NAME

SIMPLE COMMANDS & PIPELINES

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.

PRECOMMAND MODIFIERS

-

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.

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.

COMPLEX COMMANDS

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

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.

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

The arithmetic expression expr1 is evaluated first (see the section `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.

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 the section `Filename Generation'.

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 or continue 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 or continue 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'.

{
    # 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.

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 the section `Functions'.

func() { ... } 2>&1

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 the section `Conditional Expressions' for a description of exp.

ALTERNATE FORMS FOR COMPLEX COMMANDS

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
}

if true {  # Does not work!
  print yes
}

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.

function word ... [ () ] [ term ] sublist

This is a short form of function.

RESERVED WORDS

do done esac then elif else fi for case if while function repeat time until select coproc nocorrect foreach end ! [[ { } declare export float integer local readonly typeset

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

ERRORS

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 similar options

•

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

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

COMMENTS

ALIASING

A word is defined as:

•

Any plain string or glob pattern

•

Any quoted string, using any quoting method (note that the quotes must be part of the alias definition for this to be eligible)

•

Any parameter reference or command substitution

•

Any series of the foregoing, concatenated without whitespace or other tokens between them

•

Any reserved word (case, do, else, etc.)

•

With global aliasing, any command separator, any redirection operator, and `(' or `)' when not part of a glob pattern

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.

When POSIX_ALIASES is set, only plain unquoted strings are eligible for aliasing. The alias builtin does not reject ineligible aliases, but they are not expanded.

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.

alias echobar='echo bar'; echobar

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

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

QUOTING

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 the first character of $histchars (default `!').

REDIRECTION

The following may appear anywhere in a simple command or may precede or follow a complex command. Expansion occurs before word or digit is used except as noted below. If the result of substitution on word produces more than one filename, redirection occurs for each separate filename in turn.

< word

Open file word for reading as standard input. It is an error to open a file in this fashion if it does not exist.

<> word

Open file word for reading and writing as standard input. If the file does not exist then it is created.

> word

Open file word for writing as standard output. If the file does not exist then it is created. If the file exists, and the CLOBBER option is unset, this causes an error; otherwise, it is truncated to zero length.

>| word

>! word

Same as >, except that the file is truncated to zero length if it exists, regardless of CLOBBER.

>> word

Open file word for writing in append mode as standard output. If the file does not exist, and the CLOBBER and APPEND_CREATE options are both unset, this causes an error; otherwise, the file is created.

>>| word

>>! word

Same as >>, except that the file is created if it does not exist, regardless of CLOBBER and APPEND_CREATE.

<<[-] word

The shell input is read up to a line that is the same as word, or to an end-of-file. No parameter expansion, command substitution or filename generation is performed on word. The resulting document, called a here-document, becomes the standard input.

<<< word

Perform shell expansion on word and pass the result to standard input. This is known as a here-string. Compare the use of word in here-documents above, where word does not undergo shell expansion.

<& number

>& number

The standard input/output is duplicated from file descriptor number (see dup2(2)).

<& -

>& -

Close the standard input/output.

<& p

>& p

The input/output from/to the coprocess is moved to the standard input/output.

>& word

&> word

(Except where `>& word' matches one of the above syntaxes; `&>' can always be used to avoid this ambiguity.) Redirects both standard output and standard error (file descriptor 2) in the manner of `> word'. Note that this does not have the same effect as `> word 2>&1' in the presence of multios (see the section below).

>&| word

>&! word

&>| word

&>! word

Redirects both standard output and standard error (file descriptor 2) in the manner of `>| word'.

>>& word

&>> word

Redirects both standard output and standard error (file descriptor 2) in the manner of `>> word'.

>>&| word

>>&! word

&>>| word

&>>! word

Redirects both standard output and standard error (file descriptor 2) in the manner of `>>| word'.

If one of the above is preceded by a digit, then the file descriptor referred to is that specified by the digit instead of the default 0 or 1. The order in which redirections are specified is significant. The shell evaluates each redirection in terms of the (file descriptor, file) association at the time of evaluation. For example:

... 1>fname 2>&1

first associates file descriptor 1 with file fname. It then associates file descriptor 2 with the file associated with file descriptor 1 (that is, fname). If the order of redirections were reversed, file descriptor 2 would be associated with the terminal (assuming file descriptor 1 had been) and then file descriptor 1 would be associated with file fname.

