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 23-12. Local variable visibility

   1 #!/bin/bash
   2 # 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                          # 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 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.

Caution

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 and recursion.

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. [5]


Example 23-14. 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=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 (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-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 in a script.

Notes

[1]

Otherwise known as redundancy.

[2]

Otherwise known as tautology.

[3]

Otherwise known as a metaphor.

[4]

Otherwise known as a recursive function.

[5]

Too many levels of recursion may crash a script with a segfault.
   1 #!/bin/bash
   2 
   3 #  Warning: Running this script could possibly lock up your system!
   4 #  If you're lucky, it will segfault before using up all available memory.
   5 
   6 recursive_function ()		   
   7 {
   8 echo "$1"     # Makes the function do something, and hastens the segfault.
   9 (( $1 < $2 )) && recursive_function $(( $1 + 1 )) $2;
  10 #  As long as 1st parameter is less than 2nd,
  11 #+ increment 1st and recurse.
  12 }
  13 
  14 recursive_function 1 50000  # Recurse 50,000 levels!
  15 #  Most likely segfaults (depending on stack size, set by ulimit -m).
  16 
  17 #  Recursion this deep might cause even a C program to segfault,
  18 #+ by using up all the memory allotted to the stack.
  19 
  20 
  21 echo "This will probably not print."
  22 exit 0  # This script will not exit normally.
  23 
  24 #  Thanks, Stйphane Chazelas.

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