Shell functions are a way to group commands for later execution using a single name for the group. They are executed just like a "regular" command. When the name of a shell function is used as a simple command name, the list of commands associated with that function name is executed. Shell functions are executed in the current shell context; no new process is created to interpret them.
Functions are declared using this syntax:
[ function ] name () compound-command [ redirections ]
This defines a shell function named name. The reserved
word function is optional.
If the function reserved
word is supplied, the parentheses are optional.
The body of the function is the compound command
compound-command (see Compound Commands).
That command is usually a list enclosed between { and }, but
may be any compound command listed above.
compound-command is executed whenever name is specified as the
name of a command.
Any redirections (see Redirections) associated with the shell function
are performed when the function is executed.
A function definition may be deleted using the -f option to the
unset builtin (see Bourne Shell Builtins).
The exit status of a function definition is zero unless a syntax error occurs or a readonly function with the same name already exists. When executed, the exit status of a function is the exit status of the last command executed in the body.
Note that for historical reasons, in the most common usage the curly braces
that surround the body of the function must be separated from the body by
blanks or newlines.
This is because the braces are reserved words and are only recognized
as such when they are separated from the command list
by whitespace or another shell metacharacter.
Also, when using the braces, the list must be terminated by a semicolon,
a ‘&’, or a newline.
When a function is executed, the arguments to the
function become the positional parameters
during its execution (see Positional Parameters).
The special parameter ‘#’ that expands to the number of
positional parameters is updated to reflect the change.
Special parameter 0 is unchanged.
The first element of the FUNCNAME variable is set to the
name of the function while the function is executing.
All other aspects of the shell execution
environment are identical between a function and its caller
with these exceptions:
the DEBUG and RETURN traps
are not inherited unless the function has been given the
trace attribute using the declare builtin or
the -o functrace option has been enabled with
the set builtin,
(in which case all functions inherit the DEBUG and RETURN traps),
and the ERR trap is not inherited unless the -o errtrace
shell option has been enabled.
See Bourne Shell Builtins, for the description of the
trap builtin.
If the builtin command return
is executed in a function, the function completes and
execution resumes with the next command after the function
call.
Any command associated with the RETURN trap is executed
before execution resumes.
When a function completes, the values of the
positional parameters and the special parameter ‘#’
are restored to the values they had prior to the function's
execution. If a numeric argument is given to return,
that is the function's return status; otherwise the function's
return status is the exit status of the last command executed
before the return.
Variables local to the function may be declared with the
local builtin. These variables are visible only to
the function and the commands it invokes.
Function names and definitions may be listed with the
-f option to the declare or typeset
builtin commands (see Bash Builtins).
The -F option to declare or typeset
will list the function names only
(and optionally the source file and line number, if the extdebug
shell option is enabled).
Functions may be exported so that subshells
automatically have them defined with the
-f option to the export builtin
(see Bourne Shell Builtins).
Note that shell functions and variables with the same name may result
in multiple identically-named entries in the environment passed to the
shell's children.
Care should be taken in cases where this may cause a problem.
Functions may be recursive. No limit is placed on the number of recursive calls.