' EXIT
are called. This is one simple way out of the hole you can sometimes get yourself into when you have a parameter which contains the name of another parameter, instead of some data, and you want to get your hands on the data:
  # somewhere above...
  origdata='I am data.'
  # but all you know about is
  paramname=origdata
  # so to extract the data you can do...
  eval data=\$$paramname
Now $data contains the value you want. Make sure you understand the series of expansions going on: this sort of thing can get very confusing. First the command line is expanded just as normal. This turns the argument to eval into `data=$origdata'. The `$' that's still there was quoted by a backslash; the backslash was stripped and the `$' left; the $paramname was evaluated completely separately --- quoted characters like the \$ don't have any effect on expansions --- to give origdata. Eval calls the new line `data=$origdata' as a command in its own right, with the now obvious effect. If you're even slightly confused, the best thing to do is simply to quote everything you don't want to be immediately expanded:
  eval 'data=$'$paramname
or even
  eval 'data=${'$paramname'}'
may perhaps make your intentions more obvious.

It's possible when you're starting out to confuse `eval' with the `...` and $(...) commands, which also take the command in the middle `...' and evaluate it as a command line. However, these two (they're identical except for the syntax) then insert the output of that command back into the command line, while eval does no such thing; it has no effect at all on where input and output go. Conversely, the two forms of command substitution don't do an extra level of expansion. Compare:

  % foo='print bar'
  % eval $foo
  bar
with
  % foo='print bar'
  % echo $($foo)
  zsh: command not found: print bar

The $(...) substitution took $foo as the command line. As you are now painfully aware, zsh doesn't split scalar parameters, so this was turned into the single word `print bar', which isn't a command. The blank line is `echo' printing the empty result of the failed substitution.

3.2.11: More precommand modifiers: exec, noglob

Sometimes you want to run a command instead of the shell. This sometimes happens when you write a shell script to process the arguments to an external command, or set parameters for it, then call that command. For example:

  export MOZILLA_HOME=/usr/local/netscape
  netscape "$@"
Run as a script, this sets an environment variable, then starts netscape. However, as always the shell waits for the command to finish. That's rather wasteful here, since there's nothing more for the shell to do; you'd rather it simply magically turned into the netscape command. You can actually do this:
  export MOZILLA_HOME=/usr/local/netscape
  exec netscape "$@"
`exec' tells the shell that it doesn't need to wait; it can just make the command to run replace the shell. So this only uses a single process.

Normally, you should be careful not to use exec interactively, since normally you don't want the shell to go away. One legitimate use is to replace the current zsh with a brand new one if (say) you've set a whole load of options you don't like and want to restore the ones you usually have on startup:

  exec zsh
Or you may have the bad taste to start a completely different shell altogether. Conversely, a good piece of news about exec is that it is common to all shells, so you can use it from another shell to start zsh in the way I've just shown.

Like `command' and `builtin', `exec' is a `precommand modifier' in that it alters the way a command line is interpreted. Here's one more:

  noglob print *
If you've remembered what `glob' means, this is all fairly obvious. It instructs the shell not to turn the `*' into a list of all the files in the directory, but instead to let well alone. You can do this by quoting the `*', of course; often noglob is used as part of an alias to set up commands where you never need filename generation and don't want to have to bother quoting everything. However, note that noglob has no effect on any other type of expansion: parameter expansion and backquote (`....`) expansion, for example, happen as normal; the only thing that doesn't is turning patterns into a list of matching files. So it doesn't take away the necessity of knowing the rules of shell expansion. If you need that, the best thing to do is to use read or vared (see below) to read a line into a parameter, which you pass to your function:
  read -r param
  print $param
The -r makes sure $param is the unadulterated input.

3.2.12: Testing things

I told you in the last chapter that the right way to write tests in zsh was using the `[[ ... ]]' form, and why. So you can ignore the two builtins `test' and `[', even though they're the ones that resemble the Bourne shell. You can safely write

  if [[ $foo = '' ]]; then
    print The parameter foo is empty.  O, misery me.
  fi
or
  if [[ -z $foo ]]; then
    print Alack and alas, foo still has nothing in it.
  fi
instead of monstrosities like
  if test x$foo != x; then
    echo The emptiness of foo.  Yet are we not all empty\?
  fi
because even if $foo does expand to an empty string, which is what is implied if the tests are true, `[[ ... ]]' remembers there was something there and gets the syntax right. Rather than a builtin, this is actually a reserved word --- in fact it has to be, to be syntactically special --- but you probably aren't too bothered about the difference.

There are two sorts of tests, both shown above: those with three arguments, and those with two. The three-argument forms all have some comparison in the middle; in addition to `=' (or `==', which means the same here, and which according to the manual page we should be using, though none of us does), there are `!=' (not equal), `<', `>', `<=' and `>='. All these do string comparisons, i.e. they compare the sort order of the strings.

Since there are better ways of sorting things in zsh, the `=' and `!=' forms are by far the most common. Actually, they do something a bit more than string comparison: the expression on the right can be a pattern. The patterns understood are just the same as for matching filenames, except that `/' isn't special, so it can be matched by a `*'. Note that, because `=' and `!=' are treated specially by the shell, you shouldn't quote the patterns: you might think that unless you do, they'll be turned into file names, but they won't. So

  if [[ biryani = b* ]]; then
    print Word begins with a b.
  fi
works. If you'd written 'b*', including the quotes, it wouldn't have been treated as a pattern; it would have tested for a string which was exactly the two letters `b*' and nothing else. Pattern matching like this can be very powerful. If you've done any Bourne shell programming, you may remember the only way to use patterns there was via the `case' construction: that's still in zsh (see below), and uses the same sort of patterns, but the test form shown above is often more useful.

Then there are other three-argument tests which do numeric comparison. Rather oddly, these use letters rather than mathematical symbols: `-eq', `-lt' and `-le' compare if two numbers are equal, less than, or less than or equal, to one another. You can guess what `-gt' and `-ge' do. Note this is the other way round to Perl, which much more logically uses `==' to test for equality of numbers (not `=', since that's always an assignment operator in Perl) and `eq' (minus the minus) to test for equality of strings. Unfortunately we're now stuck with it this way round. If you are only comparing numbers, it's better to use the `(( ... ))' expression, because that has a proper understanding of arithmetic. However,

  if [[ $number -gt 3 ]]; then
    print Wow, that\'s big
  fi
and
  if (( $number > 3 )); then
    print Wow, that\'s STILL big
  fi
are essentially equivalent. In the second case, the status is zero (true) if the number in the expression was non-zero (sorry if I'm confusing you again) and vice versa. This means that
  if (( 3 )); then
    print It seems that 3 is non-zero, Watson.
  fi
is a perfectly valid test. As in C, the test operators in arithmetic return 1 for true and 0 for false, i.e. `$number > 3' is 1 if $number is greater than 3 and 0 otherwise; the inversion to shell logic, zero for true, only occurs at the final step when the expression has been completely evaluated and the `(( ... ))' command returns. At least with `[[ ... ]]' you don't need to worry about the extra negation; you can simply think in logical terms (although that's hard enough for a lot of people).

Finally, there are a few other odd comparisons in the three-argument form:

  if [[ file1 -nt file2 ]]; then
    print file1 is newer than file2
  fi
does the test implied by the example; there is also `-ot' to test for an older file, and there is also the little-used `-ef' which tests for an `equivalent file', meaning that they refer to the same file --- in other words, are linked; this can be a hard or a symbolic link, and in the second case it doesn't matter which of the two is the symbolic link. (If you were paying attention above, you'll know it can't possibly matter in the first case.)

In addition to these tests, which are pretty recognisable from most programming languages --- although you'll just have to remember that the `=' family compares strings and not numbers --- there are another set which are largely peculiar to UNIXy scripting languages. These are all in the form of a hyphen followed by a letter as the test, which always takes a single argument. I showed one: `-z $var' tests whether `$var' has zero length. It's opposite is `-n $var' which tests for non-zero length. Perhaps this is as good a time as any to point out that the arguments to these commands can be any single word expression, not just variables or filenames. You are quite at liberty to test

  if [[ -z "$var is sqrt(`print bibble`)" ]]; then
    print Flying pig detected.
  fi
if you like. In fact, the tests are so eager to make sure that they only have a one word argument that they will treat things like arrays, which usually return a whole set of words, as if they were in double quotes, joining the bits with spaces:
  array=(two words)
  if [[ $array = 'two words' ]]; then
    print "The array \$array is OK.  O, joy."
  fi

Apart from `-z' and `-n', most of the two-argument tests are to do with files: `-e' tests that the file named next exists, whatever type of file it is (it might be a directory or something weirder); `-f' tests if it exists and is a regular file (so it isn't a directory or anything weird this time); `-x' tests whether you can execute it. There are all sorts of others which are listed in the manual page for various properties of files. Then there are a couple of others: `-o <option>' you've met and tests whether the option is set, and `-t <fd>' tests whether the file descriptor is attached to a terminal. A file descriptor is a number which for the shell must be between 0 and 9 inclusive (others may exist, but you can't access them directly); 0 is the standard input, 1 the standard output, and 2 the channel on which errors are usually printed. Hence `[[ -t 0 ]]' tests whether the input is coming from a terminal.

There are only four other things making up tests. `&&' and `||' mean logical `and' and `or', `!' negates the effect of a test, and parentheses `( ... )' can be used to surround a set of tests which are to be treated as one. These are all essentially the same as in C. So

  if [[ 3 -gt 2 && ( me > you || ! -z bah ) ]]; then
    print will I, won\'t I...
  fi
will, because 3 is numerically greater than 2; the expression in parentheses is evaluated and though `me' actually comes before `you' in the alphabet, so the first test fails, `-z bah' is false because you gave it a non-empty string, and hence `! -z bah' is true. So both sides of the `&&' are true and the test succeeds.

3.2.13: Handling options to functions and scripts

It's often convenient to have your own functions and scripts process single-letter options in the way a lot of builtin commands (as well as a great many other UNIX-style commands) do. The shell provides a builtin for this called `getopts'. This should always be called in some kind of loop, usually a `while' loop. It's easiest to explain by example.

  testopts() {
    # $opt will hold the current option
    local opt
    while getopts ab: opt; do
      # loop continues till options finished
      # see which pattern $opt matches...
      case $opt in
        (a)
           print Option a set
           ;;
        (b)
           print Option b set to $OPTARG
           ;;
	# matches a question mark
	# (and nothing else, see text)
        (\?)
           print Bad option, aborting.
           return 1
           ;;
      esac
    done
    (( OPTIND > 1 )) && shift $(( OPTIND - 1 ))
    print Remaining arguments are: $*
  }
There's quite a lot here if you're new to shell programming. You might want to read the stuff on structures like while and case below and then come back and look at this. First let's see what it does.
  % testopts -b foo -a -- args
  Option b set to foo
  Option a set
  Remaining arguments are: args

Here's what's happening. `getopts ab: opt' is the argument to the `while'. That means that the getopts gets run; if it succeeds (returns status zero), then the loop after the `do' is run. When that's finished, the getopts command is run again, and so on until it fails (returns a non-zero status). It will do that when there are no more options left on the command line. So the loop processes the options one by one. Each time through, the number of the next argument to look at is left in the parameter $OPTIND, so this gradually increases; that's how getopts knows how far it has got.

The first argument to the getopts is `ab:'. That means `a' is an option which doesn't take an argument, while `b' is an argument which takes a single argument, signified by the colon after it. You can have any number of single-letter (or even digit) arguments, which are case-sensitive; for example `ab:c:ABC:' are six different options, three with arguments. If the option found has an argument, that is stored in the parameter $OPTARG; getopts then increments $OPTIND by however much is necessary, which may be 2 or just 1 since `-b foo' and `-bfoo' are both valid ways of giving the argument.

If an option is found, we use the `case' mechanism to find out what it was. The idea of this is simple, even if the syntax has the look of an 18th-century French chateau: the argument `$opt' is tested against all of the patterns in the `pattern)' lines until one matches, and the commands are executed until the next `;;'. It's the shell's equivalent of C's `switch'. In this example, we just print out what the getopts brought in. Note the last line, which is called if $opt is a question mark --- it's quoted because `?' on its own can stand for any single character. This is how getopts signals an unknown option. If you try it, you'll see that getopts prints its own error message, so ours was unnecessary: you can turn the former off by putting a colon right at the start of the list of options, making it `:ab:' here.

Actually, having this last pattern as an unquoted `?' isn't such a bad idea. Suppose you add a letter to the list that getopts should handle and forget to add a corresponding item in the case list for it. If the last item matches any character, you will get the behaviour for an unhandled option, which is probably the best thing to do. Otherwise nothing in the case list will match, the shell will sail blithely on to the next call to getopts, and when you try to use the function with the new option you will be left wondering quite what happened to it.

The last piece of the getopts jigsaw is the next line, which tests if $OPTIND is larger than 1, i.e. an option was found and $OPTIND was advanced --- it is automatically set to 1 at the start of every function or script. If it was, the `shift' builtin with a numeric argument, but no array name, moves the positional parameters, i.e. the function's arguments, to shift away the options that have been processed. The print in the next line shows you that only the remaining non-option arguments are left. You don't need to do that --- you can just start using the remaining arguments from $argv[$OPTIND] on --- but it's a pretty good way of doing it.

In the call, I showed a line with `--' in it: that's the standard way of telling getopts that the options are finished; even if later words start with a -, they are not treated as options. However, getopts stops anyway when it reaches a word not beginning with `-', so that wasn't necessary here. But it works anyway.

You can do all of what getopts does without that much difficulty with a few extra lines of shell programming, of course. The best argument for using getopts is probably that it allows you to group single-letter options, e.g. `-abc' is equivalent to `-a -b -c' if none of them was defined to have an argument. In this case, getopts has to remember the position inside the word on the command line for you to call it next, since the `a' `b' and `c' still appear on different calls. Rather unsatisfactorily, this is hidden inside the shell (as it is in other shells --- we haven't fixed all of everybody else's bad design decisions); you can't get at it or reset it without altering $OPTIND. But if you read the small print at the top of the guide, you'll find I carefully avoided saying everything was satisfactory.

While we're at it, why do blocks starting with `if' and `then' end with `fi', and blocks starting with `case' end with `esac', while those starting with `while' and `do' end with `done', not `elihw' (perfectly pronounceable in Welsh, after all) or `od'? Don't ask me.

