ZSHMISC(1)

NAME
zshmisc - everything and then some

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 value). If two pipelines are separated by `||', the second is executed only if the first fails (returns a nonzero value). 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 value. If it does not, the value of the sublist is that return value, else it is the value 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
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.

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

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.

exec
    The command is executed in the parent shell without forking.

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

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

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 value is zero, the then list is executed. If each elif list returns nonzero, 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 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'. If the list that is executed is terminated with ;& rather than ;;, the following list is also executed. This continues until either a list is terminated with ;; or the esac is reached.

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.

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'.

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

ALTERNATE FORMS FOR COMPLEX COMMANDS
Many of zsh's complex commands have alternate forms. These particular versions of complex commands should be considered deprecated and may be removed in the future. The versions in the previous section should be preferred instead.

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.

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 the IGNORE_BRACES option is not set.

COMMENTS
In noninteractive 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.

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. But there is nothing to prevent an alias being defined for \foo as well.

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 `$'.

REDIRECTION
If a command is followed by & and job control is not active, then the default standard input for the command is the empty file /dev/null. Otherwise, the environment for the execution of a command contains the file descriptors of the invoking shell as modified by input/output specifications.

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.

<> 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, even if CLOBBER is unset.

>> word
    Open file word for writing in append mode as standard output. If the file does not exist, and the CLOBBER option is 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, even if CLOBBER is unset.

<<[-] 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.

    If any character of word is quoted with single or double quotes or a `\', no interpretation is placed upon the characters of the document. Otherwise, parameter and command substitution occurs, `\' followed by a newline is removed, and `\' must be used to quote the characters `\', `$', ``' and the first character of word.

    If <<- is used, then all leading tabs are stripped from word and from the document.

<<< word
    Perform shell expansion on word and pass the result to standard input. This is known as a here-string.

<& 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 |'.

For output redirections only, if word is of the form `>(list)' then the output is piped to the command represented by list. See Process Substitution in zshexpn(1).

MULTIOS
If the user tries to open a file descriptor for writing more than once, the shell opens the file descriptor as a pipe to a process that copies its input to all the specified outputs, similar to tee, provided the MULTIOS option is set, as it is by default. Thus:

    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.

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, similar to cat, provided the MULTIOS option is set. Thus

    sort <foo <fubar

or even

    sort <f{oo,ubar}

is equivalent to `cat foo fubar | sort'.

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 foo > bar > baz

when MULTIOS is unset will truncate bar, and write `foo' 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
When a simple command consists of one or more redirection operators and zero or more parameter assignments, but no command name, zsh can behave in several ways.

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
If a command name contains no slashes, the shell attempts to locate it. If there exists a shell function by that name, the function is invoked as described in the section `Functions'. If there exists a shell builtin by that name, the builtin is invoked.

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.

FUNCTIONS
Shell functions are defined with the function reserved word or the special syntax `funcname ()'. Shell functions are read in and stored internally. Alias names are resolved when the function is read. Functions are executed like commands with the arguments passed as positional parameters. (See the section `Command Execution'.)

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

A function can be marked as undefined using the autoload builtin (or `functions -u' or `typeset -fu'). Such a function has no body. When the function is first executed, the shell searches for its definition using the elements of the fpath variable. Thus to define functions for autoloading, a typical sequence is:

    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.

    If element already includes a .zwc extension (i.e. the extension was explicitly given by the user), element is searched for the definition of the function without comparing its age to that of other files; in fact, there does not need to be any directory named element without the suffix. Thus including an element such as `/usr/local/funcs.zwc' in fpath will speed up the search for functions, with the disadvantage that functions included must be explicitly recompiled by hand before the shell notices any changes.

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

SPECIAL FUNCTIONS
The following functions, if defined, have special meaning to the shell:

chpwd
    Executed whenever the current working directory is changed.

periodic
    If the parameter PERIOD is set, this function is executed every $PERIOD seconds, just before a prompt.

precmd
    Executed before each prompt.

preexec
    Executed just after a command has been read and is about to be executed. If the history mechanism is active (and the line was not discarded from the history buffer), the string that the user typed is passed as the first argument, otherwise it is an empty string. The actual command that will be executed (including expanded aliases) is passed in two different forms: the second argument is a single-line, size-limited version of the command (with things like function bodies elided); the third argument contains the full text that is being executed.

