On-line Guides

How To Guides

General System Admin

Linux Security

Linux Filesystems

Web Servers

Graphics & Desktop

PC Hardware

Windows

Problem Solutions

Advanced Bash-Scripting Guide:
Prev	Chapter 23. Functions	Next

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 23-12. Local variable visibility

#!/bin/bash # Global and local variables inside a function. func () { local loc_var=23 # Declared as local variable. echo # Uses the 'local' builtin. echo "\"loc_var\" in function = $loc_var" global_var=999 # Not declared as local. # Defaults to global. echo "\"global_var\" in function = $global_var" } func # Now, to see if local variable "loc_var" exists outside function. echo echo "\"loc_var\" outside function = $loc_var" # $loc_var outside function = # No, $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 # In contrast to C, a Bash variable declared inside a function #+ is local *only* if declared as such.

Caution
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 help 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 23-13. Recursion, using a local variable

#!/bin/bash
# factorial
# ---------
# Does bash permit recursion?
# Well, yes, but...
# It's so slow that you gotta have rocks in your head to try it.
MAX_ARG=5
E_WRONG_ARGS=65
E_RANGE_ERR=66
if [ -z "1ドル" ]
then
 echo "Usage: `basename 0ドル` number"
 exit $E_WRONG_ARGS
fi
if [ "1ドル" -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 $E_RANGE_ERR
fi 
fact ()
{
 local number=1ドル
 # Variable "number" must be declared as local,
 #+ otherwise this doesn't work.
 if [ "$number" -eq 0 ]
 then
 factorial=1 # Factorial of 0 = 1.
 else
 let "decrnum = number - 1"
 fact $decrnum # Recursive function call (the function calls itself).
 let "factorial = $number * $?"
 fi
 return $factorial
}
fact 1ドル
echo "Factorial of 1ドル 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
# Warning: Running this script could possibly lock up your system!
# If you're lucky, it will segfault before using up all available memory.
recursive_function ()		 
{
echo "1ドル" # Makes the function do something, and hastens the segfault.
(( 1ドル < 2ドル )) && recursive_function $(( 1ドル + 1 )) 2ドル;
# As long as 1st parameter is less than 2nd,
#+ increment 1st and recurse.
}
recursive_function 1 50000 # Recurse 50,000 levels!
# Most likely segfaults (depending on stack size, set by ulimit -m).
# Recursion this deep might cause even a C program to segfault,
#+ by using up all the memory allotted to the stack.
echo "This will probably not print."
exit 0 # This script will not exit normally.
# Thanks, St�phane Chazelas.

Prev	Home	Next
Complex Functions and Function Complexities	Up	Recursion Without Local Variables

Published under the terms of the GNU General Public License Design by Interspire