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
1 #!/bin/bash 2 3 func () 4 { 5 local loc_var=23 # Declared as local variable. 6 echo # Uses the 'local' builtin. 7 echo "\"loc_var\" in function = $loc_var" 8 global_var=999 # Not declared as local. 9 # Defaults to global. 10 echo "\"global_var\" in function = $global_var" 11 } 12 13 func 14 15 # Now, to see if local variable "loc_var" exists outside function. 16 17 echo 18 echo "\"loc_var\" outside function = $loc_var" 19 # $loc_var outside function = 20 # Nope, $loc_var not visible globally. 21 echo "\"global_var\" outside function = $global_var" 22 # $global_var outside function = 999 23 # $global_var is visible globally. 24 echo 25 26 exit 0 27 # In contrast to C, a Bash variable declared inside a function 28 #+ 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.
1 #!/bin/bash 2 3 func () 4 { 5 global_var=37 # Visible only within the function block 6 #+ before the function has been called. 7 } # END OF FUNCTION 8 9 echo "global_var = $global_var" # global_var = 10 # Function "func" has not yet been called, 11 #+ so $global_var is not visible here. 12 13 func 14 echo "global_var = $global_var" # global_var = 37 15 # 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
1 #!/bin/bash
2
3 # factorial
4 # ---------
5
6
7 # Does bash permit recursion?
8 # Well, yes, but...
9 # You gotta have rocks in your head to try it.
10
11
12 MAX_ARG=5
13 E_WRONG_ARGS=65
14 E_RANGE_ERR=66
15
16
17 if [ -z "$1" ]
18 then
19 echo "Usage: `basename $0` number"
20 exit $E_WRONG_ARGS
21 fi
22
23 if [ "$1" -gt $MAX_ARG ]
24 then
25 echo "Out of range (5 is maximum)."
26 # Let's get real now.
27 # If you want greater range than this,
28 # rewrite it in a real programming language.
29 exit $E_RANGE_ERR
30 fi
31
32 fact ()
33 {
34 local number=$1
35 # Variable "number" must be declared as local,
36 # otherwise this doesn't work.
37 if [ "$number" -eq 0 ]
38 then
39 factorial=1 # Factorial of 0 = 1.
40 else
41 let "decrnum = number - 1"
42 fact $decrnum # Recursive function call.
43 let "factorial = $number * $?"
44 fi
45
46 return $factorial
47 }
48
49 fact $1
50 echo "Factorial of $1 is $?."
51
52 exit 0 |
See also Example A-17 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.