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).

Note: 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!"; }
#

Simple functions

Functions are called, triggered, simply by invoking their names. A function call is equivalent to a command.

#!/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."
} # 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.

Note: Functions may not be empty!

#!/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.

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".

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

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?
# It's a devious way to obfuscate the code in a script.

See also TODO Example A-56

Note: What happens when different versions of the same function appear in a script?

#  As Yan Chen points out,
#  when a function is defined multiple times,
#  the final version is what is invoked.
#  This is not, however, particularly useful.
func ()
{
  echo "First version of func ()."
}
func ()
{
  echo "Second version of func ()."
}
func   # Second version of func ().
exit $?
#  It is even possible to use functions to override
#+ or preempt system commands.
#  Of course, this is *not* advisable.