3.2.14: Random file control things

We're now down into the odds and ends. If you know UNIX at all, you will already be familiar with the umask command and its effect on file creation, but as it is a builtin I will describe it here. Create a file and look at it:

  % touch tmpfile
  % ls -l tmpfile
  -rw-r--r--    1 pws   pws    0 Jul 19 21:19 tmpfile
(I've shortened the output line for the TeX version of this document.) You'll see that the permissions are read for everyone, write-only for the owner. How did the command (touch, not a builtin, creates an empty file if there was none, or simply updates the modification time of an existing file) know what permissions to set?
  % umask
  022
  % umask 077
  % rm tmpfile; touch tmpfile
  % ls -l tmpfile
  -rw-------    1 pws   pws    0 Jul 19 21:22 tmpfile
umask was how it knew. It gives an octal number corresponding to the permissions which should not be given to a newly created file (only newly created files are affected; operations on existing files don't involve umask). Each digit is made up of a 4 for read, 2 for write, 1 for executed, in the same order that ls shows for permissions: user, then group, then everyone else. (On this Linux/GNU-based system, like many others, users have their own groups, so both are called `pws'.) So my original `022' specified that everything should be allowed except writing for group and other, while `077' disallowed any operation by group and other. These are the two most common settings, although here `002' or `007' would be useful because of the way groups are specific to users, making it easier to grant permission to specific other users to write in my directories. (Except there aren't any other users.)

You can also use chmod-like permission changing expressions in umask. So

  % umask go+rx
would restore group and other permissions for reading and executing, hence returning the mask to 022. Note that because it is adding permissions, just like chmod does, it is removing numbers from the umask.

You might have wondered about execute permissions, since `touch' didn't give any, even where it was allowed by umask. That's because only operations which create executable programmes, such as running a compiler and linker, set that bit; the normal way of opening a new file --- internally, the UNIX open function, with the O_CREAT flag set --- doesn't touch it. For the same reason, if you create shell scripts which you want to be able to execute by typing the name, you have to make them executable yourself:

  % chmod +x myscript
and, indeed, you can think of chmod as umask's companion for files which already exist. It doesn't need to be a builtin, because the files you are operating on are external to zsh; umask, on the other hand, operates when you create a file from within zsh or any child process, so needs to be a builtin. The fact that it's inherited means you can set umask before you start an editor, and files created by that editor will reflect the permissions.

Note that the value set by umask is also inherited and used by chmod. In the example of chmod I gave, I didn't see which type of execute permission to add; chmod looks at my umask and decides based on that --- in other words, with 022, everybody would be allowed to execute myscript, while with 077, only I would, because of the 1's in the number: (0+0+0)+(4+2+1)+(4+2+1). Of course, you can be explicit with chmod and say `chmod u+x myscript' and so on.

Something else that may or may not be obvious: if you run a script by passing it as an argument to the shell,

  % zsh myscript
what matters is read permission. That's what the shell's doing to the script to find the commands, after all. Execute permission applies when the system (or, in some cases, including zsh, the parent shell where you typed `myscript') has to decide whether to find a shell to run the script by itself.

3.2.15: Don't watch this space, watch some other

Finally for builtins, some things which really belong elsewhere. There are three commands you use to control the shell's editor. These will be described in chapter 4, where I talk all about the editor.

The bindkey command allows you to attach a set of keystrokes to a command. It understands an abbreviated notation for the keystrokes.

  % bindkey '^Xc' copy-prev-word
This binds the keystrokes consisting of Ctrl held down with x, then c, to the command which copies the previous word on the line to the current position. The commands are listed in the zshzle manual page. bindkey can also do things with keymaps, which are a complete set of mappings between keys and commands like the one I showed.

The vared command is an extremely useful builtin for editing a shell variable. Usually much the easiest way to change $path (or $PS1, or whatever) is to run `vared path': note the lack of a `$', since otherwise you would be editing whatever $path was expanded to. This is because very often you want to leave most of what's there and just change the odd character or word. Otherwise, you would end up doing this with ordinary parameter substitutions, which are a lot more complicated and error prone. Editing a parameter is exactly like editing a command line, except without the prompt at the start.

Finally, there is the zle command. This is the most mysterious, as it offers a fairly low-level interface to the line editor; you use it to define your own editing commands. So I'll leave this alone for now.

3.2.16: And also

There is one more standard builtin that I haven't covered: zmodload, which allows you to manipulate add-on packages for zsh. Many extras are supplied with the shell which aren't normally loaded to keep down the use of memory and to avoid having too many rarely used builtins, etc., getting in the way. In the last chapter I will talk about some of these. To be more honest, a lot of the stuff in between actually uses these addons, generically referred to as modules --- the line editor, zle, is itself a separate module, though heavily dependent on the main shell --- and you've probably forgotten I mentioned above using `zmodload zsh/mathfunc' to load mathematical functions.

3.3: Functions

Now it's time to look at functions in more detail. The various issues to be discussed are: loading functions, handling parameters, compiling functions, and repairing bike tyres when the rubber solution won't stick to the surface. Unfortunately I've already used so much space that I'll have to skip the last issue, however topical it might be for me at the moment.

3.3.1: Loading functions

Well, you know what happens now. You can define functions on the command line:

  fn() {
    print I am a function
  }
which you call under the name `fn'. As you type, the shell knows that it's in the middle of a function definition, and prompts you until you get to the closing brace.

Alternatively, and much more normally, you put a file called fn somewhere in a directory listed in the $fpath array. At this point, you need to be familiar with the KSH_AUTOLOAD option described in the last chapter. From now on, I'm just going to assume your autoloadable function files contain just the body of the function, i.e. KSH_AUTOLOAD is not set. Then the file fn would contain:

  print I am a function
and nothing else.

Recent versions of zsh, since 3.1.6, set up $fpath for you. It contains two parts, although the second may have multiple directories. The first is, typically, /usr/local/share/zsh/site-functions, although the prefix may vary. This is empty unless your system administrator has put something in it, which is what it's there for.

The remaining part may be either a single directory such as /usr/local/share/zsh/3.1.9/functions, or a whole set of directories starting with that path. It simply depends whether the person installing zsh wanted to keep all the functions in the same directory, or have them sorted according to what they do. These directories are full of functions. However, none of the functions is autoloaded automatically, so unless you specifically put `autoload ...' in a startup file, the shell won't actually take any notice of them. As you'll see, part of the path is the shell version. This makes it very easy to keep multiple versions of zsh with functions which use features that may be different between the two versions. By the way, if these directories don't exist, you should check $fpath to see if they are in some other location, and if you can't find any correspondence between what's in $fpath and what's on the disk even when you start the shell with zsh -f to suppress loading of startup files, complain to the system administrator: he or she has either not installed them properly, or has made /etc/zshenv stomp on $fpath, both of which are thoroughly evil things to do. (/etc/zshrc, /etc/zprofile and /etc/zlogin shouldn't stomp on $fpath either, of course. In fact, they shouldn't do very much; that's up to the user.)

One point about autoload is the `-U' option. This turns off the use of any aliases you have defined when the function is actually loaded --- the flag is remembered with the name of the function for future reference, rather than being interpreted immediately by the autoload command. Since aliases can pretty much redefine any command into any other, and are usually interpreted while a function is being defined or loaded, you can see that without this flag there is fair scope for complete havoc.

   alias ls='echo Your ls command has been requisitioned.'
   lsroot() {
     ls -l /
   }
   lsroot
That's not what the function writer intended. (Yes, I know it actually is, because I wrote it to show the problem, but that's not what I meant.) So -U is recommended for all standard functions, where you have no easy way of telling quite what could be run inside.

Recently, the functions for the new completion system (described in chapter 6) have been changing the fastest. They either begin with comp or an underscore, `_'. If the functions directory is subdivided, most of the subdirectories refer to this. There are various other classes of functions distributed with the shell:

  • Functions beginning zf are associated with zftp, a builtin system for FTP transfers. Traditional FTP clients, ones which don't use a graphical interface, tend to be based around a set of commands on a command line --- exactly what zsh is good at. This also makes it very easy to write macros for FTP jobs --- they're just shell functions. This is described in the final chapter along with other modules. It's based around a single builtin, zftp, which is loaded from the module zsh/zftp.

  • Functions beginning prompt, which may be in the Prompts subdirectory, are part of a `prompt themes' system which makes it easy for you to switch between preexisting prompts. You load it with `autoload -U promptinit; promptinit'. Then `prompt -h' will tell you what to do next. If you have new completion loaded (with `autoload -U compinit; compinit', what else) the arguments to `prompt' can be listed with ^D and completed with a TAB; they are various sorts of prompt which you may or may not like.

  • Functions with long names and hyphens, like predict-on and incremental-complete-word. These are editor functions; you use them with
      zle -N predict-on
      bindkey <keystroke> predict-on
    
    Here, the predict-on function automatically looks back in the history list for matching lines as you type. You should also bind predict-off, which is loaded when predict-on is first called. incremental-complete-word is a fairly simple attempt at showing possible completions for the current word as you type; it could do with improving.

  • Everything else; these may be in the Misc subdirectory. These are a very mixed bag which you should read to see if you like any. One of the most useful is zed, which allows you to edit a small file (it's really just a simple front-end to vared). The run-help file shows you the sort of thing you might want to define for use with the \eh (run-help) keystroke. is-at-least is a function for finding out if the version of the shell running is recent enough, assuming you know what version you need for a given feature. Several of the other functions refer to the old completion system --- which you won't need, since you will be reading chapter 6 and using the new completion system, of course.
  • If you have your own functions --- and if you use zsh a lot, you almost certainly will eventually --- it's a good idea to add your own personal directory to the front of $fpath, so that everything there takes precedence over the standard functions. That allows you to override a completion function very easily, just by copying it and editing it. I tend to do something like this in my .zshenv:

      [[ $fpath = *pws* ]] || fpath=(~pws/bin/fns $fpath)
    
    to protect against the possibility that the directory I want to add is already there, in case I source that startup file again, and there are other similar ways. (You may well find your own account isn't called pws, however.)

    Chances are you will always want your own functions to be autoloaded. There is an easy way of doing this: put it just after the line I showed above:

      autoload ${fpath[1]}/*(:t)
    
    The ${fpath[1]}/* expands to all the files in the directory at the head of the $fpath array. The (:t) is a `glob modifier': applied to a filename generation pattern, it takes the tail (basename) of all the files in the list. These are exactly the names of the functions you want to autoload. It's up to you whether you want the -U argument here.

    3.3.2: Function parameters

    I covered local parameters in some detail when I talked about typeset, so I won't talk about that here. I didn't mention the other parameters which are effectively local to a function, the ones that pass down the arguments to the function, so here is more detail. They work pretty much identically in scripts.

    There are basically two forms. There is the form inherited from Bourne shell via Korn shell, with the typically uninformative names: $#, $*, $@ and the numerical parameters $1 etc. --- as high a number as the shell will parse is allowed, not just single digits. Then there is the form inherited from the C shell: $ARGC and $argv. I'll mainly use the Bourne shell versions, which are far more commonly used, and come back to some oddities of the C shell variety at the end.

    $# tells you how many arguments were passed to the function, while $* gives those arguments as an array. This was the only array available in the Bourne shell, otherwise there would probably have been a more obvious way of doing it. To get the size and the number of elements of the array you don't use ${#*} and ${*[1]} etc. (well, you usually don't --- zsh is typically permissive here), you use $1, $2. Despite the syntax, these are rather like ordinary array elements; if you refer to one off the end, you will get an empty string, but no error, unless you have the option NO_UNSET set. It is this not-quite array which gets shifted if you use the shift builtin without an argument: the old $1 disappears, the old $2 becomes $1, and so on, while $# is reduced by one. If there were no arguments ($# was zero), nothing happens.

    The form $@ is very similar to $*, and you can use it in place of that in most contexts. There is one place where they differ. Let's define a function which prints the number of its arguments, then the arguments.

      args() {
        print $# $*
      }
    
    Now some arguments. We'll do this for the current shell --- it's a slightly odd idea, that you can set the arguments for what's already running, or that an interactive shell has arguments at all, but nonetheless it's true:
      set arguments to the shell
      print $*
    
    sets $* and hence prints the message `arguments to the shell'. We now pass these arguments on to the function in two different ways:
      args $*
      args $@
    
    This outputs
      4 arguments to the shell
      4 arguments to the shell
    
    -- no surprises so far. Remember, too, that zsh doesn't split words on spaces unless you ask it too. So:
      % set 'one word'
      % args $*
      1 one word
      % args $@
      1 one word
    

    Now here's the difference:

      % set two words
      % args "$*"
      1 two words
      % args "$@"
      2 two words
    
    In quotes, "$*" behaves as a normal array, joining the words with spaces. However, "$@" doesn't --- it still behaves as if it was unquoted. You can't see from the arguments themselves in this case, but you can from the digit giving the number of arguments the function has.

    This probably seems pretty silly. Why quote something to have it behave like an unquoted array? The original answer lies back in Bourne shell syntax, and relates to the vexed question of word splitting. Suppose we turn on Bourne shell behaviour, and try the example of a word with spaces again:

      % setopt shwordsplit
      % set 'one word'
      % args $*
      2 one word
      % args $@
      2 one word
      % args "$*"
      1 one word
      % args "$@"
      1 one word
    
    Aha! This time "$@" kept the single word with the space intact. In other words, "$@" was a slightly primitive mechanism for suppressing splitting of words, while allowing the splitting of arrays into elements. In zsh, you would probably quite often use $*, not "$@", safe in the knowledge that nothing was split until you asked it to be; and if you wanted it split, you would use the special form of substitution ${=*} which does that:
      % unsetopt shwordsplit
      % args $*
      1 one word
      % args ${=*}
      2 one word
    
    (I can't tell you why the `=' was chosen for this purpose, except that it consists of two split lines, or in an assignment it splits two things, or something.) This works with any parameter, whether scalar or array, quoted or unquoted.

    However, that's actually not quite the whole story. There are times when the shell removes arguments, because there's nothing there:

      % set hello '' there
      % args $*
      2 hello there
    
    The second element of the array was empty, as if you'd typed
      2=
    
    --- yes, you can assign to the individual positional parameters directly, instead of using set. When the array was expanded on the command line, the empty element was simply missed out altogether. The same happens with all empty variables, including scalars:
      % empty=
      % args $empty
      0
    
    But there are times when you don't want that, any more than you want word splitting --- you want all arguments passed just as you gave them. This is another side effect of the "$@" form.
      % args "$@"
      3 hello there
    
    Here, the empty element was passed in as well. That's why you often find "$@" being used in zsh when wordsplitting is already turned off.

    Another note: why does the following not work like the example with $*?