The `|&' command separator described in Simple Commands & Pipelines in zshmisc(1) is a shorthand for `2>&1 |'.

The various forms of process substitution, `<(list)', and `=(list)' for input and `>(list)' for output, are often used together with redirection. For example, if word in an output redirection is of the form `>(list)' then the output is piped to the command represented by list. See Process Substitution in zshexpn(1).

OPENING FILE DESCRIPTORS USING PARAMETERS

... {myfd}>&1

This opens a new file descriptor that is a duplicate of file descriptor 1 and sets the parameter myfd to the number of the file descriptor, which will be at least 10. The new file descriptor can be written to using the syntax >&$myfd. The file descriptor remains open in subshells and forked external executables.

The syntax {varid}>&-, for example {myfd}>&-, may be used to close a file descriptor opened in this fashion. Note that the parameter given by varid must previously be set to a file descriptor in this case.

It is an error to open or close a file descriptor in this fashion when the parameter is readonly. However, it is not an error to read or write a file descriptor using <&$param or >&$param if param is readonly.

If the option CLOBBER is unset, it is an error to open a file descriptor using a parameter that is already set to an open file descriptor previously allocated by this mechanism. Unsetting the parameter before using it for allocating a file descriptor avoids the error.

Note that this mechanism merely allocates or closes a file descriptor; it does not perform any redirections from or to it. It is usually convenient to allocate a file descriptor prior to use as an argument to exec. The syntax does not in any case work when used around complex commands such as parenthesised subshells or loops, where the opening brace is interpreted as part of a command list to be executed in the current shell.

The following shows a typical sequence of allocation, use, and closing of a file descriptor:

integer myfd
exec {myfd}>~/logs/mylogfile.txt
print This is a log message. >&$myfd
exec {myfd}>&-

Note that the expansion of the variable in the expression >&$myfd occurs at the point the redirection is opened. This is after the expansion of command arguments and after any redirections to the left on the command line have been processed.

MULTIOS

date >foo >bar

writes the date to two files, named `foo' and `bar'. Note that a pipe is an implicit redirection; thus

date >foo | cat

writes the date to the file `foo', and also pipes it to cat.

Note that the shell opens all the files to be used in the multio process immediately, not at the point they are about to be written.

Note also that redirections are always expanded in order. This happens regardless of the setting of the MULTIOS option, but with the option in effect there are additional consequences. For example, the meaning of the expression >&1 will change after a previous redirection:

date >&1 >output

In the case above, the >&1 refers to the standard output at the start of the line; the result is similar to the tee command. However, consider:

date >output >&1

As redirections are evaluated in order, when the >&1 is encountered the standard output is set to the file output and another copy of the output is therefore sent to that file. This is unlikely to be what is intended.

If the MULTIOS option is set, the word after a redirection operator is also subjected to filename generation (globbing). Thus

: > *

will truncate all files in the current directory, assuming there's at least one. (Without the MULTIOS option, it would create an empty file called `*'.) Similarly, you can do

echo exit 0 >> *.sh

If the user tries to open a file descriptor for reading more than once, the shell opens the file descriptor as a pipe to a process that copies all the specified inputs to its output in the order specified, provided the MULTIOS option is set. It should be noted that each file is opened immediately, not at the point where it is about to be read: this behaviour differs from cat, so if strictly standard behaviour is needed, cat should be used instead.

Thus

sort <foo <fubar

or even

sort <f{oo,ubar}

is equivalent to `cat foo fubar | sort'.

Expansion of the redirection argument occurs at the point the redirection is opened, at the point described above for the expansion of the variable in >&$myfd.

Note that a pipe is an implicit redirection; thus

cat bar | sort <foo

is equivalent to `cat bar foo | sort' (note the order of the inputs).

If the MULTIOS option is unset, each redirection replaces the previous redirection for that file descriptor. However, all files redirected to are actually opened, so

echo Hello > bar > baz

when MULTIOS is unset will truncate `bar', and write `Hello' into `baz'.

