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6. Shell Grammar


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6.1 Simple Commands & Pipelines

A simple command is a sequence of optional parameter assignments followed by blank-separated words, with optional redirections interspersed. The first word is the command to be executed, and the remaining words, if any, are arguments to the command. If a command name is given, the parameter assignments modify the environment of the command when it is executed. The value of a simple command is its exit status, or 128 plus the signal number if terminated by a signal. For example,

 
echo foo

is a simple command with arguments.

A pipeline is either a simple command, or a sequence of two or more simple commands where each command is separated from the next by ‘|’ or ‘|&’. Where commands are separated by ‘|’, the standard output of the first command is connected to the standard input of the next. ‘|&’ is shorthand for ‘2>&1 |’, which connects both the standard output and the standard error of the command to the standard input of the next. The value of a pipeline is the value of the last command, unless the pipeline is preceded by ‘!’ in which case the value is the logical inverse of the value of the last command. For example,

 
echo foo | sed 's/foo/bar/'

is a pipeline, where the output (‘foo’ plus a newline) of the first command will be passed to the input of the second.

If a pipeline is preceded by ‘coproc’, it is executed as a coprocess; a two-way pipe is established between it and the parent shell. The shell can read from or write to the coprocess by means of the ‘>&p’ and ‘<&p’ redirection operators or with ‘print -p’ and ‘read -p’. A pipeline cannot be preceded by both ‘coproc’ and ‘!’. If job control is active, the coprocess can be treated in other than input and output as an ordinary background job.

A sublist is either a single pipeline, or a sequence of two or more pipelines separated by ‘&&’ or ‘||’. If two pipelines are separated by ‘&&’, the second pipeline is executed only if the first succeeds (returns a zero status). If two pipelines are separated by ‘||’, the second is executed only if the first fails (returns a nonzero status). Both operators have equal precedence and are left associative. The value of the sublist is the value of the last pipeline executed. For example,

 
dmesg | grep panic && print yes

is a sublist consisting of two pipelines, the second just a simple command which will be executed if and only if the grep command returns a zero status. If it does not, the value of the sublist is that return status, else it is the status returned by the print (almost certainly zero).

A list is a sequence of zero or more sublists, in which each sublist is terminated by ‘;’, ‘&’, ‘&|’, ‘&!’, or a newline. This terminator may optionally be omitted from the last sublist in the list when the list appears as a complex command inside ‘(...)’ or ‘{...}’. When a sublist is terminated by ‘;’ or newline, the shell waits for it to finish before executing the next sublist. If a sublist is terminated by a ‘&’, ‘&|’, or ‘&!’, the shell executes the last pipeline in it in the background, and does not wait for it to finish (note the difference from other shells which execute the whole sublist in the background). A backgrounded pipeline returns a status of zero.

More generally, a list can be seen as a set of any shell commands whatsoever, including the complex commands below; this is implied wherever the word ‘list’ appears in later descriptions. For example, the commands in a shell function form a special sort of list.


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6.2 Precommand Modifiers

A simple command may be preceded by a precommand modifier, which will alter how the command is interpreted. These modifiers are shell builtin commands with the exception of nocorrect which is a reserved word.

-

The command is executed with a ‘-’ prepended to its argv[0] string.

builtin

The command word is taken to be the name of a builtin command, rather than a shell function or external command.

command [ -pvV ]

The command word is taken to be the name of an external command, rather than a shell function or builtin. If the POSIX_BUILTINS option is set, builtins will also be executed but certain special properties of them are suppressed. The -p flag causes a default path to be searched instead of that in $path. With the -v flag, command is similar to whence and with -V, it is equivalent to whence -v.

exec [ -cl ] [ -a argv0 ]

The following command together with any arguments is run in place of the current process, rather than as a sub-process. The shell does not fork and is replaced. The shell does not invoke TRAPEXIT, nor does it source zlogout files. The options are provided for compatibility with other shells.

The -c option clears the environment.

The -l option is equivalent to the - precommand modifier, to treat the replacement command as a login shell; the command is executed with a - prepended to its argv[0] string. This flag has no effect if used together with the -a option.

