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8. 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 7. 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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8.1 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:

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.

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.

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...


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. 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. This is done so that

eval "$(functions)"

produces a reasonable result.

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

autoload +X myfunc

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8.2 Special Functions

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

Executed whenever the current working directory is changed.

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

Executed before each prompt.

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 what what is being executed.

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.

Executed after each command.

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

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

 # code


trap '
 # code

are equivalent.

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