Functions

The function reserved word is used to define shell functions. 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 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 fpath variable will be searched for a file with the same name as the function.

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. It would normally define the function in question, but may also perform initialisation. It is executed in the context of the function execution, and may therefore define local parameters.

Otherwise, the function is defined such that its body is the complete contents of the file. This form allows the file to be used directly as an executable shell script. Initialisation code can be executed, but only as part of the first function execution, so the function would have to redefine itself to avoid reinitialising on the next execution.

If this processing of the file results in the function being fully defined, the function itself is then executed.

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, the string to be executed is passed as an argument.

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.

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