| Advanced Bash-Scripting Guide: A complete guide to shell scripting, using Bash | ||
|---|---|---|
| Prev | Chapter 23. Functions | Next |
Local variables make recursion possible.
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 a=23
6 echo
7 echo "a in function = $a"
8 echo
9 }
10
11 func
12
13 # Now, see if local 'a' exists outside function.
14
15 echo "a outside function = $a" # Nope, 'a' not visible globally.
16 echo
17
18 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
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-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.
| [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.
|