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.
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" ]
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 (the function calls itself).
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.
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.
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" # Makes the function do something, and hastens the segfault.
(( $1 < $2 )) && recursive_function $(( $1 + 1 )) $2;
# 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.
