23.2. Local Variables and Recursion

Local variables make recursion possible.

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 a=23 echo echo "a in function = $a" echo } func # Now, see if local 'a' exists outside function. echo "a outside function = $a" # Nope, 'a' not visible globally. echo exit 0

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 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-11 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.