      % args hello '' there
      3 hello there
    
    The quotes were kept here. Why? The reason is that the shell doesn't elide an argument if there were quotes, even if the result was empty: instead, it provides an empty string. So this empty string was passed as the second argument. Look back at:
      set hello '' there
    
    Although you probably didn't think about it at the time, the same thing was happening here. Only with the '' did we get an empty string assigned to $2; later, this was missed out when passing $* to the function. The same difference occurs with scalars:
      % args $empty
      0
      % args "$empty"
      1
    
    The $empty expanded to an empty string in each case. In the first case it was unquoted and was removed; this is like passing an empty part of $* to a command. In the second case, the quotes stopped that from being removed completely; this is similar to setting part of $* to an empty string using ''.

    That's all thoroughly confusing the first time round. Here's a table to try and make it a bit clearer.

                           |   Number of arguments
                           |     if $* contains...
                           |  (two words)
    Expression   Word      |       'one word'
    on line   splitting?   |             empty string
    --------------------------------------------------
    $*             n       |     2     1     0
    $@             n       |     2     1     0
    "$*"           n       |     1     1     1
    "$@"           n       |     2     1     1
                           |                    
    $*             y       |     2     2     0
    $@             y       |     2     2     0
    "$*"           y       |     1     1     1
    "$@"           y       |     2     1     1
                           |                    
    ${=*}          n       |     2     2     0
    ${=@}          n       |     2     2     0
    "${=*}"        n       |     2     2     1
    "${=@}"        n       |     2     2     1
    
    On the left is shown the expression to be passed to the function, and in the three right hand columns the number of arguments the function will get if the positional parameters are set to an array of two words, a single word with a space in the middle, or a single word which is an empty string (the effect of `set -- ''') respectively. The second column shows whether word splitting is in effect, i.e. whether the SH_WORD_SPLIT option is set. The first four lines show the normal zsh behaviour; the second four show the normal sh/ksh behaviour, with word splitting turned on --- only the case where a word has a space in it changes, and then only when no quotes are supplied. The final four show what happens when you use the `${=..}' method to turn on word splitting, for convenience: that's particularly simple, since it always forces words to be split, even inside quotation marks.

    I would recommend that anyone not wedded to the Bourne shell behaviour use the top set as standard: in particular, `$*' for normal array behaviour with removal of empty items, `"$@"' for normal array behaviour with empty items left as empty items, and `"$*"' for turning arrays into single strings. If you need word-splitting, you should use `${=*}' or `"${=@}"' for splitting with/without removal of empty items (obviously there's no counterpart to the quoted-array behaviour here). Then keep SH_WORD_SPLIT turned off. If you are wedded to the Bourne shell behaviour, you're on your own.

    It's a bug

    There's a bug in handling of the the form ${1+"$@"}. This looks rather an arcane and unlikely combination, but actually it is commonly used to get around a bug in some versions of the Bourne shell (which is not in zsh): that "$@" generates a single empty argument if there are no arguments. The form shown tests whether there is a first argument, and if so substitutes "$@", else it doesn't substitute anything, avoiding the bug.

    Unfortunately, in zsh, when shwordsplit is set --- which is the time you usually run across attempts like this to standardise the way different shells work --- this will actually cause too much word-splitting. The way the shell is written at the moment, the embedded "$@" will force extra splitting on spaces inside the arguments. So if the first argument is `one word', and shwordsplit is set, ${1+"$@"} produces two words `one' and `word'.

    Oliver Kiddle spotted a way of getting round this which has been adapted for use in the GNU autoconf package: in your initialisation code, have

      [ x$ZSH_VERSION != x ] && alias -g '${1+"$@"}'='"$@"'
    
    This uses a global alias to turn ${1+"$@"} wherever it occurs as a single word into "$@" which doesn't have the problem. Aliasing occurs so early in processing that the fact that most of the characters have a special meaning to the shell is irrelevant; the shell behaves as if it read in "$@". The only catch is that for this to work the script or function must use exactly the character string ${1+"$@"}, with no leading or trailing word characters (whitespace, obviously, or characters which terminate parsing such as `;' are all right). Some day, we may fix the underlying bug, but it's not very easy with the way the parameter substitution code is written at the moment.

    Parameters inherited from csh

    The final matter is the C-shell syntax. There are two extra variables but, luckily, there is not much extra in the way of complexity. $ARGC is essentially identical to $#, and $argv corresponds to $*, but is a real array this time, so instead of $1 you have ${argv[1]} and so on. They use the convention that scalars used by the shell are all uppercase, while arrays are all lowercase. This feature is probably the only reason anyone would need these variants. For example, ${argv[2,-1]} means all arguments from the second to the last, inclusive: negative indices count from the end, and a comma indicates a slice of an array, so that ${argv[1,-1]} is always the same as the full array. Otherwise, my advice would be to stick with the Bourne shell variants, however cryptic they may look at first sight, for the usual reason that zsh isn't really like the C shell and if you pretend it is, you will come a cropper sooner or later.

    It looks like you're missing "$@", but actually you can do that with "${argv[@]}". This, like negative indices and slices, works with all arrays.

    There's one slight oddity with $ARGC and $argv, which isn't really a deliberate feature of the shell at all, but just in case you run into it: although the values in them are of course local to functions, the variables $ARGC and $argv themselves are actually treated like global variables. That means if you apply a typeset -g command to them, it will affect the behaviour of $ARGC and $argv in all functions, even though they have different values. It's probably not a good idea to rely on this behaviour.

    nusubsect(Arguments to all commands work the same)

    I've been a little tricky here, because I've been talking about two levels of functions at once: $* and friends as set in the current function, or even at the top level, as well as how they are passed down to commands such as my args function. Of course, in the second case the same behaviour applies to all commands, not just functions. What I mean is, in

      fn() {
        cat $*
        cat "$*"
      }
    
    the `cat' command will see the differences in behaviour between the two calls just as args would. That should be obvious.

    It's not a bug

    Let me finally mention again a feature I noted in passing:

      1='first argument'
    
    sets the first command argument for the current shell or function, independently of any others. People sometimes complain that
      1000000='millionth argument'
    
    suddenly makes the shell use a lot more memory. That's not a bug at all: you've asked the shell to set the millionth element of an array, but not any others, so the shell creates an array a million elements long with the first 999,999 empty, except for any arguments which were already set. It's not surprising this takes up a lot of memory.

    3.3.3: Compiling functions

    Since version 3.1.7, it has been possible to compile functions to their internal format. It doesn't make the functions run any faster, it just reduces their loading time; the shell just has to bring the function into memory, then it `runs it as it does any other function. On many modern computers, therefore, you don't gain a great deal from this. I have to admit I don't use it, but there are other definite advantages.

    Note that when I say `compiled' I don't mean the way a C compiler, say, would take a file and turn it into the executable code which the processor understands; here, it's simply the format that the shell happens to use internally --- it's useless without a suitable version of zsh to run it. Also, it's no use thinking you can hide your code from prying eyes this way, like you can to some extent with an ordinary compiler (disassembling anything non-trivial from scratch being a time-consuming job): first of all, ordinary command lines appear inside the compiled files, except in slightly processed form, and secondly running `functions' on a compiled function which has been loaded will show you just as much as it would if the function had been loaded normally.

    One other advantage is that you can create `digest' files, which are sets of functions stored in a single file. If you often use a large fraction of those files, or they are small, or you like the function itself to appear when you run `functions' rather than a message saying it hasn't been loaded, then this works well. In fact, you can compile all the functions in a single directory in one go. You might think this uses a lot of memory, but often zsh will simply `memory map' the file, which means rather than reserving extra main memory for it and reading it in --- the obvious way of reading files --- it will tell the operating system to make the file available as if it were memory, and the system will bring it into memory piece by piece, `paging' the file as it is needed. This is a very efficient way of doing it. Actually, zsh supports both this method and the obvious method of simply reading in the file (as long as your operating system does); this is described later on.

    A little extra, in case you're interested: if you read in a file normally, the system will usually reserve space on a disk for it, the `swap', and do paging from there. So in this case you still get the saving of main memory --- this is standard in all modern operating systems. However, it's not as efficient: first of all, you had to read the file in in the first place. Secondly it eats up swap space, which is usually a fixed amount of disk, although if you've got enough main memory, the system probably won't bother allocating swap. Thirdly --- this is probably the clincher for standard zsh functions on a large system --- if the file is directly mapped read-only, as it is in this case, the system only needs one place in main memory, plus the single original file on disk, to keep the function, which is very much more efficient. With the other method, you would get multiple copies in both main memory and (where necessary) swap. This is how the system treats directly executable programmes like the shell itself --- the data is specific to each process, but the programme itself can be shared because it doesn't need to be altered when it's running.

    Here's a simple example.

      % echo 'echo hello, world' >hw
      % zcompile hw
      % ls
      hw    hw.zwc
      % rm hw
      % fpath=(. $fpath)
      % autoload hw
      % hw
      hello, world
    
    We created a simple `hello, world' function, and compiled it. This produces a file called `hw.zwc'. The extension stands for `Z-shell Word Code', because it's based on the format of words (integers longer than a single byte) used internally by the shell. Then we made sure the current directory was in our $fpath, and autoloaded the function, which ran as expected. We deleted the original file for demonstration purposes, but as long as the `.zwc' file is newer, that will be used, so you don't need to remove the originals in normal use. In fact, you shouldn't, because you will lose any comments and formatting information in it; you can regenerate the function itself with the `functions' command (try it here), but the shell only remembers the information actually needed to run the commands. Note that the function was in the zsh autoload format, not the ksh one, in this case (but see below).

    And there's more

    Now some bells and whistles. Remember the KSH_AUTOLOAD thing? When you compile a function, you can specify which format --- native zsh or ksh emulation --- will be used for loading it next time, by using the option -k or -z, instead of the default, which is to examine the option (as would happen if you were autoloading directly from the file). Then you don't need to worry about that option. So, for example, you could compile all the standard zsh functions using `zcompile -z' and save people the trouble of making sure they are autoloaded correctly.

    You can also specify that aliases shouldn't be expanded when the files are compiled by using -U: this has roughly the same effect as saying autoload -U, since when the shell comes to load a compiled file, it will never expand aliases, because the internal format assumes that all processing of that kind has already been done. The difference in this case is if you don't specify -U: then the aliases found when you compile the file, not when you load the function from it, will be used.

    Now digest files. Here's one convenient way of doing it.

      % ls ~/tmp/fns
      hw1   hw2
      % fpath=(~/tmp/fns $fpath)
      % cd ~/tmp
      % zcompile fns fns/*
      % ls
      fns   fns.zwc
    
    We've made a directory to put functions in, ~/tmp/fns, and stuck some random files in it. The zcompile command, this time, was given several arguments: a filename to use for the compiled functions, and then a list of functions to compile into it. The new file, fns.zwc, sits in the same directory where the directory fns, found in $fpath, is. The shell will actually search the digest file instead of the directory. More precisely, it will search both, and see which is the more recent, and use that as the function. So now
      % autoload hw1
      % hw1
      echo hello, first world
    

    You can test what's in the digest file with:

      % zcompile -t fns
      zwc file (read) for zsh-3.1.9-dev-3
      fns/hw1
      fns/hw2
    
    Note that the names appear as you gave them on the command line, i.e. with fns/ in front. Only the basenames are important for autoloading functions. The note `(read)' in the first line means that zsh has marked the functions to be read into the shell, rather than memory mapped as discussed above; this is easier for small functions, particularly if you are liable to remove or alter a file which is mapped, which will confuse the shell. It usually decides which method to use based on size; you can force memory mapping by giving the -M option. Memory mapping doesn't work on all systems (currently including Cygwin).

    I showed this for compiling files, but you can actually tell the shell to output compiled functions --- in other words, it will look along $fpath and compile the functions you specify. I find compiling files easier, when I do it at all, since then I can use patterns to find them as I did above. But if you want to do it the other way, you should note two other options: -a will compile files by looking along $fpath, while -c will output any functions already loaded by the shell (you can combine the two to use either). The former is recommended, because then you don't lose any information which was present in the autoload file, but not in the function stored in memory ---- this is what would happen if the file defined some extra widgets (in the non-technical sense) which weren't part of the function called subsequently.

    If you're perfectly happy with the shell only searching a digest file, and not comparing the datestamp with files in the directory, you can put that directly into your $fpath, i.e. ~/tmp/fns.zwc in this case. Then you can get rid of the original directory, or archive it somewhere for reuse.

    You can compile scripts, too. Since these are in the same format as a zsh autoload file, you don't need to do anything different from compiling a single function. You then run (say) script.zwc by typing `zsh script' --- note that you should omit the .zwc, as zsh decides if there's a compiled version of a script by explicitly appending the suffix. What's more, you can run it using `.' or `source' in just the same way (`. script') --- this means you can compile your startup files if you find they take too long to run through; the shell will spot a ~/.zshrc.zwc as it would any other sourceable file. It doesn't make much sense to use the memory mapping method in this case, since once you've sourced the files you never want to run them again, so you might as well specify `zcompile -R' to use the reading (non-memory-mapping) method explicitly.

    If you ever look inside a .zwc file, you will see that the information is actually included twice. That's because systems differ about the order in which numbers are stored: some have the least significant byte first (notably Intel and some versions of Mips) and some the most significant (notably SPARC and Cambridge Consultants' XAP processor, which is notable here mainly because I spend my working hours programming for it --- you can't run zsh on it). Since zsh uses integers a great deal in the compiled code, it saves them in both possible orders for ease of use. Why not just save it for the machine where you compiled it? Then you wouldn't be able to share the files across a heterogeneous network --- or even worse, if you made a distribution of compiled files, they would work on some machines, and not on others. Think how Emacs users would complain if the .elc files that arrived weren't the right ones. (Worse, think how the vi users would laugh.) The shell never reads or maps in the version it doesn't use, however; only extra disk space is used.

    A little -Xtra help

    There are two final autoloading issues you might want to know about. In versions of zsh since 3.1.7, you will see that when you run functions on a function which is marked for autoload but hasn't yet been loaded, you get:

    afunctionmarkedforautoloadwhichhasntbeenloaded () {
            # undefined
            builtin autoload -XU
    }
    
    The `# undefined' is just printed to alert you that this was a function marked as autoloadable by the autoload command: you can tell, because it's the only time functions will emit a comment (though there might be other `#' characters around). What's interesting is the autoload command with the -X option. That option means `Mark me for autoloading and run me straight away'. You can actually put it in a function yourself, and it will have the same effect as running `autoload' on a not-yet-existent function. Obviously, the autoload command will disappear as soon as you do run it, to be replaced by the real contents. If you put this inside a file to be autoloaded, the shell will complain --- the alternative is rather more unpalatable.