There is a problem when an output multio is attached to an external program. A simple example shows this:

cat file >file1 >file2
cat file1 file2

Here, it is possible that the second `cat' will not display the full contents of file1 and file2 (i.e. the original contents of file repeated twice).

The reason for this is that the multios are spawned after the cat process is forked from the parent shell, so the parent shell does not wait for the multios to finish writing data. This means the command as shown can exit before file1 and file2 are completely written. As a workaround, it is possible to run the cat process as part of a job in the current shell:

{ cat file } >file >file2

Here, the {...} job will pause to wait for both files to be written.

REDIRECTIONS WITH NO COMMAND

If the parameter NULLCMD is not set or the option CSH_NULLCMD is set, an error is caused. This is the csh behavior and CSH_NULLCMD is set by default when emulating csh.

If the option SH_NULLCMD is set, the builtin `:' is inserted as a command with the given redirections. This is the default when emulating sh or ksh.

Otherwise, if the parameter NULLCMD is set, its value will be used as a command with the given redirections. If both NULLCMD and READNULLCMD are set, then the value of the latter will be used instead of that of the former when the redirection is an input. The default for NULLCMD is `cat' and for READNULLCMD is `more'. Thus

< file

shows the contents of file on standard output, with paging if that is a terminal. NULLCMD and READNULLCMD may refer to shell functions.

COMMAND EXECUTION

Otherwise, the shell searches each element of $path for a directory containing an executable file by that name. If the search is unsuccessful, the shell prints an error message and returns a nonzero exit status.

If execution fails because the file is not in executable format, and the file is not a directory, it is assumed to be a shell script. /bin/sh is spawned to execute it. If the program is a file beginning with `#!', the remainder of the first line specifies an interpreter for the program. The shell will execute the specified interpreter on operating systems that do not handle this executable format in the kernel.

If no external command is found but a function command_not_found_handler exists the shell executes this function with all command line arguments. The return status of the function becomes the status of the command. If the function wishes to mimic the behaviour of the shell when the command is not found, it should print the message `command not found: cmd' to standard error and return status 127. Note that the handler is executed in a subshell forked to execute an external command, hence changes to directories, shell parameters, etc. have no effect on the main shell.

FUNCTIONS

Functions execute in the same process as the caller and share all files and present working directory with the caller. A trap on EXIT set inside a function is executed after the function completes in the environment of the caller.

The return builtin is used to return from function calls.

Function identifiers can be listed with the functions builtin. Functions can be undefined with the unfunction builtin.

AUTOLOADING FUNCTIONS

fpath=(~/myfuncs $fpath)
autoload myfunc1 myfunc2 ...

The usual alias expansion during reading will be suppressed if the autoload builtin or its equivalent is given the option -U. This is recommended for the use of functions supplied with the zsh distribution. Note that for functions precompiled with the zcompile builtin command the flag -U must be provided when the .zwc file is created, as the corresponding information is compiled into the latter.

For each element in fpath, the shell looks for three possible files, the newest of which is used to load the definition for the function:

element.zwc

A file created with the zcompile builtin command, which is expected to contain the definitions for all functions in the directory named element. The file is treated in the same manner as a directory containing files for functions and is searched for the definition of the function. If the definition is not found, the search for a definition proceeds with the other two possibilities described below.

element/function.zwc

A file created with zcompile, which is expected to contain the definition for function. It may include other function definitions as well, but those are neither loaded nor executed; a file found in this way is searched only for the definition of function.

element/function

A file of zsh command text, taken to be the definition for function.

In summary, the order of searching is, first, in the parents of directories in fpath for the newer of either a compiled directory or a directory in fpath; second, if more than one of these contains a definition for the function that is sought, the leftmost in the fpath is chosen; and third, within a directory, the newer of either a compiled function or an ordinary function definition is used.

If the KSH_AUTOLOAD option is set, or the file contains only a simple definition of the function, the file's contents will be executed. This will normally define the function in question, but may also perform initialization, which is executed in the context of the function execution, and may therefore define local parameters. It is an error if the function is not defined by loading the file.

