Local Variables

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23.2. Local Variables

What makes a variable "local"?

local variables

A variable declared as local is one that is visible only within the block of code in which it appears. It has local "scope". In a function, a local variable has meaning only within that function block.

Example 23-8. Local variable visibility

#!/bin/bash
func ()
{
local loc_var=23	# Declared local.
echo
echo "\"loc_var\" in function = $loc_var"
global_var=999	# Not declared local.

echo "\"global_var\" in function = $global_var"

}

func

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

echo

echo "\"loc_var\" outside function = $loc_var"

#"loc_var" outside function =

#Nope, $loc_var not visible globally.

echo "\"global_var\" outside function = $global_var"

#"global_var" outside function = 999

#$global_var is visible globally.

echo

exit 0

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Local Variables

Before a function is called, all variables declared within the function are invisible outside the body of the function, not just those explicitly declared as local.

#!/bin/bash
func ()
{
global_var=37	# Visible only within the function block
	#+ before the function has been called.
}	# END OF FUNCTION

echo "global_var = $global_var" # global_var =

# Function "func" has not yet been called, #+ so $global_var is not visible here.

func

echo "global_var = $global_var" # global_var = 37

#Has been set by function call.

23.2.1.Local variables make recursion possible.

Local variables permit recursion, [1] but this practice generally involves much computational overhead and is definitely not recommended in a shell script. [2]

Example 23-9. 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

E_WRONG_ARGS=65

E_RANGE_ERR=66

if [ -z "$1" ] then

echo "Usage: `basename $0` number" exit $E_WRONG_ARGS

fi

if [ "$1" -gt $MAX_ARG ] then

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Local Variables

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 $E_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	#	Factorial	of 0 = 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

See also Example A-16 for an example of recursion in a script. Be aware that recursion is resource-intensive and executes slowly, and is therefore generally not appropriate to use in a script.

Notes

[1]Herbert Mayer defines recursion as "...expressing an algorithm by using a simpler version of that same algorithm..." A recursive function is one that calls itself.

[2]Too many levels of recursion may crash a script with a segfault.

#!/bin/bash

recursive_function ()

{

(( $1 < $2 )) && f $(( $1 + 1 )) $2;

# As long as 1st parameter is less than 2nd, #+ increment 1st and recurse.

}

recursive_function 1 50000 # Recurse 50,000 levels!

#Segfaults, of course.

#Recursion this deep might cause even a C program to segfault,

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Local Variables

#+ by using up all the memory allotted to the stack.

# Thanks, S.C.

exit 0 # This script will not exit normally.

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