    Note also the -U option was set in that example: that simply means that I used autoload with the -U option when I originally told the shell to autoload the function.

    There's another option, +X, the complete opposite of -X. This one can only be used with autoload outside the function you're loading, just as -X was only meaningful inside. It means `load the file immediately, but don't run it', so it's a more active (or, as they say nowadays, since they like unnecessarily long words, proactive) form of autoload. It's useful if you want to be able to run the functions command to see the function, but don't want to run the function itself.

    Special functions

    I'm in danger of simply quoting the manual, but there are various functions with a special meaning to the shell (apart from TRAP... functions, which I've already covered). That is, the functions themselves are perfectly normal, but the shell will run them automatically on certain occasions if they happen to exist, and silently skip them if they don't.

    The two most frequently used are chpwd and precmd. The former is called whenever the directory changes, either via cd, or pushd, or an AUTO_CD --- you could turn the first two into functions, and avoid needing chpwd but not the last. Here's how to force an xterm, or a similar windowing terminal, to put the current directory into the title bar.

      chpwd() {
        [[ -t 1 ]] || return
        case $TERM in
          (sun-cmd) print -Pn "\e]l%~\e\\"
            ;;
          (*xterm*|rxvt|(dt|k|E)term) print -Pn "\e]2;%~\a"
            ;;
        esac
      }
    
    The first line tests that standard output is really a terminal --- you don't want to print the string in the middle of a script which is directing its output to a file. Then we look to see if we have a sun-cmd terminal, which has its own sui generis sequence for putting a string into the title bar, or something which recognises xterm escape sequences. In either case, the special sequences (a bit like termcap sequences as discussed for echotc) are interpreted by the terminal, and instead of being printed out cause it to put the string in the middle into the title bar. The string here is `%~': I added the -P option to print so it would expand prompt escapes. I could just have used $PWD, but this way has the useful effect of shortening your home directory, or any other named directory, into ~-notation, which is a bit more readable. Of course, you can put other stuff there if you like, or, if you're really sophisticated, put in a parameter $HEADER and define that elsewhere.

    If programmes other than the shell alter what appears in the xterm title bar, you might consider changing that chwpd function to precmd. The function precmd is called just before every prompt; in this case it will restore the title line after every command has run. Some people make the mistake of using it to set up a prompt, but there are enough ways of getting varying information into a fixed prompt string that you shouldn't do that unless you have very odd things in your prompt. It's a big nuisance having to redefine precmd to alter your prompt --- especially if you don't know it's there, since then your prompt apparently magically returns to the same format when you change it. There are some good reasons for using precmd, too, but most of them are fairly specialised. For example, on one system I use it to check if there is new input from a programme which is sending data to the shell asynchronously, and if so printing it out onto the terminal. This is pretty much what happens with job control notification if you don't have the NOTIFY option set.

    The name precmd is a bit of a misnomer: preprompt would have been better. It usurps the name more logically applied to the function actually called preexec, which is run after you finished editing a command line, but just before the line is executed. preexec has one additional feature: the line about to be executed is passed down as an argument. You can't alter what's going to be executed by editing the parameter, however: that has been suggested as an upgrade, but it would make it rather easy to get the shell into a state where you can't execute any commands because preexec always messes them up. It's better, where possible, to write function front-ends to specific commands you want to handle specially. For example, here's my ls function:

      local ls
      if [[ -n $LS_COLORS ]]; then
        ls=(ls --color=auto)
      else
        ls=(ls -F)
      fi
      command $ls $*
    
    This handles GNU and non-GNU versions of ls. If $LS_COLORS is set, it assumes we are using GNU ls, and hence colouring (or colorizing, in geekspeak) is available. Otherwise, it uses the standard option -F to show directories and links with a special symbol. Then it uses command to run the real ls --- this is a key thing to remember any time you use a function front-end to a command. I could have done this another way: test in my initialisation files which version of ls I was using, then alias ls to one of the two forms. But I didn't.

    Apart from the trap functions, there is one remaining special function. It is periodic, which is executed before a prompt, like precmd, but only every now and then, in fact every $PERIOD seconds; it's up to you to set $PERIOD when you defined periodic. If $PERIOD isn't set, or is zero, nothing happens. Don't get $PERIOD confused with $SECONDS, which just counts up from 0 when the shell starts.

    3.4: Aliases

    Aliases are much simpler than functions. In the C shell and its derivatives, there are no functions, so aliases take their place and can have arguments, which involve expressions rather like those which extract elements of previous history lines with `!'. Zsh's aliases, like ksh's, don't take arguments; you have to use functions for that. However, there are things aliases can do which functions can't, so sometimes you end up using both, for example

      zfget() {
        # function to retrieve a file by FTP,
        # using globbing on the remote host
      }
      alias zfget='noglob zfget'
    
    The function here does the hard work; this is a function from the zftp function suite, supplied with the shell, which retrieves a file or set of files from another machine. The function allows patterns, so you can retrieve an entire directory with `zfget *'. However, you need to avoid the `*' being expanded into the set of files in the current directory on the machine you're logged into; this is where the alias comes in, supplying the `noglob' in front of the function. There's no way of doing this with the function alone; by the time the function is called, the `*' would already have been expanded. Of course you could quote it, but that's what we're trying to avoid. This is a common reason for using the alias/function combination.

    Remember to include the `=' in alias definition, necessary in zsh, unlike csh and friends. If you do:

      alias zfget noglob zfget
    
    they are treated as a list of aliases. Since none has the `=' and a definition, the shell thinks you want to list the definitions of the listed words; I get the output
      zfget='noglob zfget'
      zfget='noglob zfget'
    
    since zfget was aliased as before, but noglob wasn't aliased and was skipped, although the failed alias lookup caused status 1 to be returned. Remember that the alias command takes as many arguments as you like; any with `=' is a definition, any without is a request to print the current definition.

    Aliases can in fact be allowed to expand to almost anything the shell understands, not just sets of words. That's because the text retrieved from the alias is put back into the input, and reread more or less as if you'd typed it. That means you can get away with strange combinations like

      alias tripe="echo foo | sed 's/foo/bar/' |"
      tripe cat
    
    which is interpreted exactly the same way as
      echo foo | sed 's/foo/bar/' | cat
    
    where the word `foo' is sent to the stream editor, which alters it to `bar' (`s/old/new/' is sed's syntax for a substitution), and passes it on to `cat', which simply dumps the output. It's useless, of course, but it does show what can lurk behind an apparently simple command if it happens to be an alias. It is usually not a good idea to do this, due to the potential confusion.

    As the manual entry explains, you can prevent an alias from being expanded by quoting it. This isn't like quoting any other expansion, though; there's no particular important character which has to be interpreted literally to stop the expansion. The point is that because aliases are expanded early on in processing of the command line, looking up an alias is done on a string without quotes removed. So if you have an alias `drivel', none of the strings `\drivel', `'d'rivel', or `drivel""' will be expanded as the alias: they all would have the same effect as proper commands, after the quotes are removed, but as aliases they appear different. The manual entry also notes that you can actually make aliases for any of these special forms, e.g. `alias '\drivel'=...' (note the quotes, since you need the backslash to be passed down to the alias command). You would need a pretty good reason to do so.

    Although my `tripe' example was silly, you know from the existence of `precommand modifiers' that it's sometimes useful to have a special command which precedes a command line, like noglob or the non-shell command nice. Since they have commands following, you would probably expect aliases to be expanded there, too. But this doesn't work:

      % alias foo='echo an alias for foo'
      % noglob foo
      zsh: command not found: foo
    
    because the foo wasn't in command position. The way round this is to use a special feature: aliases whose definitions end in a space force the next word along to be looked up as a possible alias, too:
      % alias noglob='noglob '
      % noglob foo
      an alias for foo
    
    which is useful for any command which can take a command line after it. This also shows another feature of aliases: unlike functions, they remember that you have already called an alias of a particular name, and don't look it up again. So the `noglob' which comes from expanding the alias is not treated as an alias, but as the ordinary precommand modifier.

    You may be a little mystified about this difference. A simple answer is that it's useful that way. It's sometimes useful for functions to call themselves; for example if you are handling a directory hierarchy in one go you might get a function to examine a directory, do something for every ordinary file, and for every directory file call itself with the new directory name tacked on. Aliases are too simple for this to be a useful feature. Another answer is that it's particularly easy to mark aliases as being `in use' while they are being expanded, because it happens while the strings inside them are being examined, before any commands are called, where things start to get complicated.

    Lastly, there are `global aliases'. If aliases can get you into a lot of trouble, global aliases can get you into a lot of a lot of trouble. They are defined with the option -g and are expanded not just in command position, but anywhere on the command line.

      alias -g L='| less'
      echo foo L
    
    This turns into `echo foo | less'. It's a neat trick if you don't mind your command lines having only a minimal amount to do with what is actually executed.

    I already pointed out that alias lookups are done so early that aliases are expanded when you define functions:

      % alias hello='echo I have been expanded'
      % fn() {
      function>  hello
      function> }
      % which fn
      fn () {
              echo I have been expanded
      }
    
    You can't stop this when typing in functions directly, except by quoting part of the name you type. When autoloading, the -U option is available, and recommended for use with any non-trivial function.

    A brief word about that `function>' which appears to prompt you while you are editing a function; I mentioned this in the previous chapter but here I want to be clearer about what's going on. While you are being prompted like that, the shell is not actually executing the commands you are typing in. Only when it is satisfied that it has a complete set of commands will it go away and execute them (in this case, defining the function). That means that it won't always spot errors until right at the end. Luckily, zsh has multi-line editing, so if you got it wrong you should just be able to hit up-arrow and edit what you typed; hitting return will execute the whole thing in one go. If you have redefined $PS2 (or $PROMPT2), or you have an old version of the shell, you may not see the full prompt, but you will usually see something ending in `>' which means the same.

    3.5: Command summary

    As a reminder, the shell looks up commands in this order:

  • aliases, which will immediately be interpreted again as texts for commands, possible even other aliases; they can be deleted with `unalias',

  • reserved words, those special to the shell which often need to be interpreted differently from ordinary commands due to the syntax, although they can be disabled if you really need to,

  • functions; these can also be disabled, although it's usually easier to `unfunction' them,

  • builtin commands, which can be disabled, or called as a builtin by putting `builtin' in front,

  • external commands, which can be called as such, even if the name clashes with one of the above types, by putting `command' in front.
  • 3.6: Expansions and quotes

    As I keep advertising, there will be a whole chapter dedicated to the subject of shell expansions and what to do with them. However, it's a rather basic subject, which definitely comes under the heading of basic shell syntax, so I shall here list all the forms of expansion. As given in the manual, there are five stages.

    3.6.1: History expansion

    This is the earliest, and is only done on an interactive command line, and only if you have not set NO_BANG_HIST. It was described in the section `The history mechanism; types of history' in the previous chapter. It is almost independent of the shell's processing of the command line; it takes place as the command line is read in, not when the commands are interpreted. However, in zsh it is done late enough that the `!'s can be quoted by putting them in single quotes:

      echo 'Hello!!'
    
    doesn't insert the previous line at that point, but
      echo "Hello!!"
    
    does. You can always quote active `!'s with a backslash, so
      echo "Hello\!\!"
    
    works, with or without the double quotes. Amusingly, since single quotes aren't special in double quotes, if you set the HIST_VERIFY option, which puts the expanded history line back on the command line for possible further editing, and try the first two of the three possibilities above in order, then keep hitting return, you will find ever increasing command lines:
      % echo 'Hello!!'
      Hello!!
      % echo "Hello!!"
      % echo "Helloecho 'Hello!!'"
      % echo "Helloecho 'Helloecho 'Hello!!''"
      % echo "Helloecho 'Helloecho 'Helloecho 'Hello!!'''"
    
    and if you understand why, you have a good grasp of how quotes work.

    There's another way of quoting exclamation marks in a line: put a `!"' in it. It can appear anywhere (as long as it's not in single quotes) and will be removed from the line, but it has the effect of disabling any subsequent exclamation marks till the end of the line. This is the only time quote marks which are significant to the shell (i.e. are not themselves quoted) don't have to occur in a matching pair.

    Note that as exclamation marks aren't active in any text read non-interactively --- and this includes autoloaded functions and sourced files, such as startup files, read inside interactive shells --- it is an error to quote any `!'s in double quotes in files. This will simply pass on the backslashes to the next level of parsing. Other forms of quoting are all right: `\!', because any character quoted with a backslash is treated as itself, and '!' because single quotes can quote anything anyway.

    3.6.2: Alias expansion

    As discussed above, alias expansion also goes on as the command line is read, so is to a certain extent similar to history expansion. However, while a history expansion may produce an alias for expansion, `!'s in the text resulting from alias expansions are normal characters, so it can be thought of as a later phase (and indeed it's implemented that way).

    3.6.3: Process, parameter, command, arithmetic and brace expansion

    There are a whole group of expansions which are done together, just by looking at the line constructed from the input after history and alias expansion and reading it from left to right, picking up any active expansions as the line is examined. Whenever a complete piece of expandable text is found, it is expanded; the text is not re-examined, except in the case of brace expansion, so none of these types of expansion is performed on any resulting text. Whether later forms of expansion --- in other words, filename generation and filename expansion are performed --- is another matter, depending largely on the GLOB_SUBST option as discussed in the previous chapter. Here's a brief summary of the different types.

    Process substitution

    There are three forms that result in a command line argument which refers to a file from or to which input or output is taken: `<(process)' runs the process which is expected to generate output which can be used as input by a command; `>(process)' runs the process which will take input to it; and `=(process)' acts like the first one, but it is guaranteed that the file is a plain file.

    This probably sounds like gobbledygook. Here are some simple examples.

      cat < <(echo This is output)
    
    (There are people in the world with nothing better to do than compile lists of dummy uses of the `cat' command, as in that example, and pour scorn on them, but I'll just have to brave it out.) What happens is that the command `echo This is output' is run, with the obvious result. That output is not put straight into the command line, as it would be with command substitution, to be described shortly. Instead, the command line is given a filename which, when read, gets that output. So it's more like:
      echo This is output >tmpfile
      cat < tmpfile
      rm tmpfile
    
    (note that the temporary file is cleaned up automatically), except that it's more compact. In this example I could have missed out the remaining `<', since cat does the right thing with a filename, but I put it there to emphasise the fact that if you want to redirect input from the process substitution you need an extra `<', over and above the one in the substitution syntax.