TRAPNAL
    If defined and non-null, this function will be executed whenever the shell catches a signal SIGNAL, where NAL is a signal name as specified for the kill builtin. The signal number will be passed as the first parameter to the function.

    If a function of this form is defined and null, the shell and processes spawned by it will ignore SIGNAL.

TRAPDEBUG
    Executed after each command.

TRAPEXIT
    Executed when the shell exits, or when the current function exits if defined inside a function.

TRAPZERR
    Executed whenever a command has a non-zero exit status. However, the function is not executed if the command occurred in a sublist followed by `&&' or `||'; only the final command in a sublist of this type causes the trap to be executed.

The functions beginning `TRAP' may alternatively be defined with the trap builtin: this may be preferable for some uses, as they are then run in the environment of the calling process, rather than in their own function environment. Apart from the difference in calling procedure and the fact that the function form appears in lists of functions, the forms

    TRAPNAL() {
     # code
    }

and

    trap '
     # code

are equivalent.

JOBS
If the MONITOR option is set, an interactive shell associates a job with each pipeline. It keeps a table of current jobs, printed by the jobs command, and assigns them small integer numbers. When a job is started asynchronously with `&', the shell prints a line which looks like:

    [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. 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
    Any job whose command line begins with string.
%?string
    Any 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.

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
The INT and QUIT signals for an invoked command are ignored if the command is followed by `&' and the MONITOR option is not active. Otherwise, signals have the values inherited by the shell from its parent (but see the TRAPNAL special functions in the section `Functions').

ARITHMETIC EVALUATION
The shell can perform integer and floating point arithmetic, either using the builtin let, or via a substitution of the form $((...)). For integers, the shell is usually compiled to use 8-byte precision where this is available, otherwise precision is 4 bytes. This can be tested, for example, by giving the command `print - $(( 12345678901 ))'; if the number appears unchanged, the precision is at least 8 bytes. Floating point arithmetic is always double precision.

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 "..."'. 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. 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.

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.

If the C_BASES option is set, hexadecimal numbers 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.

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

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 ASCII value of this character and an expression of the form `#foo' gives the ASCII value of the first character of the value of the parameter foo. Note that this is different from the expression `$#foo', a standard parameter substitution which gives the length of the parameter foo. `#\' 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 down to the next integer.

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 down to the next integer.

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 conditional expression is used with the [[ compound command to test attributes of files and to compare strings. Each expression can be constructed from one or more of the following unary or binary 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.

-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 `==' form is the preferred one. The `=' form is for backward compatibility and should be considered obsolete.

string != pattern
    true if string does not match pattern.

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.

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.

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. However, 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.

PROMPT EXPANSION
Prompt sequences undergo a special form of expansion. This type of expansion is also available using the -P option to the print builtin.

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. Some escapes take an optional integer argument, which should appear between the `%' and the next character of the sequence. The following escape sequences are recognized:

  Special characters
%%  A `%'.

%)  A `)'.

  Login information
%l  The line (tty) the user is logged in on, without `/dev/' prefix. If the name starts with `/dev/tty', that prefix is stripped.

%M  The full machine hostname.

%m  The hostname up to the first `.'. An integer may follow the `%' to specify how many components of the hostname are desired. With a negative integer, trailing components of the hostname are shown.

%n  $USERNAME.

%y  The line (tty) the user is logged in on, without `/dev/' prefix. This does not treat `/dev/tty' names specially.

  Shell state
%#  A `#' if the shell is running with privileges, a `%' if not. Equivalent to `%(!.#.%%)'. The definition of `privileged', for these purposes, is that either the effective user ID is zero, or, if POSIX.1e capabilities are supported, that at least one capability is raised in either the Effective or Inheritable capability vectors.

%?  The return code of the last command executed just before the prompt.

%_  The status of the parser, i.e. the shell constructs (like `if' and `for') that have been started on the command line. If given an integer number that many strings will be printed; zero or negative or no integer means print as many as there are. This is most useful in prompts PS2 for continuation lines and PS4 for debugging with the XTRACE option; in the latter case it will also work non-interactively.

%d
%/  Present working directory ($PWD). If an integer follows the `%', it specifies a number of trailing components of $PWD to show; zero means the whole path. A negative integer specifies leading components, i.e. %-1d specifies the first component.

