Local Variables

Advanced Bash-Scripting Guide:
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# 24.2. 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. [1]

  Example 24-12. Local variable visibility

      #!/bin/bash
      # ex62.sh: Global and local variables inside a function.

      func ()
      {
        local loc_var=23       # Declared as local variable.
        echo                   # Uses the 'local' builtin.
        echo "\"loc_var\" in function = $loc_var"
        global_var=999         # Not declared as local.
                               # Therefore, defaults to global.
        echo "\"global_var\" in function = $global_var"
      }

      func

      # Now, to see if local variable "loc_var" exists outside the function.

      echo
      echo "\"loc_var\" outside function = $loc_var"
                                            # $loc_var outside function =
                                            # No, $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
      #  In contrast to C, a Bash variable declared inside a function
      #+ 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.
      #!/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.

  | 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.
      #!/bin/bash

      echo "==OUTSIDE Function (global)=="
      t=$(exit 1)
      echo $?      # 1
                   # As expected.
      echo

      function0 ()
      {

      echo "==INSIDE Function=="
      echo "Global"
      t0=$(exit 1)
      echo $?      # 1
                   # As expected.

      echo
      echo "Local declared & assigned in same command."
      local t1=$(exit 1)
      echo $?      # 0
                   # Unexpected!
      #  Apparently, the variable assignment takes place before
      #+ the local declaration.
      #+ The return value is for the latter.

      echo
      echo "Local declared, then assigned (separate commands)."
      local t2
      t2=$(exit 1)
      echo $?      # 1
                   # As expected.

      }

      function0

## 24.2.1. 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. [6]

Example 24-15. Recursion, using a local variable

    #!/bin/bash

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

    # Does bash permit recursion?
    # Well, yes, but...
    # It's so slow that you gotta have rocks in your head to try it.

    MAX_ARG=5
    E_WRONG_ARGS=85
    E_RANGE_ERR=86

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

    if [ "$1" -gt $MAX_ARG ]
    then
      echo "Out of range ($MAX_ARG 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 (the function calls itself).
        let "factorial = $number * $?"
      fi

      return $factorial
    }

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

    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.

### Notes

[1] | However, as Thomas Braunberger points out, a local variable declared in a function is also visible to functions called by the parent function.
    #!/bin/bash

    function1 ()
    {
      local func1var=20

      echo "Within function1, \$func1var = $func1var."

      function2
    }

    function2 ()
    {
      echo "Within function2, \$func1var = $func1var."
    }

    function1

    exit 0

    # Output of the script:

    # Within function1, $func1var = 20.
    # Within function2, $func1var = 20.
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.
    #!/bin/bash

    #  Warning: Running this script could possibly lock up your system!
    #  If you're lucky, it will segfault before using up all available memory.

    recursive_function ()
    {
    echo "$1"     # Makes the function do something, and hastens the segfault.
    (( $1 < $2 )) && recursive_function $(( $1 + 1 )) $2;
    #  As long as 1st parameter is less than 2nd,
    #+ increment 1st and recurse.
    }

    recursive_function 1 50000  # Recurse 50,000 levels!
    #  Most likely segfaults (depending on stack size, set by ulimit -m).

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

    echo "This will probably not print."
    exit 0  # This script will not exit normally.

    #  Thanks, Stéphane Chazelas.

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