    Here's an example for the corresponding output substitution:

      echo This is output > \ 
      >(sed 's/output/rubbish/' >outfile)
    
    which is a perfectly foul example, but works essentially like:
      echo This is output >tmpfile
      sed 's/output/rubbish/' <tmpfile >outfile
    
    There's an obvious relationship to pipes here, and in fact this example could be better written,
      echo This is output | sed 's/output/rubbish/' >outfile
    
    A good example of an occasion where the output process substitution can't be replaced by a pipe is when it's on the error output, and standard output is being piped:
      ./myscript 2> >(grep -v idiot >error.log) |
          process-output >output.log
    
    a little abstract, but here the main point of the script `myscript' is to produce some output which undergoes further processing on the right-hand side of the pipe. However, we want to process the error output here, by filtering out occurrences of lines which use the word `idiot', before dumping those errors into a file error.log. So we get an effect similar to having two pipelines at once, one for output and one for error. Note again the two `>' signs present next to one another to get that effect.

    Finally, the `=(process)' form. Why do we need this as well as the one with `<'? To understand that, you need to know a little of how zsh tries to implement the latter type efficiently. Most modern UNIX-like systems have `named pipes', which are essentially files that behave like the `|' on the command line: one process writes to the file, another reads from it, and the effect is essentially that data goes straight through. If your system has them, you will usually find the following demonstration works:

      % mknod tmpfile p
      % echo This is output >tmpfile &
      [2] 1507
      % read line <tmpfile
      %
      [2]  + 1507 done       echo This is output >> tmpfile
      % print -- $line
      This is output
      %
    
    The syntax to create a named pipe is that rather strange `mknod' command, with `p' for pipe. We stick this in the background, because it won't do anything yet: you can't write to the pipe when there's no-one to read it (a fundamental rule of pipes which isn't quite as obvious as it may seem, since it is possible for data to lurk in the pipe, buffered, before the process reading from it extracts it), so we put that in the background to wait for action. This comes in the next line, where we read from the pipe: that allows the echo to complete and exit. Then we print out the line we've read.

    The problem with pipes is that they are just temporary storage spaces for data on the way through. In particular, you can't go back to the beginning (in C-speak, `you can't seek backwards on a pipe') and re-read what was there. Sometimes this doesn't matter, but some commands, such as editors, need that facility. As the `<' process substitution is implemented with named pipes (well, maybe), there is also the `=' form, which produces a real, live temporary file, probably in the `/tmp' directory, containing the output from the file, and then puts the name of that file on the command line. The manual notes, unusually helpfully, that this is useful with the `diff' command for comparing the output of two processes:

      diff =(./myscript1) =(./myscript2)
    
    where, presumably, the two scripts produce similar, but not identical, output which you want to compare.

    I said `well, maybe' in that paragraph because there's another way zsh can do `<' process substitutions. Many modern systems allow you to access a file with a name like `/dev/fd/0' which corresponds to file descriptor 0, in this case standard input: to anticipate the section on redirection, a `file descriptor' is a number assigned to a particular input or output stream. This method allows you to access it as a file; and if this facility is available, zsh will use it to pass the name of the file in process substitution instead of using a named pipe, since in this case it doesn't have to create a temporary file; the system does everything. Now, if you are really on the ball, you will realise that this doesn't get around the problem of pipes --- where is data on this file descriptor going to come from? The answer is that it will either have to come from a real temporary file --- which is pointless, because that's what we wanted to avoid --- or from a pipe opened from some process --- which is equivalent to the named pipe method, except with just a file descriptor instead of a name. So even if zsh does it this way, you still need the `=' form for programmes which need to go backwards in what they're reading.

    Parameter substitution

    You've seen enough of this already. This comes from a `$' followed either by something in braces, or by alphanumeric characters forming the name of the parameter: `$foo' or `${foo}', where the second form protects the expansion from any other strings at the ends and also allows a veritable host of extra things to appear inside the braces to modify the substitution. More detail will be held over to till chapter 5; there's a lot of it.

    Command substitution

    This has two forms, $(process) and `process`. They function identically; the first form has two advantages: substitutions can be nested, since the end character is different from the start character, and (because it uses a `$') it reminds you that, like parameter substitutions, command substitutions can take place inside double-quoted strings. In that case, like most other things in quotes, the result will be a single word; otherwise, the result is split into words on any field separators you have defined, usually whitespace or the null character. I'll use the args function again:

      % args() { print $# $*; }
      % args $(echo two words)
      2 two words
      % args "$(echo one word)"
      1 one word
    
    The first form will split on newlines, not just spaces, so an equivalent is
      % args $(echo two; echo words)
      2 two words
    
    Thus entire screens of text will be flattened out into a single line of single-word command arguments. By contrast, with the double quotes no processing is done whatsoever; the entire output is put verbatim into one command argument, with newlines intact. This means that the quite common case of wanting a single complete line from a file per command argument has to be handled by trickery; zsh has such trickery, but that's the stuff of chapter 5.

    Note the difference from process substitution: no intermediate file name is involved, the output itself goes straight onto the command line. This form of substitution is considerably more common, and, unlike the other, is available in all UNIX shells, though not in all shells with the more modern form `$(...)'.

    The rule that the command line is evaluated only once, left to right, is adhered to here, but it's a little more complicated in this case since the expression being substituted is scanned as a complete command line, so can include anything a command usually can, with all the rules of quoting and expansion being applied. So if you get confused about what a command substitution is actually up to, you should extract the commands from it and think of them as a command line in their own right. When you've worked out what that's doing, decide what it's output will be, and that's the result of the substitution. You can ignore any error output; that isn't captured, so will go straight to the terminal. If you want to ignore it, use the standard trick (see below) `2>/dev/null' inside the command substitution --- not on the main command line, where it won't work because substitutions are performed before redirection of the main command line, and in any case that will have the obvious side effect of changing the error output from the command line itself.

    The only real catch with command substitution is that, as it is run as a separate process --- even if it only involves shell builtins --- no effects other than the output will percolate back to the main shell:

      % print $(bar=value; print bar is $bar)
      bar is value
      % print bar is $bar
      bar is
    
    There is maybe room for a form of substitution that runs inside the shell, instead; however, with modern computers the overhead in starting the extra process is pretty small --- and in any case we seem to have run out of new forms of syntax.

    Once you know and are comfortable with command substitution, you will probably start using it all the time, so there is one good habit to get into straight away. A particularly common use is simply to put the contents of a file onto the command line.

      # Don't do this, do the other.
      process_cmd `cat file_arguments`
    
    But there's a shortcut.
      # Do do this, don't do the other
      process_cmd $(<file_arguments)
    
    It's not only less writing, it's more efficient: zsh spots the special syntax, with the < immediately inside the parentheses, reads the file directly without bothering to start `cat', and inserts its contents: no external process is involved. You shouldn't confuse this with `null redirections' as described below: the syntax is awfully similar, unfortunately, but the feature shown here is not dependent on that other feature being enabled or set up in a particular way. In fact, this feature works in ksh, which doesn't have zsh's null redirections.

    You can quote the file-reading form too, of course: in that case, the contents of the file `cmd_arguments' would be passed as just one argument, with newlines and spaces intact.

    Sometimes, the rule about splitting the result of a command substitution can get you into trouble:

      % typeset foo=`echo words words`
      % print $foo
      words
    
    You probably expected the command substitution not to be split here. but it was, and the shell executed typeset with the arguments `foo=words' and `words'. That's because in zsh arguments to typeset are treated pretty much normally, except for some jiggery pokery with tildes described below. Other shells do this differently, and zsh (from 4.0.2 and 4.1.1) provides a compatibility option, KSH_TYPESET. In earlier versions you need to use quotes:
      % typeset foo="`echo words words`"
      % print $foo
      words words
    

    A really rather technical afterword: using `$(cat file_arguments)', you might have counted two extra processes to be started, one being the usual one for a command substitution, and another the `cat' process, since that's an external command itself. That would indeed be the obvious way of doing it, but in fact zsh has an optimisation in cases like this: if it knows the shell is about to exit --- in this case, the forked process which is just interpreting the command line for the substitution --- it will not bother to start a new process for the last command, and here just replaces itself with the cat. So actually there's only one extra process here. Obviously, an interactive shell is never replaced in this way, since clairvoyance is not yet a feature of the shell.

    Arithmetic substitution

    Arithmetic substitution is easy to explain: everything I told you about the (( ... )) command under numerical parameters, above, applies to arithmetic substitution. You simply bang a `$' in front, and it becomes an expansion.

      % print $(( 32 + 2 * 5 ))
      42
    
    You can perform everything inside arithmetic substitution that you can inside the builtin, including assignments; the only difference is that the status is not set, instead the value is put directly onto the command line in place of the original expression. As in C, the value of an assignment is the value being assigned, `$(( param = 3 + 2))' substitutes the value 5 as well as assigning it to $param.

    By the way, there's an extra level of substitution involved in all arithmetic expansions, since scalar parameters are subject to arithmetic expansion when they're read in. This is simple if they only contain numbers, but less obvious if they contain complete expressions:

      % foo=3+5
      % print $(( foo + 2))
      10
    
    The foo was evaluated into 8 before it was substituted in. Note this means there were two evaluations: this doesn't work:
      % foo=3+
      % print $(( foo 2 ))
      zsh: bad math expression: operand expected at `'
    
    --- the complaint here is about the missing operand after the `+' in the $foo. However the following does work:
      % foo=3+
      % print $(( $foo 2 ))
      5
    
    That's because the scalar $foo is turned into 3+ first. This is more logical than you might think: with the rule about left to right evaluation, the $foo is picked up inside the $((...)) and expanded as an ordinary parameter substitution while the argument of $((...)) is being scanned. Then the complete argument `3+ 2' is expanded as an arithmetical expression. (Unfortunately, zsh isn't always this logical; there could easily be cases where we haven't thought it through --- you should feel free to bring these to our attention.)

    There's an older form with single square brackets instead of double parentheses; there is now no reason to use it, as it's non-standard, but you may sometimes still meet it.

    Brace expansion

    Brace expansion is a feature acquired from the C shell and it's relatives, although some versions of ksh have it, as it's a compile time option there. It's a useful way of saving you from typing the same thing twice on a single command line:

      % print -l {foo,bar}' is used far too often in examples'
      foo is used far too often in examples
      bar is used far too often in examples
    
    `print' is given two arguments which it is told to print out one per line. The text in quotes is common to both, but one has `foo' in front, while the other has `bar' in front. The brace expression can equally be in the middle of an argument: for example, a common use of this among programmers is for similarly named source files:
      % print zle_{tricky,vi,word}.c
      zle_tricky.c zle_vi.c zle_word.c
    
    As you see, you're not limited to two; you can have any number. You can quote a comma if you need a real one:
      % print -l \`{\,,.}\'' is a punctuation character'
      `,' is a punctuation character
      `.' is a punctuation character
    
    The quotes needed quoting with a backslash to get them into the output. The second comma is the active one for the braces.

    You can nest braces. Once again, this is done left to right. In

      print {now,th{en,ere{,abouts}}}
    
    the first argument of the outer brace is `now', and the second is `th{en,ere{,abouts}}'. This brace expands to `then' and then the expansion of `there{,abouts}', which is `there thereabouts' --- there's nothing to stop you having an empty argument. Putting this all together, we have
      print now then there thereabouts
    

    There's more to know about brace expansion, which will appear in chapter 5 on clever expansions.

    3.6.4: Filename Expansion

    It's a shame the names `filename expansion' and `filename generation' sound so similar, but most people just refer to `~ and = expansion' and `globbing' respectively, which is all that is meant by the two. The first is by far the simpler. The rule is: unquoted `~'s at the beginning of words perform expansion of named directories, which may be your home directory:

      % print ~
      /home/pws
    
    some user's home directory:
      % print ~root
      /root
    
    (that may turn up `/' on your system), a directory named directly by you:
      % t=/tmp
      % print ~t
      /tmp
    
    a directory you've recently visited:
      % pwd
      /home/pws/zsh/projects/zshguide
      % print ~+
      /home/pws/zsh/projects/zshguide
      % cd /tmp
      % print ~-
      /home/pws/zsh/projects/zshguide
    
    or a directory in your directory stack:
      % pushd /tmp
      % pushd ~
      % pushd /var/tmp
      % print ~2
      /tmp
    
    These forms were discussed above. There are various extra rules. You can add a `/' after any of them, and the expansions still take place, so you can use them to specify just the first part of a longer expression (as you almost certainly have done with a simple `~'). If you quote the `~' in any of the ways quoting normally takes place, the expansion doesn't happen.

    A ~ in the middle of the word means something completely different, if you have the EXTENDED_GLOB option set; if you don't, it doesn't mean anything. There are a few exceptions here; assignments are a fairly natural one:

      % foo=~pws
      % print $foo
      /home/pws
    
    (note that the `~pws', being unquoted, was expanded straight away at the assignment, not at the print statement). But the following works too:
      % PATH=$PATH:~pws/bin
    
    because colons are special in assignments. Note that this happens even if the variable isn't a colon-separated path; the shell doesn't know what use you're going to make of all the different variables.

    The companion of `~' is `=', which again has to occur at the start of a word or assignment to be special. The remainder of the word (here the entire remainder, because directory paths aren't useful) is taken as the name of an external command, and the word is expanded to the complete path to that command, using $PATH just as if the command were to be executed:

      % print =ls
      /bin/ls
    
    and, slightly confusingly,
      % foo==ls
      % print $foo
      /bin/ls
    
    where the two `='s have two different meanings. This form is useful in a number of cases. For example, you might want to look at or edit a script which you know is in your path; the form
      % vi =scriptname
    
    is more convenient than the more traditional
      % vi `whence -p ls`
    
    where I put the `-p' in to force whence to follow the path, ignoring builtins, functions, etc. This brings us to another use for `=' expansion,
      % =ls
    
    is a neat and extremely short way of referring to an external command when ls is usually a function. It has some of the same effect as `command ls', but is easier to type.

    In versions up to and including 4.0, this syntax will also expand aliases, so you need to be a bit careful if you really want a path to an external command:

      % alias foo='ls -F'
      % print =foo
      ls -F
    
    (Path expansion is done in preference, so you are safe if you use ls, unless your $PATH is strange.) Putting `=foo' at the start of the command line doesn't work, and the reason why bears examination: =-expansion occurs quite late on, after ordinary alias expansion and word splitting, so that the result is the single word `ls -F', where the space is part of the word, which probably doesn't mean anything (and if it does, don't lend me your computer when I need something done in a hurry). It's probably already obvious that alias expansion here is more trouble than it's worth. A less-than-exhaustive search failed to find anyone who liked this feature, and it has been removed from the shell from 4.1, so that `='-expansion now only expands paths to external commands.