The -a option is used to specify explicitly the argv[0] string (the name of the command as seen by the process itself) to be used by the replacement command and is directly equivalent to setting a value for the ARGV0 environment variable.

nocorrect

Spelling correction is not done on any of the words. This must appear before any other precommand modifier, as it is interpreted immediately, before any parsing is done. It has no effect in non-interactive shells.

noglob

Filename generation (globbing) is not performed on any of the words.


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6.3 Complex Commands

A complex command in zsh is one of the following:

if list then list [ elif list then list ] ... [ else list ] fi

The if list is executed, and if it returns a zero exit status, the then list is executed. Otherwise, the elif list is executed and if its status is zero, the then list is executed. If each elif list returns nonzero status, the else list is executed.

for name ... [ in word ... ] term do list done

where term is at least one newline or ;. Expand the list of words, and set the parameter name to each of them in turn, executing list each time. If the in word is omitted, use the positional parameters instead of the words.

More than one parameter name can appear before the list of words. If N names are given, then on each execution of the loop the next N words are assigned to the corresponding parameters. If there are more names than remaining words, the remaining parameters are each set to the empty string. Execution of the loop ends when there is no remaining word to assign to the first name. It is only possible for in to appear as the first name in the list, else it will be treated as marking the end of the list.

for (( [expr1] ; [expr2] ; [expr3] )) do list done

The arithmetic expression expr1 is evaluated first (see Arithmetic Evaluation). The arithmetic expression expr2 is repeatedly evaluated until it evaluates to zero and when non-zero, list is executed and the arithmetic expression expr3 evaluated. If any expression is omitted, then it behaves as if it evaluated to 1.

while list do list done

Execute the do list as long as the while list returns a zero exit status.

until list do list done

Execute the do list as long as until list returns a nonzero exit status.

repeat word do list done

word is expanded and treated as an arithmetic expression, which must evaluate to a number n. list is then executed n times.

The repeat syntax is disabled by default when the shell starts in a mode emulating another shell. It can be enabled with the command ‘enable -r repeat

case word in [ [(] pattern [ | pattern ] ... ) list (;;|;&|;|) ] ... esac

Execute the list associated with the first pattern that matches word, if any. The form of the patterns is the same as that used for filename generation. See Filename Generation.

If the list that is executed is terminated with ;& rather than ;;, the following list is also executed. The rule for the terminator of the following list ;;, ;& or ;| is applied unless the esac is reached.

If the list that is executed is terminated with ;| the shell continues to scan the patterns looking for the next match, executing the corresponding list, and applying the rule for the corresponding terminator ;;, ;& or ;|. Note that word is not re-expanded; all applicable patterns are tested with the same word.

select name [ in word ... term ] do list done

where term is one or more newline or ; to terminate the words. Print the set of words, each preceded by a number. If the in word is omitted, use the positional parameters. The PROMPT3 prompt is printed and a line is read from the line editor if the shell is interactive and that is active, or else standard input. If this line consists of the number of one of the listed words, then the parameter name is set to the word corresponding to this number. If this line is empty, the selection list is printed again. Otherwise, the value of the parameter name is set to null. The contents of the line read from standard input is saved in the parameter REPLY. list is executed for each selection until a break or end-of-file is encountered.

( list )

Execute list in a subshell. Traps set by the trap builtin are reset to their default values while executing list.

{ list }

Execute list.

{ try-list } always { always-list }

First execute try-list. Regardless of errors, or break, continue, or return commands encountered within try-list, execute always-list. Execution then continues from the result of the execution of try-list; in other words, any error, or break, continue, or return command is treated in the normal way, as if always-list were not present. The two chunks of code are referred to as the ‘try block’ and the ‘always block’.

Optional newlines or semicolons may appear after the always; note, however, that they may not appear between the preceding closing brace and the always.

An ‘error’ in this context is a condition such as a syntax error which causes the shell to abort execution of the current function, script, or list. Syntax errors encountered while the shell is parsing the code do not cause the always-list to be executed. For example, an erroneously constructed if block in try-list would cause the shell to abort during parsing, so that always-list would not be executed, while an erroneous substitution such as ${*foo*} would cause a run-time error, after which always-list would be executed.