Otherwise, the function body (with no surrounding `funcname() {...}') is taken to be the complete contents of the file. This form allows the file to be used directly as an executable shell script. If processing of the file results in the function being re-defined, the function itself is not re-executed. To force the shell to perform initialization and then call the function defined, the file should contain initialization code (which will be executed then discarded) in addition to a complete function definition (which will be retained for subsequent calls to the function), and a call to the shell function, including any arguments, at the end.

For example, suppose the autoload file func contains

func() { print This is func; }
print func is initialized

then `func; func' with KSH_AUTOLOAD set will produce both messages on the first call, but only the message `This is func' on the second and subsequent calls. Without KSH_AUTOLOAD set, it will produce the initialization message on the first call, and the other message on the second and subsequent calls.

It is also possible to create a function that is not marked as autoloaded, but which loads its own definition by searching fpath, by using `autoload -X' within a shell function. For example, the following are equivalent:

myfunc() {
  autoload -X
}
myfunc args...

and

unfunction myfunc   # if myfunc was defined
autoload myfunc
myfunc args...

In fact, the functions command outputs `builtin autoload -X' as the body of an autoloaded function. This is done so that

eval "$(functions)"

produces a reasonable result. A true autoloaded function can be identified by the presence of the comment `# undefined' in the body, because all comments are discarded from defined functions.

To load the definition of an autoloaded function myfunc without executing myfunc, use:

autoload +X myfunc

ANONYMOUS FUNCTIONS

Arguments to the function may be specified as words following the closing brace defining the function, hence if there are none no arguments (other than $0) are set. This is a difference from the way other functions are parsed: normal function definitions may be followed by certain keywords such as `else' or `fi', which will be treated as arguments to anonymous functions, so that a newline or semicolon is needed to force keyword interpretation.

Note also that the argument list of any enclosing script or function is hidden (as would be the case for any other function called at this point).

Redirections may be applied to the anonymous function in the same manner as to a current-shell structure enclosed in braces. The main use of anonymous functions is to provide a scope for local variables. This is particularly convenient in start-up files as these do not provide their own local variable scope.

For example,

variable=outside
function {
  local variable=inside
  print "I am $variable with arguments $*"
} this and that
print "I am $variable"

outputs the following:

I am inside with arguments this and that
I am outside

Note that function definitions with arguments that expand to nothing, for example `name=; function $name { ... }', are not treated as anonymous functions. Instead, they are treated as normal function definitions where the definition is silently discarded.

SPECIAL FUNCTIONS

zshaddhistory() {
  print -sr -- ${1%%$'\n'}
  fc -p .zsh_local_history
}

TRAPINT() {
  print "Caught SIGINT, aborting."
  return $(( 128 + $1 ))
}

TRAPNAL() { 
 # code
}

trap '
 # code
' NAL

JOBS

[1] 1234

indicating that the job which was started asynchronously was job number 1 and had one (top-level) process, whose process ID was 1234.

If a job is started with `&|' or `&!', then that job is immediately disowned. After startup, it does not have a place in the job table, and is not subject to the job control features described here.

If you are running a job and wish to do something else you may hit the key ^Z (control-Z) which sends a TSTP signal to the current job: this key may be redefined by the susp option of the external stty command. The shell will then normally indicate that the job has been `suspended', and print another prompt. You can then manipulate the state of this job, putting it in the background with the bg command, or run some other commands and then eventually bring the job back into the foreground with the foreground command fg. A ^Z takes effect immediately and is like an interrupt in that pending output and unread input are discarded when it is typed.

A job being run in the background will suspend if it tries to read from the terminal.

Note that if the job running in the foreground is a shell function, then suspending it will have the effect of causing the shell to fork. This is necessary to separate the function's state from that of the parent shell performing the job control, so that the latter can return to the command line prompt. As a result, even if fg is used to continue the job the function will no longer be part of the parent shell, and any variables set by the function will not be visible in the parent shell. Thus the behaviour is different from the case where the function was never suspended. Zsh is different from many other shells in this regard.

One additional side effect is that use of disown with a job created by suspending shell code in this fashion is delayed: the job can only be disowned once any process started from the parent shell has terminated. At that point, the disowned job disappears silently from the job list.

