zsh/Doc/Zsh/func.yo

texinode(Functions)(Jobs & Signals)(Command Execution)(Top)
chapter(Functions)
ifzman(\
sect(Functions)
)\
cindex(functions)
findex(function)
Shell functions are defined with the tt(function) reserved word or the
special syntax `var(funcname) tt(())'.
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 noderef(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 tt(EXIT) set inside a function
is executed after the function completes in the environment
of the caller.

findex(return, use of)
The tt(return) builtin is used to return from function calls.

findex(functions, use of)
Function identifiers can be listed with the tt(functions) builtin.
findex(unfunction, use of)
Functions can be undefined with the tt(unfunction) builtin.
sect(Autoloading Functions)
findex(autoload, use of)
cindex(autoloading functions)
cindex(functions, autoloading)

A function can be marked as em(undefined) using the tt(autoload) builtin
(or `tt(functions -u)' or `tt(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 tt(fpath) variable.  Thus to define
functions for autoloading, a typical sequence is:

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

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

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

startitem()
item(var(element)tt(.zwc))(
A file created with the tt(zcompile) builtin command, which is expected to
contain the definitions for all functions in the directory named
var(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 the other two possibilities described below.

If var(element) already includes a tt(.zwc) extension (i.e. the extension
was explicitly given by the user), var(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 var(element)
without the suffix.  Thus including an element such as
`tt(/usr/local/funcs.zwc)' in tt(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.  )
item(var(element)tt(/)var(function)tt(.zwc))( A file created with
tt(zcompile), which is expected to contain the definition for
var(function).  It may include other function definitions as well, but
those are neither loaded nor executed; a file found in this way is searched
em(only) for the definition of var(function).  )
item(var(element)tt(/)var(function))( A file of zsh command text, taken to
be the definition for var(function).  ) enditem()

In summary, the order of searching is, first, directories in tt(fpath),
with the earliest directory containing a function definition being used;
within that directory, the newest of the three possibilities --- a compiled
directory, a compiled function, or an ordinary function defition --- is
used.

pindex(KSH_AUTOLOAD, use of)
If the tt(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 `var(funcname)tt(()
{)var(...)tt(}) 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 tt(func) contains

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

then `tt(func; func)' with tt(KSH_AUTOLOAD) set will produce both messages
on the first call, but only the message `tt(This is func)' on the second
and subsequent calls.  Without tt(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 tt(fpath), by using
`tt(autoload -X)' within a shell function.  For example, the following are
equivalent:

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

and

example(unfunction myfunc   # if myfunc was defined
autoload myfunc
myfunc args...)

In fact, the tt(functions) command outputs `tt(builtin autoload -X)' as
the body of an autoloaded function.  A true autoloaded function can be
identified by the presence of the comment `tt(# undefined)' in the body,
because all comments are discarded from defined functions.  This is done
so that

example(eval "$(functions)")

produces a reasonable result.

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

example(autoload +X myfunc)

sect(Special Functions)
The following functions, if defined, have special meaning to
the shell:

startitem()
findex(chpwd)
item(tt(chpwd))(
Executed whenever the current working directory is changed.
)
findex(periodic)
item(tt(periodic))(
vindex(PERIOD)
If the parameter tt(PERIOD)
is set, this function is executed every tt($PERIOD)
seconds, just before a prompt.
)
findex(precmd)
item(tt(precmd))(
Executed before each prompt.
)
findex(preexec)
item(tt(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.
)
item(tt(TRAP)var(NAL))(
cindex(signals, trapping)
cindex(trapping signals)
If defined and non-null,
this function will be executed whenever the shell
catches a signal tt(SIG)var(NAL), where var(NAL) is a signal
name as specified for the tt(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 tt(SIG)var(NAL).
)
findex(TRAPDEBUG)
item(tt(TRAPDEBUG))(
Executed after each command.
)
findex(TRAPEXIT)
item(tt(TRAPEXIT))(
Executed when the shell exits,
or when the current function exits if defined inside a function.
)
findex(TRAPZERR)
item(tt(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
`tt(&&)' or `tt(||)'; only the final command in a sublist of this type
causes the trap to be executed.
)
enditem()

The functions beginning `tt(TRAP)' may alternatively be defined with the
tt(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

example(TRAPNAL() { 
 # code
})

and

example(trap '
 # code
' NAL)

are equivalent.