zsh: 9 Functions

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# 9 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 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.

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## 9.1 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

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## 9.2 Anonymous Functions

If no name is given for a function, it is ‘anonymous’ and is handled specially. Either form of function definition may be used: a ‘`()`’ with no preceding name, or a ‘`function`’ with an immediately following open brace. The function is executed immediately at the point of definition and is not stored for future use. The function name is set to ‘`(anon)`’.

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.

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## 9.3 Special Functions

Certain functions, if defined, have special meaning to the shell.

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### 9.3.1 Hook Functions

For the functions below, it is possible to define an array that has the same name as the function with ‘`_functions`’ appended. Any element in such an array is taken as the name of a function to execute; it is executed in the same context and with the same arguments as the basic function. For example, if `$chpwd_functions` is an array containing the values ‘`mychpwd`’, ‘`chpwd_save_dirstack`’, then the shell attempts to execute the functions ‘`chpwd`’, ‘`mychpwd`’ and ‘`chpwd_save_dirstack`’, in that order. Any function that does not exist is silently ignored. A function found by this mechanism is referred to elsewhere as a ‘hook function’. An error in any function causes subsequent functions not to be run. Note further that an error in a `precmd` hook causes an immediately following `periodic` function not to run (though it may run at the next opportunity).

`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. Note that if multiple functions are defined using the array `periodic_functions` only one period is applied to the complete set of functions, and the scheduled time is not reset if the list of functions is altered. Hence the set of functions is always called together.

`precmd`

  Executed before each prompt. Note that precommand functions are not re-executed simply because the command line is redrawn, as happens, for example, when a notification about an exiting job is displayed.

`preexec`

  Executed just after a command has been read and is about to be executed. If the history mechanism is active (regardless of whether the line was 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.

`zshaddhistory`

  Executed when a history line has been read interactively, but before it is executed. The sole argument is the complete history line (so that any terminating newline will still be present).

  If any of the hook functions returns status 1 (or any non-zero value other than 2, though this is not guaranteed for future versions of the shell) the history line will not be saved, although it lingers in the history until the next line is executed, allowing you to reuse or edit it immediately.

  If any of the hook functions returns status 2 the history line will be saved on the internal history list, but not written to the history file. In case of a conflict, the first non-zero status value is taken.

  A hook function may call ‘`fc -p` ...’ to switch the history context so that the history is saved in a different file from the that in the global `HISTFILE` parameter. This is handled specially: the history context is automatically restored after the processing of the history line is finished.

  The following example function works with one of the options `INC_APPEND_HISTORY` or `SHARE_HISTORY` set, in order that the line is written out immediately after the history entry is added. It first adds the history line to the normal history with the newline stripped, which is usually the correct behaviour. Then it switches the history context so that the line will be written to a history file in the current directory.

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

`zshexit`

  Executed at the point where the main shell is about to exit normally. This is not called by exiting subshells, nor when the `exec` precommand modifier is used before an external command. Also, unlike `TRAPEXIT`, it is not called when functions exit.

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### 9.3.2 Trap Functions

The functions below are treated specially but do not have corresponding hook arrays.

`TRAP`NAL

  If defined and non-null, this function will be executed whenever the shell catches a signal `SIG`NAL, 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 `SIG`NAL.

  The return status from the function is handled specially. If it is zero, the signal is assumed to have been handled, and execution continues normally. Otherwise, the shell will behave as interrupted except that the return status of the trap is retained.

  Programs terminated by uncaught signals typically return the status 128 plus the signal number. Hence the following causes the handler for `SIGINT` to print a message, then mimic the usual effect of the signal.

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

  The functions `TRAPZERR`, `TRAPDEBUG` and `TRAPEXIT` are never executed inside other traps.

`TRAPDEBUG`

  If the option `DEBUG_BEFORE_CMD` is set (as it is by default), executed before each command; otherwise executed after each command. See the description of the `trap` builtin in Shell Builtin Commands for details of additional features provided in debug traps.

`TRAPEXIT`

  Executed when the shell exits, or when the current function exits if defined inside a function. The value of `$?` at the start of execution is the exit status of the shell or the return status of the function exiting.

`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 function `TRAPERR` acts the same as `TRAPZERR` on systems where there is no `SIGERR` (this is the usual case).

The functions beginning ‘`TRAP`’ may alternatively be defined with the `trap` builtin: this may be preferable for some uses. Setting a trap with one form removes any trap of the other form for the same signal; removing a trap in either form removes all traps for the same signal. The forms

    TRAPNAL() {
     # code
    }

(’function traps’) and

    trap '
     # code
    ' NAL

(’list traps’) are equivalent in most ways, the exceptions being the following:

- Function traps have all the properties of normal functions, appearing in the list of functions and being called with their own function context rather than the context where the trap was triggered.
- The return status from function traps is special, whereas a return from a list trap causes the surrounding context to return with the given status.
- Function traps are not reset within subshells, in accordance with zsh behaviour; list traps are reset, in accordance with POSIX behaviour.

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This document was generated on December 21, 2016 using texi2any.

Zsh version 5.3.1, released on December 21, 2016.