| Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting | ||
|---|---|---|
| Prev | Chapter 24. Functions | Next | 
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
|    1 #!/bin/bash
   2 # ex62.sh: Global and local variables inside a function.
   3 
   4 func ()
   5 {
   6   local loc_var=23       # Declared as local variable.
   7   echo                   # Uses the 'local' builtin.
   8   echo "\"loc_var\" in function = $loc_var"
   9   global_var=999         # Not declared as local.
  10                          # Therefore, defaults to global. 
  11   echo "\"global_var\" in function = $global_var"
  12 }  
  13 
  14 func
  15 
  16 # Now, to see if local variable "loc_var" exists outside the function.
  17 
  18 echo
  19 echo "\"loc_var\" outside function = $loc_var"
  20                                       # $loc_var outside function = 
  21                                       # No, $loc_var not visible globally.
  22 echo "\"global_var\" outside function = $global_var"
  23                                       # $global_var outside function = 999
  24                                       # $global_var is visible globally.
  25 echo				      
  26 
  27 exit 0
  28 #  In contrast to C, a Bash variable declared inside a function
  29 #+ 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. 
 | 
|  | 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. 
 | 
| 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
|    1 #!/bin/bash
   2 
   3 #               factorial
   4 #               ---------
   5 
   6 
   7 # Does bash permit recursion?
   8 # Well, yes, but...
   9 # It's so slow that you gotta have rocks in your head to try it.
  10 
  11 
  12 MAX_ARG=5
  13 E_WRONG_ARGS=85
  14 E_RANGE_ERR=86
  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 $MAX_ARG ]
  24 then
  25   echo "Out of range ($MAX_ARG 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 (the function calls itself).
  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 | 
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.
| [1] | However, as Thomas Braunberger points out, a local variable declared in a function is also visible to functions called by the parent function. 
 This is documented in the Bash manual: "Local can only be used within a function; it makes the variable name have a visible scope restricted to that function and its children." [emphasis added] The ABS Guide author considers this behavior to be a bug. | |
| [2] | Otherwise known as redundancy. | |
| [3] | Otherwise known as tautology. | |
| [4] | Otherwise known as a metaphor. | |
| [5] | Otherwise known as a recursive function. | |
| [6] | Too many levels of recursion may crash a script with a segfault. 
 |