The same behaviour is found when the shell is executing code as the right hand side of a pipeline or any complex shell construct such as if, for, etc., in order that the entire block of code can be managed as a single job. Background jobs are normally allowed to produce output, but this can be disabled by giving the command `stty tostop'. If you set this tty option, then background jobs will suspend when they try to produce output like they do when they try to read input.

When a command is suspended and continued later with the fg or wait builtins, zsh restores tty modes that were in effect when it was suspended. This (intentionally) does not apply if the command is continued via `kill -CONT', nor when it is continued with bg.

There are several ways to refer to jobs in the shell. A job can be referred to by the process ID of any process of the job or by one of the following:

%number

The job with the given number.

%string

The last job whose command line begins with string.

%?string

The last job whose command line contains string.

%%

Current job.

%+

Equivalent to `%%'.

%-

Previous job.

The shell learns immediately whenever a process changes state. It normally informs you whenever a job becomes blocked so that no further progress is possible. If the NOTIFY option is not set, it waits until just before it prints a prompt before it informs you. All such notifications are sent directly to the terminal, not to the standard output or standard error.

When the monitor mode is on, each background job that completes triggers any trap set for CHLD.

When you try to leave the shell while jobs are running or suspended, you will be warned that `You have suspended (running) jobs'. You may use the jobs command to see what they are. If you do this or immediately try to exit again, the shell will not warn you a second time; the suspended jobs will be terminated, and the running jobs will be sent a SIGHUP signal, if the HUP option is set.

To avoid having the shell terminate the running jobs, either use the nohup command (see nohup(1)) or the disown builtin.

SIGNALS

Certain jobs are run asynchronously by the shell other than those explicitly put into the background; even in cases where the shell would usually wait for such jobs, an explicit exit command or exit due to the option ERR_EXIT will cause the shell to exit without waiting. Examples of such asynchronous jobs are process substitution, see the section PROCESS SUBSTITUTION in the zshexpn(1) manual page, and the handler processes for multios, see the section MULTIOS in the zshmisc(1) manual page.

ARITHMETIC EVALUATION

The let builtin command takes arithmetic expressions as arguments; each is evaluated separately. Since many of the arithmetic operators, as well as spaces, require quoting, an alternative form is provided: for any command which begins with a `((', all the characters until a matching `))' are treated as a quoted expression and arithmetic expansion performed as for an argument of let. More precisely, `((...))' is equivalent to `let "..."'. The return status is 0 if the arithmetic value of the expression is non-zero, 1 if it is zero, and 2 if an error occurred.

For example, the following statement

(( val = 2 + 1 ))

is equivalent to

let "val = 2 + 1"

both assigning the value 3 to the shell variable val and returning a zero status.

Integers can be in bases other than 10. A leading `0x' or `0X' denotes hexadecimal and a leading `0b' or `0B' binary. Integers may also be of the form `base#n', where base is a decimal number between two and thirty-six representing the arithmetic base and n is a number in that base (for example, `16#ff' is 255 in hexadecimal). The base# may also be omitted, in which case base 10 is used. For backwards compatibility the form `[base]n' is also accepted.

An integer expression or a base given in the form `base#n' may contain underscores (`_') after the leading digit for visual guidance; these are ignored in computation. Examples are 1_000_000 or 0xffff_ffff which are equivalent to 1000000 and 0xffffffff respectively.

It is also possible to specify a base to be used for output in the form `[#base]', for example `[#16]'. This is used when outputting arithmetical substitutions or when assigning to scalar parameters, but an explicitly defined integer or floating point parameter will not be affected. If an integer variable is implicitly defined by an arithmetic expression, any base specified in this way will be set as the variable's output arithmetic base as if the option `-i base' to the typeset builtin had been used. The expression has no precedence and if it occurs more than once in a mathematical expression, the last encountered is used. For clarity it is recommended that it appear at the beginning of an expression. As an example:

typeset -i 16 y
print $(( [#8] x = 32, y = 32 ))
print $x $y

outputs first `8#40', the rightmost value in the given output base, and then `8#40 16#20', because y has been explicitly declared to have output base 16, while x (assuming it does not already exist) is implicitly typed by the arithmetic evaluation, where it acquires the output base 8.

