24.2. Local Variables
- 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. [1]
Example 24-12. Local variable visibility
#!/bin/bash # ex62.sh: 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. # Therefore, defaults to global. echo "\"global_var\" in function = $global_var" } func # Now, to see if local variable "loc_var" exists outside the 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 = $global_var" # global_var = 37 # Has been set by function call.
As Evgeniy Ivanov points out, when declaring and setting a local variable in a single command, apparently the order of operations is to first set the variable, and only afterwards restrict it to local scope. This is reflected in the return value.
#!/bin/bash echo "==OUTSIDE Function (global)==" t=$(exit 1) echo $? # 1 # As expected. echo function0 () { echo "==INSIDE Function==" echo "Global" t0=$(exit 1) echo $? # 1 # As expected. echo echo "Local declared & assigned in same command." local t1=$(exit 1) echo $? # 0 # Unexpected! # Apparently, the variable assignment takes place before #+ the local declaration. #+ The return value is for the latter. echo echo "Local declared, then assigned (separate commands)." local t2 t2=$(exit 1) echo $? # 1 # As expected. } function0
24.2.1. Local variables and recursion.
Recursion is an interesting and sometimes useful form of self-reference. Herbert Mayer defines it as ". . . expressing an algorithm by using a simpler version of that same algorithm . . ." Consider a definition defined in terms of itself, [2] an expression implicit in its own expression, [3] a snake swallowing its own tail, [4] or . . . a function that calls itself. [5] Example 24-13. Demonstration of a simple recursive function
Example 24-14. Another simple demonstration
|
Local variables are a useful tool for writing recursive code, but this practice generally involves a great deal of computational overhead and is definitely not recommended in a shell script. [6]
Example 24-15. 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=85
E_RANGE_ERR=86
if [ -z "$1" ]
then
echo "Usage: `basename $0` number"
exit $E_WRONG_ARGS
fi
if [ "$1" -gt $MAX_ARG ]
then
echo "Out of range ($MAX_ARG 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 |
Also see Example A-15 for an example of recursion in a script. Be aware that recursion is resource-intensive and executes slowly, and is therefore generally not appropriate in a script.