An error condition can be tested and reset with the special integer variable TRY_BLOCK_ERROR. Outside an always-list the value is irrelevant, but it is initialised to -1. Inside always-list, the value is 1 if an error occurred in the try-list, else 0. If TRY_BLOCK_ERROR is set to 0 during the always-list, the error condition caused by the try-list is reset, and shell execution continues normally after the end of always-list. Altering the value during the try-list is not useful (unless this forms part of an enclosing always block).

Regardless of TRY_BLOCK_ERROR, after the end of always-list the normal shell status $? is the value returned from always-list. This will be non-zero if there was an error, even if TRY_BLOCK_ERROR was set to zero.

The following executes the given code, ignoring any errors it causes. This is an alternative to the usual convention of protecting code by executing it in a subshell.

 
{
    # code which may cause an error
  } always {
    # This code is executed regardless of the error.
    (( TRY_BLOCK_ERROR = 0 ))
}
# The error condition has been reset.

An exit command (or a return command executed at the outermost function level of a script) encountered in try-list does not cause the execution of always-list. Instead, the shell exits immediately after any EXIT trap has been executed.

function word ... [ () ] [ term ] { list }
word ... () [ term ] { list }
word ... () [ term ] command

where term is one or more newline or ;. Define a function which is referenced by any one of word. Normally, only one word is provided; multiple words are usually only useful for setting traps. The body of the function is the list between the { and }. See Functions.

If the option SH_GLOB is set for compatibility with other shells, then whitespace may appear between between the left and right parentheses when there is a single word; otherwise, the parentheses will be treated as forming a globbing pattern in that case.

time [ pipeline ]

The pipeline is executed, and timing statistics are reported on the standard error in the form specified by the TIMEFMT parameter. If pipeline is omitted, print statistics about the shell process and its children.

[[ exp ]]

Evaluates the conditional expression exp and return a zero exit status if it is true. See Conditional Expressions for a description of exp.


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6.4 Alternate Forms For Complex Commands

Many of zsh’s complex commands have alternate forms. These are non-standard and are likely not to be obvious even to seasoned shell programmers; they should not be used anywhere that portability of shell code is a concern.

The short versions below only work if sublist is of the form ‘{ list }’ or if the SHORT_LOOPS option is set. For the if, while and until commands, in both these cases the test part of the loop must also be suitably delimited, such as by ‘[[ ... ]]’ or ‘(( ... ))’, else the end of the test will not be recognized. For the for, repeat, case and select commands no such special form for the arguments is necessary, but the other condition (the special form of sublist or use of the SHORT_LOOPS option) still applies.

if list { list } [ elif list { list } ] ... [ else { list } ]

An alternate form of if. The rules mean that

 
if [[ -o ignorebraces ]] {
  print yes
}

works, but

 
if true {  # Does not work!
  print yes
}

does not, since the test is not suitably delimited.

if list sublist

A short form of the alternate ‘if’. The same limitations on the form of list apply as for the previous form.

for name ... ( word ... ) sublist

A short form of for.

for name ... [ in word ... ] term sublist

where term is at least one newline or ;. Another short form of for.

for (( [expr1] ; [expr2] ; [expr3] )) sublist

A short form of the arithmetic for command.

foreach name ... ( word ... ) list end

Another form of for.

while list { list }

An alternative form of while. Note the limitations on the form of list mentioned above.

until list { list }

An alternative form of until. Note the limitations on the form of list mentioned above.

repeat word sublist

This is a short form of repeat.

case word { [ [(] pattern [ | pattern ] ... ) list (;;|;&|;|) ] ... }

An alternative form of case.

select name [ in word term ] sublist

where term is at least one newline or ;. A short form of select.


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6.5 Reserved Words

The following words are recognized as reserved words when used as the first word of a command unless quoted or disabled using disable -r:

do done esac then elif else fi for case if while function repeat time until select coproc nocorrect foreach end ! [[ { }

Additionally, ‘}’ is recognized in any position if neither the IGNORE_BRACES option nor the IGNORE_CLOSE_BRACES option is set.


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6.6 Errors

Certain errors are treated as fatal by the shell: in an interactive shell, they cause control to return to the command line, and in a non-interactive shell they cause the shell to be aborted. In older versions of zsh, a non-interactive shell running a script would not abort completely, but would resume execution at the next command to be read from the script, skipping the remainder of any functions or shell constructs such as loops or conditions; this somewhat illogical behaviour can be recovered by setting the option CONTINUE_ON_ERROR.