The base may be replaced or followed by an underscore, which may itself be followed by a positive integer (if it is missing the value 3 is used). This indicates that underscores should be inserted into the output string, grouping the number for visual clarity. The following integer specifies the number of digits to group together. For example:

setopt cbases
print $(( [#16_4] 65536 ** 2 ))

outputs `0x1_0000_0000'.

The feature can be used with floating point numbers, in which case the base must be omitted; grouping is away from the decimal point. For example,

zmodload zsh/mathfunc
print $(( [#_] sqrt(1e7) ))

outputs `3_162.277_660_168_379_5' (the number of decimal places shown may vary).

If the C_BASES option is set, hexadecimal numbers are output in the standard C format, for example `0xFF' instead of the usual `16#FF'. If the option OCTAL_ZEROES is also set (it is not by default), octal numbers will be treated similarly and hence appear as `077' instead of `8#77'. This option has no effect on the output of bases other than hexadecimal and octal, and these formats are always understood on input.

When an output base is specified using the `[#base]' syntax, an appropriate base prefix will be output if necessary, so that the value output is valid syntax for input. If the # is doubled, for example `[##16]', then no base prefix is output.

Floating point constants are recognized by the presence of a decimal point or an exponent. The decimal point may be the first character of the constant, but the exponent character e or E may not, as it will be taken for a parameter name. All numeric parts (before and after the decimal point and in the exponent) may contain underscores after the leading digit for visual guidance; these are ignored in computation.

An arithmetic expression uses nearly the same syntax and associativity of expressions as in C.

In the native mode of operation, the following operators are supported (listed in decreasing order of precedence):

+ - ! ~ ++ --

unary plus/minus, logical NOT, complement, {pre,post}{in,de}crement

<< >>

bitwise shift left, right

&

bitwise AND

^

bitwise XOR

|

bitwise OR

**

exponentiation

* / %

multiplication, division, modulus (remainder)

+ -

addition, subtraction

< > <= >=

comparison

== !=

equality and inequality

&&

logical AND

|| ^^

logical OR, XOR

? :

ternary operator

= += -= *= /= %= &= ^= |= <<= >>= &&= ||= ^^= **=

assignment

,

comma operator

The operators `&&', `||', `&&=', and `||=' are short-circuiting, and only one of the latter two expressions in a ternary operator is evaluated. Note the precedence of the bitwise AND, OR, and XOR operators.

With the option C_PRECEDENCES the precedences (but no other properties) of the operators are altered to be the same as those in most other languages that support the relevant operators:

+ - ! ~ ++ --

unary plus/minus, logical NOT, complement, {pre,post}{in,de}crement

**

exponentiation

* / %

multiplication, division, modulus (remainder)

+ -

addition, subtraction

<< >>

bitwise shift left, right

< > <= >=

comparison

== !=

equality and inequality

&

bitwise AND

^

bitwise XOR

|

bitwise OR

&&

logical AND

^^

logical XOR

||

logical OR

? :

ternary operator

= += -= *= /= %= &= ^= |= <<= >>= &&= ||= ^^= **=

assignment

,

comma operator

Note the precedence of exponentiation in both cases is below that of unary operators, hence `-3**2' evaluates as `9', not `-9'. Use parentheses where necessary: `-(3**2)'. This is for compatibility with other shells.

Mathematical functions can be called with the syntax `func(args)', where the function decides if the args is used as a string or a comma-separated list of arithmetic expressions. The shell currently defines no mathematical functions by default, but the module zsh/mathfunc may be loaded with the zmodload builtin to provide standard floating point mathematical functions.

An expression of the form `##x' where x is any character sequence such as `a', `^A', or `\M-\C-x' gives the value of this character and an expression of the form `#name' gives the value of the first character of the contents of the parameter name. Character values are according to the character set used in the current locale; for multibyte character handling the option MULTIBYTE must be set. Note that this form is different from `$#name', a standard parameter substitution which gives the length of the parameter name. `#\' is accepted instead of `##', but its use is deprecated.

Named parameters and subscripted arrays can be referenced by name within an arithmetic expression without using the parameter expansion syntax. For example,

((val2 = val1 * 2))

assigns twice the value of $val1 to the parameter named val2.