    If you don't like =-expansion, you can turn it off by setting the option NO_EQUALS. One catch, which might make you want to do that, is that the commands mmv, mcp and mln, which are a commonly used though non-standard piece of free software, use `=' followed by a number to replace a pattern, for example

      mmv '*.c' '=1.old.c'
    
    renames all files ending with .c to end with .old.c. If you were not alert, you might forget to quote the second word. Otherwise, however, =' isn't very common at the start of a word, so you're probably fairly safe. For a way to do that with zsh patterns, see the discussion of the function zmv below (the answer is `zmv '(*).c' '$1.old.c'').

    Note that zsh is smart enough to complete the names of commands after an `=' of the expandable sort when you hit TAB.

    3.6.5: Filename Generation

    Filename generation is exactly the same as `globbing': the expanding of any unquoted wildcards to match files. This is only done in one directory at a time. So for example

      print *.c
    
    won't match files in a subdirectory ending in `.c'. However, it is done on all parts of a path, so
      print */*.c
    
    will match all `.c' files in all immediate subdirectories of the current directory. Furthermore, zsh has an extension --- one of its most commonly used special features --- to match files in any subdirectory at any depth, including the current directory: use two `*'s as part of the path:
      print **/*.c
    
    will match `prog.c', `version1/prog.c', `version2/test/prog.c', `oldversion/working/saved/prog.c', and so on. I will talk about filename generation and other uses of zsh's extremely powerful patterns at much greater length in chapter 5. My main thrust here is to fit it into other forms of expansion; the main thing to remember is that it comes last, after everything has already been done.

    So although you would certainly expect this to work,

      print ~/*
    
    generating all files in your home directory, you now know why: it is first expanded to `/home/pws/*' (or wherever), then the shell scans down the path until it finds a pattern, and looks in the directory it has reached (/home/pws) for matching files. Furthermore,
      foo=~/
      print $foo*
    
    works. However, as I explained in the last chapter, you need to be careful with
      foo=*
      print ~/$foo
    
    This just prints `/home/pws/*'. To get the `*' from the parameter to be a wildcard, you need to tell the shell explicitly that's what you want:
      foo=*
      print ~/${~foo}
    
    As also noted, other shells do expand the * as a wildcard anyway. The zsh attitude here, as with word splitting, is that parameters should do exactly what they're told rather than waltz off generating extra words or expansions.

    Be even more careful with arrays:

      foo=(*)
    
    will expand the * immediately, in the current directory --- the elements of the array assignment are expanded exactly like a normal command line glob. This is often very useful, but note the difference from scalar assignments, which do other forms of expansion, but not globbing.

    I'll mention a few possible traps for the unwary, which might confuse you until you are a zsh globbing guru. Firstly, parentheses actually have two uses. Consider:

      print (foo|bar)(.)
    
    The first set of parentheses means `match either foo or bar'. If you've used egrep, you will probably be familiar with this. The second, however, simply means `match only regular files'. The `(.)' is called a `globbing qualifier', because it limits the scope of any matches so far found. For example, if either or both of foo and bar were found, but were directories, they would not now be matched. There are many other possibilities for globbing qualifiers. For now, the easiest way to tell if something at the end is not a globbing qualifier is if it contains a `|'.

    The second point is about forms like this:

      print file-<1-10>.dat
    
    The `<' and `>' smell of redirection, as described next, but actually the form `<', optional start number, `-', optional finish number, `>' means match any positive integer in the range between the two numbers, inclusive; if either is omitted, there is no limit on that end, hence the cryptic but common `<->' to match any positive integer --- in other words, any group of decimal digits (bases other than ten are not handled by this notation). Older versions of the shell allowed the form `<>' as a shorthand to match any number, but the overlap with redirection was too great, as you'll see, so this doesn't work any more.

    Another two cryptic symbols are the two that do negation. These only work with the option `EXTENDED_GLOB' set: this is necessary to get the most out of zsh's patterns, but it can be a trap for the unwary by turning otherwise innocuous characters into patterns:

      print ^foo
    
    This means any file in the current directory except the file foo. One way of coming unstuck with `^' is something like
      stty kill ^u
    
    where you would hope `^u' means control with `u', i.e. ASCII character 21. But it doesn't, if EXTENDED_GLOB is set: it means `any file in the current directory except one called `u' ', which is definitely a different thing. The other negation operator isn't usually so fraught, but it can look confusing:
      print *.c~f*
    
    is a pattern of two halves; the shell tries to match `*.c', but rejects any matches which also match `f*'. Luckily, a `~' right at the end isn't special, so
     rm *.c~
    
    removes all files ending in `.c~' --- it wouldn't be very nice if it matched all files ending in `.c' and treated the final `~' as an instruction not to reject any, so it doesn't. The most likely case I can think of where you might have problems is with Emacs' numeric backup files, which can have a `~' in the middle which you should quote. There is no confusion with the directory use of `~', however: that only occurs at the beginning of a word, and this use only occurs in the middle.

    The final oddments that don't fit into normal shell globbing are forms with `#'. These also require that EXTENDED_GLOB be set. In the simplest use, a `#' after a pattern says `match this zero or more times'. So `(foo|bar)#.c' matches foo.c, bar.c, foofoo.c, barbar.c, foobarfoo.c, ... With an extra #, the pattern before (or single character, if it has no special meaning) must match at least once. The other use of `#' is in a facility called `globbing flags', which look like `(#X)' where `X' is some letter, possibly followed by digits. These turn on special features from that point in the pattern and are one of the newest features of zsh patterns; they will receive much more space in chapter 5.

    3.7: Redirection: greater-thans and less-thans

    Redirection means retrieving input from some other file than the usual one, or sending output to some other file than the usual one. The simplest examples of these are `<' and `>', respectively.

      % echo 'This is an announcement' >tempfile
      % cat <tempfile >newfile
      % cat newfile
      This is an announcement
    
    Here, echo sends its output to the file tempfile; cat took its input from that file and sent its output --- the same as its input --- to the file newfile; the second cat takes its input from newfile and, since its output wasn't redirected, it appeared on the terminal.

    The other basic form of redirection is a pipe, using `|'. Some people loosely refer to all redirections as pipes, but that's rather confusing. The input and output of a pipe are both programmes, unlike the case above where one end was a file. You've seen lots of examples already:

      echo foo | sed 's/foo/bar/'
    
    Here, echo sends its output to the programme sed, which substitutes foo by bar, and sends its own output to standard output. You can chain together as many pipes as you like; once you've grasped the basic behaviour of a single pipe, it should be obvious how that works:
      echo foo is a word | 
        sed 's/foo/bar/' | 
        sed 's/a word/an unword/'
    
    runs another sed on the output of the first one. (You can actually type it like that, by the way; the shell knows a pipe symbol can't be at the end of a command.) In fact, a single sed will suffice:
      echo foo is a word |
        sed -e 's/foo/bar/' -e 's/a word/an unword/'
    
    has the same effect in this case.

    Obviously, all three forms of redirection only work if the programme in question expects input from standard input, and sends output to standard output. You can't do:

      echo 'edit me' | vi
    
    to edit input, since vi doesn't use the input sent to it; it always deals with files. Most simple UNIX commands can be made to deal with standard input and output, however. This is a big difference from other operating systems, where getting programmes to talk to each other in an automated fashion can be a major headache.

    3.7.1: Clobber

    The word `clobber', as in the option NO_CLOBBER which I mentioned in the previous chapter, may be unfamiliar to people who don't use English as their first language. Its basic meaning is `hit' or `defeat' or `destroy', as in `Itchy and Scratchy clobbered each other with mallets'. If you do:

      % echo first go >file
      % echo second go >file
    
    then file will contain only the words `second go'. The first thing you put into the file, `first go', has been clobbered. Hence the NO_CLOBBER option: if this is set, the shell will complain when you try to overwrite the file. You can use `>|file' or `>! file' to override this. You usually can't use `>!file' because history expansion will try to expand `!file' before the shell parses the line; hence the form with the vertical bar tends to be more useful.

    3.7.2: File descriptors

    UNIX-like systems refer to different channels such as input, output and error by `file descriptors', which are small integers. Usually three are special: 0, standard input; 1, standard output; and 2, standard error. Bourne-like shells (but not csh-like shells) allow you to refer to a particular file descriptor, instead of standard input or output, by putting the integer immediately before the `<' or `>' (no space is allowed). What's more, if the `<' or `>' is followed immediately by `&', a file descriptor can follow the redirection (the one before is optional as usual). A common use is:

      % echo This message will go to standard error >&2
    
    The command sends its message to standard output, file descriptor 1. As usual, `>' redirects standard output. This time, however, it is redirected not to a file, but to file descriptor 2, which is standard error. Normally this is the same device as standard output, but it can be redirected completely separately. So:
      % { echo A message
      cursh> echo An error >&2 } >file
      An error
      % cat file
      A message
    
    Apologies for the slightly unclear use of the continuation prompt `cursh>': this guide goes into a lot of different formats, and some are a bit finicky about long lines in preformatted text. As pointed out above, the `>file' here will redirect all output from the stuff in braces, just as if it were a single command. However, the `>&2' inside redirects the output of the second echo to standard error. Since this wasn't redirected, it goes straight to the terminal.

    Note the form in braces in the previous example --- I'm going to use that in a few more examples. It simply sends something to standard output, and something else to standard error; that's its only use. Apart from that, you can treat the bit in braces as a black box --- anything which can produce both sorts of output.

    Sometimes you want to redirect both at once. The standard Bourne-like way of doing this is:

      % { echo A message
      cursh> echo An error >&2 } >file 2>&1
    
    The `>file' redirects standard output from the {...} to the file; the following 2>&1 redirects standard error to wherever standard output happens to be at that point, which is the same file. This allows you to copy two file descriptors to the same place. Note that the order is important; if you swapped the two around, `2>&1' would copy standard error to the initial destination of standard output, which is the terminal, before it got around to redirecting standard output.

    Zsh has a shorthand for this borrowed from csh-like shells:

      % { echo A message
      cursh> echo An error >&2 } >&file
    
    is exactly equivalent to the form in the previous paragraph, copying standard output and standard error to the same file. There is obviously a clash of syntax with the descriptor-copying mechanism, but if you don't have files whose names are numbers you won't run into it. Note that csh-like shells don't have the descriptor-copying mechanism: the simple `>&' and the same thing with pipes are the only uses of `&' for redirections, and it's not possible there to refer to particular file descriptors.

    To copy standard error to a pipe, there are also two forms:

      % { echo A message
      cursh> echo An error >&2 } 2>&1 | sed -e 's/A/I/'
      I message
      In error
      % { echo A message
      cursh> echo An error >&2 } |& sed -e 's/A/I/'
      I message
      In error
    
    In the first case, note that the pipe is opened before the other redirection, so that `2>&1' copies standard error to the pipe, not the original standard output; you couldn't put that after the pipe in any case, since it would refer to the `sed' command's output. The second way is like csh; unfortunately, `|&' has a different meaning in ksh (start a coprocess), so zsh is incompatible with ksh in this respect.

    You can also close a file descriptor you don't need: the form `2<&-' will close standard error for the command where it appears.

    One thing not always appreciated about redirections is that they can occur anywhere on the command line, not just at the end.

      % >file echo foo
      % cat file
      foo
    

    3.7.3: Appending, here documents, here strings, read write

    There are various other forms which use multiple `>'s and `<'s. First,

      % echo foo >file
      % echo bar >>file
      % cat file
      foo
      bar
    
    The `>>' appends to the file instead of overwriting it. Note, however, that if you use this a lot you may find there are neater ways of doing the same thing. In this example,
      % { echo foo
      cursh> echo bar } >file
      % cat file
      foo
      bar
    
    Here, `cursh>' is a prompt from the shell that it is waiting for you to close the `{' construct which executes a set of commands in the current shell. This construct can have a redirection applied to the entire sequence of commands: `>file' after the closing brace therefore redirects the output from both echos.

    In the case of input, doubling the sign has a totally different effect. The word after the << is not a file, but a string which will be used to mark in the end of input. Input is read until a line with only this string is found:

      % sed -e 's/foo/bar/' <<HERE
      heredoc> This line has foo in it.
      heredoc> There is another foo in this one.
      heredoc> HERE
      This line has a bar in it.
      There is another bar in this one.
    
    The shell prompts you with `heredoc>' to tell you it is reading a `here document', which is how this feature is referred to. When it finds the final string, in this case `HERE', it passes everything you have typed as input to the command as if it came from a file. The command in this case is the stream editor, which has been told to replace the first `foo' on each line with a `bar'. (Replacing things with a bar is a familiar experience from the city centre of my home town, Newcastle upon Tyne.)

    So far, the features are standard in Bourne-like shells, but zsh has an extension to here documents, sometimes referred to as `here strings'.

      % sed -e 's/string/nonsense/' \ 
      > <<<'This string is the entire document.'
      This nonsense is the entire document.
    
    Note that `>' on the second line is a continuation prompt, not part of the command line; it was just too long for the TeX version of this document if I didn't split it. This is a shorthand form of `here' document if you just want to pass a single string to standard input.

    The final form uses both symbols: `<>file' opens the file for reading and writing --- but only on standard input. In other words, a programme can now both read from and write to standard input. This isn't used all that often, and when you do use it you should remember that you need to open standard output explicitly to the same file:

      % echo test >/tmp/redirtest
      % sed 's/e/Z/g' <>/tmp/redirtest 1>&0
      % cat /tmp/redirtest
      tZtst
    
    As standard input (the 0) was opened for writing, you can perform the unusual trick of copying standard output (the 1) into it. This is generally not a particularly safe way of doing in-place editing, however, though it seems to work fine with sed. Note that in older versions of zsh, `<>' was equivalent to `<->', which is a pattern that matches any number; this was changed quite some time ago.

    3.7.4: Clever tricks: exec and other file descriptors

    All Bourne-like shells have two other features. First, the `command' exec, which I described above as being used to replace the shell with the command you give after it, can be used with only redirections after it. These redirections then apply permanently to the shell itself, rather than temporarily to a single command. So

      exec >file
    
    makes file the destination for standard output from that point on. This is most useful in scripts, where it's quite common to want to change the destination of all output.

