Chapter 23. Functions
Like "real" programming languages,
Bash has functions, though in a somewhat limited implementation.
A function is a subroutine, a code
block that implements a set of operations, a "black
box" that performs a specified task. Wherever there is
repetitive code, when a task repeats with only slight variations,
then consider using a function.
function function_name {
command...
}
or
function_name () {
command...
}
This second form will cheer the hearts of C programmers
(and is more portable).
As in C, the function's opening bracket may optionally appear
on the second line.
function_name ()
{
command...
}
Functions are called, triggered, simply by
invoking their names.
Example 23-1. Simple function
#!/bin/bash
funky ()
{
echo "This is a funky function."
echo "Now exiting funky function."
} # Function declaration must precede call.
# Now, call the function.
funky
exit 0 |
The function definition must precede the first call to
it. There is no method of "declaring" the function,
as, for example, in C.
f1
# Will give an error message, since function "f1" not yet defined.
declare -f f1 # This doesn't help either.
f1 # Still an error message.
# However...
f1 ()
{
echo "Calling function \"f2\" from within function \"f1\"."
f2
}
f2 ()
{
echo "Function \"f2\"."
}
f1 # Function "f2" is not actually called until this point,
#+ although it is referenced before its definition.
# This is permissable.
# Thanks, S.C. |
It is even possible to nest a function within another function,
although this is not very useful.
f1 ()
{
f2 () # nested
{
echo "Function \"f2\", inside \"f1\"."
}
}
f2 # Gives an error message.
# Even a preceding "declare -f f2" wouldn't help.
echo
f1 # Does nothing, since calling "f1" does not automatically call "f2".
f2 # Now, it's all right to call "f2",
#+ since its definition has been made visible by calling "f1".
# Thanks, S.C. |
Function declarations can appear in unlikely places, even where a
command would otherwise go.
ls -l | foo() { echo "foo"; } # Permissable, but useless.
if [ "$USER" = bozo ]
then
bozo_greet () # Function definition embedded in an if/then construct.
{
echo "Hello, Bozo."
}
fi
bozo_greet # Works only for Bozo, and other users get an error.
# Something like this might be useful in some contexts.
NO_EXIT=1 # Will enable function definition below.
[[ $NO_EXIT -eq 1 ]] && exit() { true; } # Function definition in an "and-list".
# If $NO_EXIT is 1, declares "exit ()".
# This disables the "exit" builtin by aliasing it to "true".
exit # Invokes "exit ()" function, not "exit" builtin.
# Thanks, S.C. |