An internal integer representation of a named parameter can be specified with the integer builtin. Arithmetic evaluation is performed on the value of each assignment to a named parameter declared integer in this manner. Assigning a floating point number to an integer results in rounding towards zero.

Likewise, floating point numbers can be declared with the float builtin; there are two types, differing only in their output format, as described for the typeset builtin. The output format can be bypassed by using arithmetic substitution instead of the parameter substitution, i.e. `${float}' uses the defined format, but `$((float))' uses a generic floating point format.

Promotion of integer to floating point values is performed where necessary. In addition, if any operator which requires an integer (`&', `|', `^', `<<', `>>' and their equivalents with assignment) is given a floating point argument, it will be silently rounded towards zero except for `~' which rounds down.

Users should beware that, in common with many other programming languages but not software designed for calculation, the evaluation of an expression in zsh is taken a term at a time and promotion of integers to floating point does not occur in terms only containing integers. A typical result of this is that a division such as 6/8 is truncated, in this being rounded towards 0. The FORCE_FLOAT shell option can be used in scripts or functions where floating point evaluation is required throughout.

Scalar variables can hold integer or floating point values at different times; there is no memory of the numeric type in this case.

If a variable is first assigned in a numeric context without previously being declared, it will be implicitly typed as integer or float and retain that type either until the type is explicitly changed or until the end of the scope. This can have unforeseen consequences. For example, in the loop

for (( f = 0; f < 1; f += 0.1 )); do
# use $f
done

if f has not already been declared, the first assignment will cause it to be created as an integer, and consequently the operation `f += 0.1' will always cause the result to be truncated to zero, so that the loop will fail. A simple fix would be to turn the initialization into `f = 0.0'. It is therefore best to declare numeric variables with explicit types.

CONDITIONAL EXPRESSIONS

-a file

true if file exists.

-b file

true if file exists and is a block special file.

-c file

true if file exists and is a character special file.

-d file

true if file exists and is a directory.

-e file

true if file exists.

-f file

true if file exists and is a regular file.

-g file

true if file exists and has its setgid bit set.

-h file

true if file exists and is a symbolic link.

-k file

true if file exists and has its sticky bit set.

-n string

true if length of string is non-zero.

-o option

true if option named option is on. option may be a single character, in which case it is a single letter option name. (See the section `Specifying Options'.)

-p file

true if file exists and is a FIFO special file (named pipe).

-r file

true if file exists and is readable by current process.

-s file

true if file exists and has size greater than zero.

-t fd

true if file descriptor number fd is open and associated with a terminal device. (note: fd is not optional)

-u file

true if file exists and has its setuid bit set.

-v varname

true if shell variable varname is set.

-w file

true if file exists and is writable by current process.

-x file

true if file exists and is executable by current process. If file exists and is a directory, then the current process has permission to search in the directory.

-z string

true if length of string is zero.

-L file

true if file exists and is a symbolic link.

-O file

true if file exists and is owned by the effective user ID of this process.

-G file

true if file exists and its group matches the effective group ID of this process.

-S file

true if file exists and is a socket.

-N file

true if file exists and its access time is not newer than its modification time.

file1 -nt file2

true if file1 exists and is newer than file2.

file1 -ot file2

true if file1 exists and is older than file2.

file1 -ef file2

true if file1 and file2 exist and refer to the same file.

string = pattern

string == pattern

true if string matches pattern. The two forms are exactly equivalent. The `=' form is the traditional shell syntax (and hence the only one generally used with the test and [ builtins); the `==' form provides compatibility with other sorts of computer language.

string != pattern

true if string does not match pattern.

string =~ regexp

true if string matches the regular expression regexp. If the option RE_MATCH_PCRE is set regexp is tested as a PCRE regular expression using the zsh/pcre module, else it is tested as a POSIX extended regular expression using the zsh/regex module. Upon successful match, some variables will be updated; no variables are changed if the matching fails.

string1 < string2

true if string1 comes before string2 based on ASCII value of their characters.

string1 > string2

true if string1 comes after string2 based on ASCII value of their characters.

exp1 -eq exp2

true if exp1 is numerically equal to exp2. Note that for purely numeric comparisons use of the ((...)) builtin described in the section `ARITHMETIC EVALUATION' is more convenient than conditional expressions.

exp1 -ne exp2

true if exp1 is numerically not equal to exp2.

exp1 -lt exp2

true if exp1 is numerically less than exp2.

exp1 -gt exp2

true if exp1 is numerically greater than exp2.

exp1 -le exp2

true if exp1 is numerically less than or equal to exp2.

exp1 -ge exp2

true if exp1 is numerically greater than or equal to exp2.