    The second feature is that you can use file descriptors which haven't even been opened yet, as long as they are single digits --- in other words, you can use numbers 3 to 9 for your own purposes. This can be combined with the previous feature for some quite clever effects:

      exec 3>&1               
      # 3 refers to stdout
      exec >file
      # stdout goes to `file', 3 untouched
          # random commands output to `file'
      exec 1>&3               
      # stdout is now back where it was
      exec 3>&-
      # file descriptor 3 closed to tidy up
    
    Here, file descriptor 3 has been used simply as a placeholder to remember where standard output was while we temporarily divert it. This is an alternative to the `{...} >file' trick. Note that you can put more than one redirection on the exec line: `exec 3>&1 >file' also works, as long as you keep the order the same.

    3.7.5: Multios

    Multios allow you to do an implicit `cat' (concatenate files) on input and `tee' (send the same data to different files) on output. They depend on the option MULTIOS being set, which it is by default. I described this in the last chapter in discussing whether or not you should have the option set, so you can look at the examples there.

    Here's one fact I didn't mention. You use output multios like this:

      command-generating-output >file1 >file2
    
    where the command's output is copied to both files. This is done by a process forked off by the shell: it simply sits waiting for input, then copies it to all the files in its list. There's a problem in all versions of the shell to date (currently 4.0.6): this process is asynchronous, so you can't rely on it having finished when the shell starts executing the next command. In other words, if you look at file1 or file2 immediately after the command has finished, they may not yet contain all the output because the forked process hasn't finished writing to it.

    This is really a bug, but for the time being you will have to live with it as it's quite complicated to fix in all cases. Multios are most useful as a shorthand in interactive use, like so much of zsh; in a script or function it is safer to use tee,

      command-generating-output | tee file1 file2
    
    which does the same thing, but as tee is handled as a synchronous process file1 and file2 are guaranteed to be complete when the pipeline exits.

    3.8: Shell syntax: loops, (sub)shells and so on

    3.8.1: Logical command connectors

    I have been rather cavalier in using a couple of elements of syntax without explaining them:

      true  &&  print Previous command returned true
      false  ||  print Previous command returned false
    
    The relationship between `&&' and `||' and tests is fairly obvious, but in this case they connect complete commands, not test arguments. The `&&' executes the following command if the one before succeeded, and the `||' executes the following command if the one before failed. In other words, the first is equivalent to
      if true; then
        print Previous command returned true
      fi
    
    but is more compact.

    There is a perennial argument about whether to use these or not. In the comp.unix.shell newsgroup on Usenet, you see people arguing that the `&&' syntax is unreadable, and only an idiot would use it, while other people argue that the full `if' syntax is slower and clumsier, and only an idiot would use that for a simple test; but Usenet is like that, and both answers are a bit simplistic. On the one hand, the difference in speed between the two forms is minute, probably measurable in microseconds rather than milliseconds on a modern computer; the scheduling of the shell process running the script by the operating system is likely to make more difference if these are embedded inside a much longer script or function, as they will be. And on the other hand, the connection between `&&' and a logical `and' is so strong in the minds of many programmers that to anyone with moderate shell experience they are perfectly readable. So it's up to you. I find I use the `&&' and `||' forms for a pair of simple commands, but use `if' for anything more complicated.

    I would certainly advise you to avoid chains like:

      true || print foo && print bar || false
    
    If you try that, you will see `bar' but not `foo', which is not what a C programmer might expect. Using the usual rules of precedence, you would parse it as: either true must be true; or both the print statements must be true; or the false must be true. However, the shell parses it differently, using these rules:
  • If you encounter an `&&',
  • if the command before it (really the complete pipeline) succeeded, execute the command immediately after, and execute what follows normally
  • else if the command failed, skip the next command and any others until an `||' is encountered, or until the group of commands is ended by a newline, a semicolon, or the end of an enclosing group. Then execute whatever follows in the normal way.
  • If you encounter an `||',
  • if the command before it succeeded, skip the next command and any others until an `&&' is encountered, or until the end of the group, and execute what follows normally
  • else if the command failed, execute the command immediately after the `||'.
  • If that's hard to follow, just note that the rule is completely symmetric; a simple summary is that the logical connectors don't remember their past state. So in the example shown, the `true' succeeds, we skip `print foo' but execute `print bar' and then skip false. The expression returns status zero because the last thing it executed did so. Oddly enough, this is completely standard behaviour for shells. This is a roundabout way of saying `don't use combined chains of `&&'s and `||'s unless you think Gödel's theorem is for sissies'.

    Strictly speaking, the and's and or's come in a hierarchy of things which connect commands. They are above pipelines, which explains my remark above --- an expression like `echo $ZSH_VERSION | sed '/dev//'' is treated as a single command between any logical connectors --- and they are below newlines and semicolons --- an expression like `true && print yes; false || print no' is parsed as two distinct sets of logically connected command sequences. In the manual, a list is a complete set of commands executed in one go:

      echo foo; echo bar
    
      echo small furry animals
    
    --- a shell function is basically a glorified list with arguments and a name. A sublist is a set of commands up to a newline or semicolon, in other words a complete expression possibly involving the logical connectors:
      show -nomoreproc | 
        grep -q foo && 
        print The word '`foo'\' occurs.
    
    A pipeline is a chain of one or more commands connected by `|', for example both individual parts of the previous sublist,
      show -nomoreproc | grep -q foo
    
    and
      print The word '`foo'\' occurs.
    
    count as pipelines. A simple command is one single unit of execution with a command name, so to use the same example that includes all three of the following,
      show -nomoreproc
      grep -q foo
      print The word '`foo'\' occurs.
    

    This means that in something like

      print foo
    
    where the command is terminated by a newline and then executed in one go, the expression is all of the above --- list, sublist, pipeline and simple command. Mostly I won't need to make the formal distinction; it sometimes helps when you need to break down a complicated set of commands. It's a good idea, and usually possible, to write in such a way that it's obvious how the commands break down. It's not too important to know the details, as long as you've got a feel for how the shell finds the next command.

    3.8.2: Structures

    I've shown plenty of examples of one sort of shell structure already, the if statement:

      if [[ black = white ]]; then
        print Yellow is no colour.
      fi
    
    The main points are: the `if' itself is followed by some command whose return status is tested; a `then' follows as a new command; any number of commands may follow, as complex as you like; the whole sequence is ended by a `fi' as a command on its own. You can write the `then' on a new line if you like, I just happen to find it neater to stick it where it is. If you follow the form here, remember the semicolon before it; the then must start a separate command. (You can put another command immediately after the then without a newline or semicolon, though, although people tend not to.)

    The double-bracketed test is by far the most common thing to put here in zsh, as in ksh, but any command will do; only the status is important.

      if true; then
        print This always gets executed
      fi
      if false; then
        print This never gets executed
      fi
    
    Here, true always returns true (status 0), while false always returns false (status 1 in zsh, although some versions return status 255 --- anything nonzero will do). So the statements following the prints are correct.

    The if construct can be extended by `elif' and `else':

      read var
      if [[ $var = yes ]]; then
        print Read yes
      elif [[ $var = no ]]; then
        print Read no
      else
        print Read something else
      fi
    
    The extension is pretty straightforward. You can have as many `elif's with different tests as you like; the code following the first test to succeed is executed. If no test succeeded, and there is an `else' (there doesn't need to be), the code following that is executed. Note that the form of the `elif' is identical to that of `if', including the `then', while the else just appears on its own.

    The while-loop is quite similar to if. There are two differences: the syntax uses while, do and done instead of if, then and fi, and after the loop body is executed (if it is), the test is evaluated again. The process stops as soon as the test is false. So

      i=0
      while (( i++ < 3 )); do
        print $i
      done
    
    prints 1, then 2, then 3. As with if, the commands in the middle can be any set of zsh commands, so
      i=0
      while (( i++ < 3 )); do
        if (( i & 1 )); then
          print $i is odd
        else
          print $i is even
        fi
      done
    
    tells you that 1 and 3 are odd while 2 is even. Remember that the indentation is irrelevant; it is purely there to make the structures more easy to understand. You can write the code on a single line by replacing all the newlines with semicolons.

    There is also an until loop, which is identical to the while loop except that the loop is executed until the test is true. `until [[...' is equivalent to `while ! [[...'.

    Next comes the for loop. The normal case can best be demonstrated by another example:

      for f in one two three; do
        print $f
      done
    
    which prints out `one' on the first iteration, then `two', then `three'. The f is set to each of the three words in turn, and the body of the loop executed for each. It is very useful that the words after the `in' may be anything you would normally have on a shell command line. So `for f in *; do' will execute the body of the loop once for each file in the current directory, with the file available as $f, and you can use arrays or command substitutions or any other kind of substitution to generate the words to loop over.

    The for loop is so useful that the shell allows a shorthand that you can use on the command line: try

      for f in *; print $f
    
    and you will see the files in the current directory printed out, one per line. This form, without the do and the done, involves less typing, but is also less clear, so it is recommended that you only use it interactively, not in scripts or functions. You can turn the feature off with NO_SHORT_LOOPS.

    The case statement is used to test a pattern against a series of possibilities until one succeeds. It is really a short way of doing a series of if and elif tests on the same pattern:

      read var
      case $var in
        (yes) print Read yes
              ;;
        (no) print Read no
             ;;
        (*) print Read something else
            ;;
       esac
    
    is identical to the if/elif/else example above. The $var is compared against each pattern in turn; if one matches, the code following that is executed --- then the statement is exited; no further matches are looked for. Hence the `*' at the end, which can match anything, acts like the `else' of an if statement.

    Note the quirks of the syntax: the pattern to test must appear in parentheses. For historical reasons, you can miss out the left parenthesis before the pattern. I haven't done that mainly because unbalanced parentheses confuse the system I am using for writing this guide. Also, note the double semicolon: this is the only use of double semicolons in the shell. That explains the fact that if you type `;;' on its own the shell will report a `parse error'; it couldn't find a case to associate it with.

    You can also use alternative patterns by separating them with a vertical bar. Zsh allows alternatives with extended globbing anyway; but this is actually a separate feature, which is present in other shells which don't have zsh's extended globbing feature; it doesn't depend on the EXTENDED_GLOB option:

      read var
      case $var in
        (yes|true|1) print Reply was affirmative
                     ;;
        (no|false|0) print Reply was negative
                     ;;
        (*) print Reply was cobblers
                  ;;
      esac
    
    The first `print' is used if the value of $var read in was `yes', `true' or `1', and so on. Each of the separate items can be a pattern, with any of the special characters allowed by zsh, this time depending on the setting of the option EXTENDED_GLOB.

    The select loop is not used all that often, in my experience. It is only useful with interactive input (though the code may certainly appear in a script or function):

      select var in earth air fire water; do
        print You selected $var
      done
    
    This prints a menu; you must type 1, 2, 3 or 4 to select the corresponding item; then the body of the loop is executed with $var set to the value in the list corresponding to the number. To exit the loop hit the break key (usually ^G) or end of file (usually ^D: the feature is so infrequently used that currently there is a bug in the shell that this tells you to use `exit' to exit, which is nonsense). If the user entered a bogus value, then the loop is executed with $var set to the empty string, though the actual input can be retrieved from $REPLY. Note that the prompt printed for the user input is $PROMPT3, the only use of this parameter in the shell: all normal prompt substitutions are available.

    There is one final type of loop which is special to zsh, unlike the others above. This is `repeat'. It can be used two ways:

      % repeat 3 print Hip Hip Hooray
      Hip Hip Hooray
      Hip Hip Hooray
      Hip Hip Hooray
    
    Here, the first word after repeat is a count, which could be a variable as normal substitutions are performed. The rest of the line (or until the first semicolon) is a command to repeat; it is executed identically each time.

    The second form is a fully fledged loop, just like while:

      % repeat 3; do
      repeat> print Hip Hip Hooray
      repeat> done
      Hip Hip Hooray
      Hip Hip Hooray
      Hip Hip Hooray
    
    which has the identical effect to the previous one. The `repeat>' is the shell's prompt to show you that it is parsing the contents of a `repeat' loop.

    3.8.3: Subshells and current shell constructs

    More catching up with stuff you've already seen. The expression in parentheses here:

      % (cd ~; ls)
      <all the files in my home directory>
      % pwd
      <where I was before, not necessarily ~>
    
    is run in a subshell, as if it were a script. The main difference is that the shell inherits almost everything from the main shell in which you are typing, including options settings, functions and parameters. The most important thing it doesn't inherit is probably information about jobs: if you run jobs in a subshell, you will get no output; you can't use fg to resume a job in a subshell; you can't use `kill %n' to kill a job (though you can still use the process ID); and so on. By now you should have some feel for the effect of running in a separate process. Running a command, or set of commands, in a different directory, as in this example, is one quite common use for this construct. (In zsh 4.1, you can use jobs in a subshell; it lists the jobs running in the parent shell; this is because it is very useful to be able to pipe the output of jobs into some processing loop.)

    On the other hand, the expression in braces here:

      % {cd ~; ls}
      <all the files in my home directory>
      % pwd
      /home/pws
    
    is run in the current shell. This is what I was blathering on about in the section on redirection. Indeed, unless you need some special effect like redirecting a whole set of commands, you won't use the current-shell construct. The example here would behave just the same way if the braces were missing.

    As you might expect, the syntax of the subshell and current-shell forms is very similar. You can use redirection with both, just as with simple commands, and they can appear in most places where a simple command can appear:

      [[ $test = true ]] && {
        print Hello.
        print Well, this is exciting.
      }
    
    That would be much clearer using an `if', but it works. For some reason, you often find expressions of this form in system start-up files located in the directory /etc/rc.d or, on older systems, in files whose names begin with `/etc/rc.'. You can even do:
      if { foo=bar; [[ $foo = bar ]] }; then
        print yes
      fi
    
    but that's also pretty gross.

    One use for {...} is to make sure a whole set of commands is executed at once. For example, if you copy a set of commands from a script in one window and want them to be run in one go in a shell in another window, you can do:

       % {
       cursh>            # now paste your commands in here...
        ...
       cursh> }
    
    and the commands will only be executed when you hit return after the final `}'. This is also a workaround for some systems where cut and paste has slightly odd effects due to the way different states of the terminal are handled. The current-shell construct is a little bit like an anonymous function, although it doesn't have any of the usual features of functions --- you can't pass it arguments, and variables declared inside aren't local to that section of code.

    3.8.4: Subshells and current shells

    In case you're confused about what happens in the current shell and what happens in a subshell, here's a summary.

    The following are run in the current shell.

