Chapter 24. 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 in procedure, 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
...
}
A function may be "compacted" into a single line.
fun () { echo "This is a function"; echo; }
# ^ ^
In this case, however, a semicolon must follow the final command in the function.
fun () { echo "This is a function"; echo } # Error!
# ^ fun2 () { echo "Even a single-command function? Yes!"; }
# ^
Functions are called, triggered, simply by invoking their names. A function call is equivalent to a command.
Example 24-1. Simple functions
#!/bin/bash
# ex59.sh: Exercising functions (simple).
JUST_A_SECOND=1 funky ()
{ # This is about as simple as functions get.
echo "This is a funky function."
echo "Now exiting funky function."
Chapter 24. Functions 398
Advanced Bash-Scripting Guide
} # Function declaration must precede call.
fun ()
{ # A somewhat more complex function.
i=0
REPEATS=30
echo
echo "And now the fun really begins."
echo
sleep $JUST_A_SECOND # Hey, wait a second!
while [ $i -lt $REPEATS ]
do
echo "----------FUNCTIONS---------->"
echo "<------------ARE-------------"
echo "<------------FUN------------>"
echo
let "i+=1"
done
}
# Now, call the functions.
funky fun exit $?
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 permissible.
# Thanks, S.C.
Functions may not be empty!
Chapter 24. Functions 399
Advanced Bash-Scripting Guide
#!/bin/bash
# empty-function.sh
empty ()
{
} exit 0 # Will not exit here!
# $ sh empty-function.sh
# empty-function.sh: line 6: syntax error near unexpected token `}'
# empty-function.sh: line 6: `}'
# $ echo $?
# 2
# Note that a function containing only comments is empty.
func ()
{
# Comment 1.
# Comment 2.
# This is still an empty function.
# Thank you, Mark Bova, for pointing this out.
}
# Results in same error message as above.
# However ...
not_quite_empty ()
{
illegal_command
} # A script containing this function will *not* bomb
#+ as long as the function is not called.
not_empty ()
{
:
} # Contains a : (null command), and this is okay.
# Thank you, Dominick Geyer and Thiemo Kellner.
It is even possible to nest a function within another function, although this is not very useful.
f1 ()
{
f2 () # nested
{
echo "Function \"f2\", inside \"f1\"."
}
} f2 # Gives an error message.
# Even a preceding "declare -f f2" wouldn't help.
echo
Chapter 24. Functions 400
Advanced Bash-Scripting Guide f1 # Does nothing, since calling "f1" does not automatically call "f2".
f2 # Now, it's all right to call "f2",
#+ since its definition has been made visible by calling "f1".
# Thanks, S.C.
Function declarations can appear in unlikely places, even where a command would otherwise go.
ls -l | foo() { echo "foo"; } # Permissible, but useless.
if [ "$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 $NO_EXIT is 1, declares "exit ()".
# This disables the "exit" builtin by aliasing it to "true".
exit # Invokes "exit ()" function, not "exit" builtin.
# Or, similarly: filename=file1
[ -f "$filename" ] && foo () { rm -f "$filename"; echo "File "$filename" deleted."; } || foo () { echo "File "$filename" not found."; touch bar; } foo
# Thanks, S.C. and Christopher Head
Function names can take strange forms.
_(){ for i in {1..10}; do echo -n "$FUNCNAME"; done; echo; }
# ^^^ No space between function name and parentheses.
# This doesn't always work. Why not?
# Now, let's invoke the function.
_ # __________
# ^^^^^^^^^^ 10 underscores (10 x function name)!
# A "naked" underscore is an acceptable function name.
# In fact, a colon is likewise an acceptable function name.
:(){ echo ":"; }; :
# Of what use is this?
Chapter 24. Functions 401