( exp )

true if exp is true.

! exp

true if exp is false.

exp1 && exp2

true if exp1 and exp2 are both true.

exp1 || exp2

true if either exp1 or exp2 is true.

For compatibility, if there is a single argument that is not syntactically significant, typically a variable, the condition is treated as a test for whether the expression expands as a string of non-zero length. In other words, [[ $var ]] is the same as [[ -n $var ]]. It is recommended that the second, explicit, form be used where possible.

Normal shell expansion is performed on the file, string and pattern arguments, but the result of each expansion is constrained to be a single word, similar to the effect of double quotes.

Filename generation is not performed on any form of argument to conditions. However, it can be forced in any case where normal shell expansion is valid and when the option EXTENDED_GLOB is in effect by using an explicit glob qualifier of the form (#q) at the end of the string. A normal glob qualifier expression may appear between the `q' and the closing parenthesis; if none appears the expression has no effect beyond causing filename generation. The results of filename generation are joined together to form a single word, as with the results of other forms of expansion.

This special use of filename generation is only available with the [[ syntax. If the condition occurs within the [ or test builtin commands then globbing occurs instead as part of normal command line expansion before the condition is evaluated. In this case it may generate multiple words which are likely to confuse the syntax of the test command.

For example,

[[ -n file*(#qN) ]]

produces status zero if and only if there is at least one file in the current directory beginning with the string `file'. The globbing qualifier N ensures that the expression is empty if there is no matching file.

Pattern metacharacters are active for the pattern arguments; the patterns are the same as those used for filename generation, see zshexpn(1), but there is no special behaviour of `/' nor initial dots, and no glob qualifiers are allowed.

In each of the above expressions, if file is of the form `/dev/fd/n', where n is an integer, then the test applied to the open file whose descriptor number is n, even if the underlying system does not support the /dev/fd directory.

In the forms which do numeric comparison, the expressions exp undergo arithmetic expansion as if they were enclosed in $((...)).

For example, the following:

[[ ( -f foo || -f bar ) && $report = y* ]] && print File exists.

tests if either file foo or file bar exists, and if so, if the value of the parameter report begins with `y'; if the complete condition is true, the message `File exists.' is printed.

EXPANSION OF PROMPT SEQUENCES

If the PROMPT_SUBST option is set, the prompt string is first subjected to parameter expansion, command substitution and arithmetic expansion. See zshexpn(1).

Certain escape sequences may be recognised in the prompt string.

If the PROMPT_BANG option is set, a `!' in the prompt is replaced by the current history event number. A literal `!' may then be represented as `!!'.

If the PROMPT_PERCENT option is set, certain escape sequences that start with `%' are expanded. Many escapes are followed by a single character, although some of these take an optional integer argument that should appear between the `%' and the next character of the sequence. More complicated escape sequences are available to provide conditional expansion.

SIMPLE PROMPT ESCAPES

CONDITIONAL SUBSTRINGS IN PROMPTS

%v

The value of the first element of the psvar array parameter. Following the `%' with an integer gives that element of the array. Negative integers count from the end of the array.

%(x.true-text.false-text)

Specifies a ternary expression. The character following the x is arbitrary; the same character is used to separate the text for the `true' result from that for the `false' result. This separator may not appear in the true-text, except as part of a %-escape sequence. A `)' may appear in the false-text as `%)'. true-text and false-text may both contain arbitrarily-nested escape sequences, including further ternary expressions.

%<string<

%>string>

%[xstring]

Specifies truncation behaviour for the remainder of the prompt string. The third, deprecated, form is equivalent to `%xstringx', i.e. x may be `<' or `>'. The string will be displayed in place of the truncated portion of any string; note this does not undergo prompt expansion.