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3.18. 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. Whenever there is repetitive code, when a task repeats with only slight variations, then writing a function should be investigated.

function function-name {
command...
}

or

function-name () {
command...
}

This second form will cheer the hearts of C programmers.

The opening bracket in the function may optionally be placed on the second line, to more nearly resemble C function syntax.

function-name ()
{
command...
}

Functions are called, triggered, simply by invoking their names.

Note that the function definition must precede the first call to it. There is no method of "declaring" the function, as, for example, in C.

Example 3-64. Simple function

#!/bin/bash

funky ()
{
  echo This is a funky function.
  echo Now exiting funky function.
}

# Note: function must precede call.

# Now, call the function.

funky

exit 0

More complex functions may have arguments passed to them and return exit values to the script for further processing.

function-name $arg1 $arg2

The function refers to the passed arguments by position (as if they were positional parameters), that is, $1, $2, and so forth.

Example 3-65. Function Taking Parameters

#!/bin/bash

func2 () {
   if [ -z $1 ]
   # Checks if any params.
   then
     echo "No parameters passed to function."
     return 0
   else
     echo "Param #1 is $1."
   fi

   if [ $2 ]
   then
     echo "Parameter #2 is $2."
   fi
}
   
func2
# Called with no params
echo

func2 first
# Called with one param
echo

func2 first second
# Called with two params
echo

exit 0

Note: In contrast to certain other programming languages, shell scripts permit passing only value parameters to functions. Variable names (which are actually pointers), if passed as parameters to functions, will be treated as string literals and cannot be dereferenced. Functions interpret their arguments literally.

exit status

Functions return a value, called an exit status. The exit status may be explicitly specified by a return statement, otherwise it is the exit status of the last command in the function (0 if successful, and a non-zero error code if not). This exit status may be used in the script by referencing it as $?.

return

Terminates a function. The return statement optionally takes an integer argument, which is returned to the calling script as the "exit status" of the function, and this exit status is assigned to the variable $?.

Example 3-66. Converting numbers to Roman numerals

#!/bin/bash

# Arabic number to Roman numeral conversion
# Range 0 - 200
# It's crude, but it works.

# Extending the range and otherwise improving the script
# is left as an exercise for the reader.

# Usage: roman number-to-convert

ARG_ERR=1
OUT_OF_RANGE=200

if [ -z $1 ]
then
  echo "Usage: `basename $0` number-to-convert"
  exit $ARG_ERR
fi  

num=$1
if [ $num -gt $OUT_OF_RANGE ]
then
  echo "Out of range!"
  exit $OUT_OF_RANGE
fi  

to_roman ()
{
number=$1
factor=$2
rchar=$3
let "remainder = number - factor"
while [ $remainder -ge 0 ]
do
  echo -n $rchar
  let "number -= factor"
  let "remainder = number - factor"
done  

return $number
}

# Note: must declare function
#       before first call to it.

to_roman $num 100 C
num=$?
to_roman $num 90 LXXXX
num=$?
to_roman $num 50 L
num=$?
to_roman $num 40 XL
num=$?
to_roman $num 10 X
num=$?
to_roman $num 9 IX
num=$?
to_roman $num 5 V
num=$?
to_roman $num 4 IV
num=$?
to_roman $num 1 I

echo

exit 0
local variables

A variable declared as local is one that is visible only within the block of code in which it appears. In a shell script, this means the variable has meaning only within its own function.

Example 3-67. Local variable visibility

#!/bin/bash

func ()
{
  local a=23
  echo
  echo "a in function is $a"
  echo
}  

func

# Now, see if local 'a'
# exists outside function.

echo "a outside function is $a"
echo
# Nope, 'a' not visible globally.

exit 0

Local variables permit recursion (a recursive function is one that calls itself), but this practice usually involves much computational overhead and is definitely not recommended in a shell script.

Example 3-68. Recursion, using a local variable

#!/bin/bash

#               factorial
#               ---------


# Does bash permit recursion?
# Well, yes, but...
# You gotta have rocks in your head to try it.


MAX_ARG=5
WRONG_ARGS=1
RANGE_ERR=2


if [ -z $1 ]
then
  echo "Usage: `basename $0` number"
  exit $WRONG_ARGS
fi

if [ $1 -gt $MAX_ARG ]
then
  echo "Out of range (5 is maximum)."
  # Let's get real now...
  # If you want greater range than this, rewrite it in a real programming language.
  exit $RANGE_ERR
fi  

fact ()
{
  local number=$1
  # Variable "number" must be declared as local otherwise this doesn't work.
  if [ $number -eq 0 ]
  then
    factorial=1
  else
    let "decrnum = number - 1"
    fact $decrnum  # Recursive function call.
    let "factorial = $number * $?"
  fi

  return $factorial
}

fact $1
echo "Factorial of $1 is $?."

exit 0



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