%~  As %d and %/, but if $PWD has a named directory as its prefix, that part is replaced by a `~' followed by the name of the directory. If it starts with $HOME, that part is replaced by a `~'.

%h
%!  Current history event number.

%i  The line number currently being executed in the script, sourced file, or shell function given by %N. This is most useful for debugging as part of $PS4.

%j  The number of jobs.

%L  The current value of $SHLVL.

%N  The name of the script, sourced file, or shell function that zsh is currently executing, whichever was started most recently. If there is none, this is equivalent to the parameter $0. An integer may follow the `%' to specify a number of trailing path components to show; zero means the full path. A negative integer specifies leading components.

%c
%.
%C  Trailing component of $PWD. An integer may follow the `%' to get more than one component. Unless `%C' is used, tilde contraction is performed first. These are deprecated as %c and %C are equivalent to %1~ and %1/, respectively, while explicit positive integers have the same effect as for the latter two sequences.

  Date and time
%D  The date in yy-mm-dd format.

%T  Current time of day, in 24-hour format.

%t
%@  Current time of day, in 12-hour, am/pm format.

%*  Current time of day in 24-hour format, with seconds.

%w  The date in day-dd format.

%W  The date in mm/dd/yy format.

%D{string}
    string is formatted using the strftime function. See strftime(3) for more details. Three additional codes are available: %f prints the day of the month, like %e but without any preceding space if the day is a single digit, and %K/%L correspond to %k/%l for the hour of the day (24/12 hour clock) in the same way.

  Visual effects
%B (%b)
    Start (stop) boldface mode.

%E  Clear to end of line.

%U (%u)
    Start (stop) underline mode.

%S (%s)
    Start (stop) standout mode.

%{...%}
    Include a string as a literal escape sequence. The string within the braces should not change the cursor position. Brace pairs can nest.

  Conditional substrings
%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.

    The left parenthesis may be preceded or followed by a positive integer n, which defaults to zero. A negative integer will be multiplied by -1. The test character x may be any of the following:

    !   True if the shell is running with privileges.
    #   True if the effective uid of the current process is n.
    ?   True if the exit status of the last command was n.
    _   True if at least n shell constructs were started.
    C
    /   True if the current absolute path has at least n elements.
    c
    .
    ~   True if the current path, with prefix replacement, has at least n elements.
    D   True if the month is equal to n (January = 0).
    d   True if the day of the month is equal to n.
    g   True if the effective gid of the current process is n.
    j   True if the number of jobs is at least n.
    L   True if the SHLVL parameter is at least n.
    l   True if at least n characters have already been printed on the current line.
    S   True if the SECONDS parameter is at least n.
    T   True if the time in hours is equal to n.
    t   True if the time in minutes is equal to n.
    v   True if the array psvar has at least n elements.
    w   True if the day of the week is equal to n (Sunday = 0).

%<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 numeric argument, which in the third form may appear immediately after the `[', specifies the maximum permitted length of the various strings that can be displayed in the prompt. The string will be displayed in place of the truncated portion of any string; note this does not undergo prompt expansion.

    The forms with `<' truncate at the left of the string, and the forms with `>' truncate at the right of the string. For example, if the current directory is `/home/pike', the prompt `%8<..<%/' will expand to `..e/pike'. In this string, the terminating character (`<', `>' or `]'), or in fact any character, may be quoted by a preceding `\'; note when using print -P, however, that this must be doubled as the string is also subject to standard print processing, in addition to any backslashes removed by a double quoted string: the worst case is therefore `print -P "%<\\\\<<..."'.

    If the string is longer than the specified truncation length, it will appear in full, completely replacing the truncated string.

    The part of the prompt string to be truncated runs to the end of the string, or to the end of the next enclosing group of the `%(' construct, or to the next truncation encountered at the same grouping level (i.e. truncations inside a `%(' are separate), which ever comes first. In particular, a truncation with argument zero (e.g. `%<<') marks the end of the range of the string to be truncated while turning off truncation from there on. For example, the prompt '%10<...<%~%<<%# ' will print a truncated representation of the current directory, followed by a `%' or `#', followed by a space. Without the `%<<', those two characters would be included in the string to be truncated.