Fatal errors found in non-interactive shells include:

Failure to parse shell options passed when invoking the shell
Failure to change options with the set builtin
Parse errors of all sorts, including failures to parse

mathematical expressions

Failures to set or modify variable behaviour with typeset,

local, declare, export, integer, float

Execution of incorrectly positioned loop control structures

(continue, break)

Attempts to use regular expression with no regular expression

module available

Disallowed operations when the RESTRICTED options is set
Failure to create a pipe needed for a pipeline
Failure to create a multio
Failure to autoload a module needed for a declared shell feature
Errors creating command or process substitutions
Syntax errors in glob qualifiers
File generation errors where not caught by the option BAD_PATTERN
All bad patterns used for matching within case statements
File generation failures where not caused by NO_MATCH or
All file generation errors where the pattern was used to create a

multio

Memory errors where detected by the shell
Invalid subscripts to shell variables
Attempts to assign read-only variables
Logical errors with variables such as assignment to the wrong type
Use of invalid variable names
Errors in variable substitution syntax
Failure to convert characters in $’... expressions

similar options


If the POSIX_BUILTINS option is set, more errors associated with shell builtin commands are treated as fatal, as specified by the POSIX standard.


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6.7 Comments

In non-interactive shells, or in interactive shells with the INTERACTIVE_COMMENTS option set, a word beginning with the third character of the histchars parameter (‘#’ by default) causes that word and all the following characters up to a newline to be ignored.


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6.8 Aliasing

Every token in the shell input is checked to see if there is an alias defined for it. If so, it is replaced by the text of the alias if it is in command position (if it could be the first word of a simple command), or if the alias is global. If the text ends with a space, the next word in the shell input is treated as though it were in command position for purposes of alias expansion. An alias is defined using the alias builtin; global aliases may be defined using the -g option to that builtin.

Alias expansion is done on the shell input before any other expansion except history expansion. Therefore, if an alias is defined for the word foo, alias expansion may be avoided by quoting part of the word, e.g. \foo. Any form of quoting works, although there is nothing to prevent an alias being defined for the quoted form such as \foo as well. For use with completion, which would remove an initial backslash followed by a character that isn’t special, it may be more convenient to quote the word by starting with a single quote, i.e. ’foo; completion will automatically add the trailing single quote.

There is a commonly encountered problem with aliases illustrated by the following code:

 
alias echobar='echo bar'; echobar

This prints a message that the command echobar could not be found. This happens because aliases are expanded when the code is read in; the entire line is read in one go, so that when echobar is executed it is too late to expand the newly defined alias. This is often a problem in shell scripts, functions, and code executed with ‘source’ or ‘.’. Consequently, use of functions rather than aliases is recommended in non-interactive code.

Note also the unhelpful interaction of aliases and function definitions:

 
alias func='noglob func'
func() {
    echo Do something with $*
}

Because aliases are expanded in function definitions, this causes the following command to be executed:

 
noglob func() {
    echo Do something with $*
}

which defines noglob as well as func as functions with the body given. To avoid this, either quote the name func or use the alternative function definition form ‘function func’. Ensuring the alias is defined after the function works but is problematic if the code fragment might be re-executed.


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6.9 Quoting

A character may be quoted (that is, made to stand for itself) by preceding it with a ‘\’. ‘\’ followed by a newline is ignored.

A string enclosed between ‘$’’ and ‘’ is processed the same way as the string arguments of the print builtin, and the resulting string is considered to be entirely quoted. A literal ‘’ character can be included in the string by using the ‘\’’ escape.

All characters enclosed between a pair of single quotes (’’) that is not preceded by a ‘$’ are quoted. A single quote cannot appear within single quotes unless the option RC_QUOTES is set, in which case a pair of single quotes are turned into a single quote. For example,

 
print ''''

outputs nothing apart from a newline if RC_QUOTES is not set, but one single quote if it is set.

Inside double quotes (""), parameter and command substitution occur, and ‘\’ quotes the characters ‘\’, ‘’, ‘"’, and ‘$’.


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This document was generated by Simon Ruderich on January 19, 2014 using texi2html 1.82.
Zsh version 5.0.5, released on January 5, 2014.