    1. All shell builtins and anything which looks like one, such as a precommand modifier and tests with `[['.
    2. All complex statements and loops such as if and while. Tests and code inside the block must both be considered separately.
    3. All shell functions.
    4. All files run by `source' or `.' as well as startup files.
    5. The code inside a `{...}'.
    6. The right hand side of a pipeline: this is guaranteed in zsh, but don't rely on it for other shells.
    7. All forms of substitution except `...`, $(...), =(...), <(...) and >(...).

    The following are run in a subshell.

    1. All external commands.
    2. Anything on the left of a pipe, i.e. all sections of a pipeline but the last.
    3. The code inside a `(...)'.
    4. Substitutions involving execution of code, i.e. `...`, $(...), =(...), <(...) and >(...). (TCL fans note that this is different from the `[...]' command substitution in that language.)
    5. Anything started in the background with `&' at the end.
    6. Anything which has ever been suspended. This is a little subtle: suppose you execute a set of commands in the current shell and suspend it with ^Z. Since the shell needs to return you to the prompt, it forks a subshell to remember the commands it was executing when you interrupted it. If you use fg or bg to restart, the commands will stay in the subshell. This is a special feature of zsh; most shells won't let you interrupt anything in the current shell like that, though you can still abort it with ^C.
    With an alias, you can't tell where it will be executed --- you need to find out what it expands too first. The expansion naturally takes place in the current shell.

    Of course, if for some reason the current set of commands is already running in a subshell, it doesn't get magically returned to the current shell --- so a shell builtin on the left hand side of a pipeline is running in a subshell. However, it doesn't get an extra subshell, as an external command would. What I mean is:

      { print Hello; cat file } |
        while read line; print $line; done
    
    The shell forks, producing a subshell, to execute the left hand side of the pipeline, and that subshell forks to execute the cat external command, but nothing else in that set of commands will cause a new subshell to be created.

    (For the curious only: actually, that's not quite true, and I already pointed this out when I talked about command substitutions: the shell keeps track of occasions when it is in a subshell and has no more commands to execute. In this case it will not bother forking to create a new process for the cat, it will simply replace the subshell which is not needed any more. This can only happen in simple cases where the shell has no clearing up to do.)

    3.9: Emulation and portability

    I described the options you need to set for compatibility with ksh in the previous chapter. Here I'm more interested in the best way of running ksh scripts and functions.

    First, you should remember that because of all zsh's options you can't assume that a piece of zsh code will simply run a piece of sh or ksh code without any extra changes. Our old friend SH_WORD_SPLIT is the most common problem, but there are plenty of others. In addition to options, there are other differences which simply need to be worked around. I will list some of them a bit later. Generally speaking, Bourne shell is simple enough that zsh emulates it pretty well --- although beware in case you are using bash extensions, since to many Linux users bash is the nearest approximation to the Bourne shell they ever come across. Zsh makes no attempt to emulate bash, even though some of bash's features have been incorporated.

    To make zsh emulate ksh or sh as closely as it knows how, there are various things you can do.

    1. Invoke zsh under the name sh or ksh, as appropriate. You can do this by creating a symbolic link from zsh to sh or ksh. Then when zsh starts up all the options will be set appropriately. If you are starting that shell from another zsh, you can use the feature of zsh that tricks a programme into thinking it has a different name: `ARGV0=sh zsh' runs zsh under the name sh, just like the symbolic link method.
    2. Use `emulate ksh' at the top of the script or function you want to run. In the case of a function, it is better to run `emulate -L ksh' since this makes sure the normal options will be restored when the function exits; this is irrelevant for a script as the options cannot be propagated to the process which ran the script. You can also use the option `-R' after emulate, which forces more options to be like ksh; these extra options are generally for user convenience and not relevant to basic syntax, but in some cases you may want the extra cover provided.

      If it's possible the script may already be running under ksh, you can instead use

        [[ -z $ZSH_VERSION ]] && emulate ksh
      
      or for sh, using the simpler test command there,
        [ x$ZSH_VERSION = x ] && emulate sh
      
    Both these methods have drawbacks, and if you plan to be a heavy zsh user there's no substitute for simply getting used to zsh's own basic syntax. If you think there is some useful element of emulation we missed, however, you should certainly tell the zsh-workers mailing list about it.

    Emulation of ksh88 is much better than emulation of ksh93. Support for the latter is gradually being added, but only patchily.

    There is no easy way of converting code written for any csh-like shell; you will just have to convert it by hand. See the FAQ for some hints on converting aliases to functions.

    3.9.1: Differences in detail

    Here are some differences from ksh88 which might prove significant for ksh programmers. This is lifted straight from the corresponding section of the FAQ; it is not complete, and indeed some of the `differences' could be interpreted as bugs. Those marked `*' perform in a ksh-like manner if the shell is invoked with the name `ksh', or if `emulate ksh' is in effect.

  • Syntax:
  • * Shell word splitting.
  • * Arrays are (by default) more csh-like than ksh-like: subscripts start at 1, not 0; array[0] refers to array[1]; $array refers to the whole array, not $array[0]; braces are unnecessary: $a[1] == ${a[1]}, etc. The KSH_ARRAYS option is now available.
  • Coprocesses are established by coproc; |& behaves like csh. Handling of coprocess file descriptors is also different.
  • In cmd1 && cmd2 &, only cmd2 instead of the whole expression is run in the background in zsh. The manual implies this is a bug. Use { cmd1 && cmd2 } & as a workaround.
  • Command line substitutions, globbing etc.:
  • * Failure to match a globbing pattern causes an error (use NO_NOMATCH).
  • * The results of parameter substitutions are treated as plain text: foo="*"; print $foo prints all files in ksh but * in zsh (unset GLOB_SUBST).
  • * $PSn do not do parameter substitution by default (use PROMPT_SUBST).
  • * Standard globbing does not allow ksh-style `pattern-lists'. See chapter 5 for a list of equivalent zsh forms. The ^, ~ and # (but not |) forms require EXTENDED_GLOB. From version 3.1.3, the ksh forms are fully supported when the option KSH_GLOB is in effect.

    [1] Note that ~ is the only globbing operator to have a lower precedence than /. For example, **/foo~*bar* matches any file in a subdirectory called foo, except where bar occurred somewhere in the path (e.g. users/barstaff/foo will be excluded by the ~ operator). As the ** operator cannot be grouped (inside parentheses it is treated as *), this is the way to exclude some subdirectories from matching a **.

  • Unquoted assignments do file expansion after colons (intended for PATHs).
  • integer does not allow -i.
  • typeset and integer have special behaviour for assignments in ksh, but not in zsh. For example, this doesn't work in zsh:
            integer k=$(wc -l ~/.zshrc)
        
    
    because the return value from wc includes leading whitespace which causes wordsplitting. Ksh handles the assignment specially as a single word.
  • Command execution:
  • * There is no $ENV variable (use /etc/zshrc, ~/.zshrc; note also $ZDOTDIR).
  • $PATH is not searched for commands specified at invocation without -c.
  • Aliases and functions:
  • The order in which aliases and functions are defined is significant: function definitions with () expand aliases.
  • Aliases and functions cannot be exported.
  • There are no tracked aliases: command hashing replaces these.
  • The use of aliases for key bindings is replaced by `bindkey'.
  • * Options are not local to functions (use LOCAL_OPTIONS; note this may always be unset locally to propagate options settings from a function to the calling level).
  • Traps and signals:
  • * Traps are not local to functions. The option LOCAL_TRAPS is available from 3.1.6.
  • TRAPERR has become TRAPZERR (this was forced by UNICOS which has SIGERR).
  • Editing:
  • The options emacs, gmacs, viraw are not supported. Use bindkey to change the editing behaviour: set -o {emacs,vi} becomes bindkey -{e,v}; for gmacs, go to emacs mode and use bindkey \^t gosmacs-transpose-characters.
  • The keyword option does not exist and -k is instead interactivecomments. (keyword will not be in the next ksh release either.)
  • Management of histories in multiple shells is different: the history list is not saved and restored after each command. The option SHARE_HISTORY appeared in 3.1.6 and is set in ksh compatibility mode to remedy this.
  • \ does not escape editing chars (use ^V).
  • Not all ksh bindings are set (e.g. <ESC>#; try <ESC>q).
  • * # in an interactive shell is not treated as a comment by default.
  • Built-in commands:
  • Some built-ins (r, autoload, history, integer ...) were aliases in ksh.
  • There is no built-in command newgrp: use e.g. alias newgrp="exec newgrp"
  • jobs has no -n flag.
  • read has no -s flag.
  • Other idiosyncrasies:
  • select always redisplays the list of selections on each loop.
  • 3.9.2: Making your own scripts and functions portable

    There are also problems in making your own scripts and functions available to other people, who may have different options set.

    In the case of functions, it is always best to put `emulate -L zsh' at the top of the function, which will reset the options to the default zsh values, and then set any other necessary options. It doesn't take the shell a great deal of time to process these commands, so try and get into the habit of putting them any function you think may be used by other people. (Completion functions are a special case as the environment is already standardised --- see chapter 6 for this.)

    The same applies to scripts, since if you run the script without using the option `-f' to zsh the user's non-interactive startup files will be run, and in any case the file /etc/zshenv will be run. We urge system administrators not to set options unconditionally in that file unless absolutely necessary; but they don't always listen. Hence an emulate can still save a lot of grief.

    3.10: Running scripts

    Here are some final comments on running scripts: they apply regardless of the problems of portability, but you should certainly also be aware of what I was saying in the previous section.

    You may be aware that you can force the operating system to run a script using a particular interpreter by putting `#!' and the path to the interpreter at the top of the script. For example, a zsh script could start with

       #!/usr/local/bin/zsh
       print The arguments are $*
    
    assuming that zsh lives in the directory /usr/local/bin. Then you can run the script under its name as if it were an ordinary command. Suppose the script were called `scriptfile' and in the current directory, and you want to run it with the arguments `one two forty-three'. First you must make sure the script is executable:
      % chmod +x scriptfile
    
    and then you can run it with the arguments:
      % ./scriptfile one two forty-three
      The arguments are one two forty-three
    
    The shell treats the first line as a comment, since it begins with a `#', but note it still gets evaluated by the shell; the system simply looks inside the file to see if what's there, it doesn't change it just because the first line tells it to execute the shell.

    I put the `./' in front to refer to the current directory because I don't usually have that in my path --- this is for safety, to avoid running things which happen to have names like commands simply because they were in the current directory. But many people aren't so paranoid, and if `.' is in your path, you can omit the `./'. Hence, obviously, it can be anywhere else in your path: it is searched for as an ordinary executable.

    The shell actually provides this mechanism even on operating systems (now few and far between in the UNIX world) that don't have the feature built into them. The way this works is that if the shell found the file, and it was executable, but running it didn't work, then it will look for the #!, extract the name following and run (in this example) `/usr/local/bin/zsh <path>/scriptfile one two forty-three', where <path> is the path where the file was found. This is, in fact, pretty much what the system does if it handles it itself.

    Some shells search for scripts using the path when they are given as filenames at invocation, but zsh happens not to. In other words, `zsh scriptfile' only runs scriptfile in the current directory.

    There are two other features you may want to be aware of. Both are down to the operating system, if that is what is responsible for the `#!' trick (true of all the most common UNIX-like systems at the moment). First, you are usually allowed to supply one, but only one, argument or option in the `#!' line, thus:

      #!/usr/local/bin/zsh -f
      print other stuff here
    
    which stops startup files other than /etc/zshenv from being run, but otherwise works the same as before. If you need more options, you should combine them in the same word. However, it's usually clearer, for anything apart from -f, -i (which forces the shell into interactive mode) and a few other options which need to take effect immediately, to put a `setopt' line at the start of the body of the script. In a few versions of zsh, there was an unexpected consequence of the fact that the line would only be split once: if you accidentally left some spaces at the end of the line (e.g. `#!/usr/local/bin/zsh -f ') they would be passed down to the shell, which would report an error, which was hard to interpret. The spaces will still usually be passed down, but the shell is now smart enough to ignore spaces in an option list.

    The second point is that the length of the `#!' line which will be evaluated is limited. Often the limit is 32 characters, in total, That means if your path to zsh is long, e.g. `/home/users/psychology/research/dreams/freud/solaris_2.5/bin/zsh' the system won't be able to find the shell. Your only recourse is to find a shorter path, or execute the shell directly, or some sneakier trick such as running the script under /bin/sh and making that start zsh when it detects that zsh isn't running yet. That's a fairly nasty way of doing it, but just in case you find it necessary, here's an example:

      #!/bin/sh
    
      if [ x$ZSH_VERSION = x ]; then
        # Put the right path in here ---
        # or just rely on finding zsh in
        # $path, since `exec' handles that.
        exec /usr/local/bin/zsh $0 "$@"
      fi
    
      print $ZSH_VERSION
      print Hello, this is $0
      print with arguments $*.
    
    Note that first `$0', which passes down the name of the script that was originally executed. Running this as `testexec foo bar' gives me
      3.1.9-dev-8
      Hello, this is /home/pws/tmp/testexec
      with arguments foo bar.
    
    I hope you won't have to resort to that. By the way, really, excruciatingly old versions of zsh didn't have $ZSH_VERSION. Rather than fix the script, I suggest you upgrade the shell. Also, on some old Bourne shells you might need to replace "$@" with ${1+"$@"}, which is more careful about only putting in arguments if there were any (this is the sort of thing we'll see in chapter 5). Usually this isn't necessary.

    You can use the same trick on ancient versions of UNIX which didn't handle `#!'. On some such systems, anything with a `:' as the first character is run with the Bourne shell, so this serves as an alternative to `#!/bin/sh', while on some Berkeley systems, a plain `#' caused csh to be used. In the second case, you will need to change the syntax of the first test to be understood by both zsh and csh. I'll leave that as an exercise for the reader. If you have perl (very probable these days) you can look at the perlrun manual page, which discusses the corresponding problem of starting perl scripts from a shell, for some ideas.

    There's one other glitch you may come across. Sometimes if you type the name of a script which you know is in your path and is executable, the shell may tell you `file not found', or some equivalent message. What this usually means is that the interpreter wasn't found, because you mistyped the line after the `#!'. This confusing message isn't the shell's fault: a lot of operating systems return the same system error in this case as if the script were really not found. It's not worth the shell searching the path to see if the script is there, because in the vast majority of cases the error refers to the programme in the execution path. If the operating system returned the more natural error, `exec format error', then the shell would know that there was something wrong with the file, and could investigate; but unfortunately life's not that simple.