Advanced Bash-Scripting Guide:
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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

#!/bin/bash

func ()
{
  local loc_var=23       # Declared local.
  echo
  echo "\"loc_var\" in function = $loc_var"
  global_var=999         # Not declared local.
  echo "\"global_var\" in function = $global_var"
}

func

# Now, see if local 'a' exists outside function.

echo
echo "\"loc_var\" outside function = $loc_var"
                                      # "loc_var" outside function =
                                      # Nope, $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

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.

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

#!/bin/bash

#               factorial
#               ---------


# Does bash permit recursion?
# Well, yes, but...
# You gotta have rocks in your head to try it.


MAX_ARG=5
E_WRONG_ARGS=65
E_RANGE_ERR=66


if [ -z "$1" ]
then
  echo "Usage: `basename $0` number"
  exit $E_WRONG_ARGS
fi

if [ "$1" -gt $MAX_ARG ]
then
  echo "Out of range (5 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.
    let "factorial = $number * $?"
  fi

  return $factorial
}

fact $1
echo "Factorial of $1 is $?."

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.

Notes

[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.

#!/bin/bash

recursive_function ()
{
(( $1 < $2 )) && f $(( $1 + 1 )) $2;
#  As long as 1st parameter is less than 2nd,
#+ increment 1st and recurse.
}

recursive_function 1 50000  # Recurse 50,000 levels!
# Segfaults, of course.

#  Recursion this deep might cause even a C program to segfault,
#+ by using up all the memory allotted to the stack.

# Thanks, S.C.

exit 0  # This script will not exit normally.

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