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. [110]

Example 24.12. Local variable visibility

# 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"


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

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.

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.


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.

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.


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

function0 ()

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

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



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. [115]

Example 24.15. Recursion, using a local variable


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

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


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

if [ "$1" -gt $MAX_ARG ]
  echo "Out of range ($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

fact ()
  local number=$1
  #  Variable "number" must be declared as local,
  #+ otherwise this doesn't work.
  if [ "$number" -eq 0 ]
    factorial=1    # Factorial of 0 = 1.
    let "decrnum = number - 1"
    fact $decrnum  # Recursive function call (the function calls itself).
    let "factorial = $number * $?"

  return $factorial

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

exit 0

Also see Example A.15, “Generating prime numbers using the modulo operator” 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.

[110] However, as Thomas Braunberger points out, a local variable declared in a function is also visible to functions called by the parent function.


function1 ()
  local func1var=20

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


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


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.

[111] Otherwise known as redundancy.

[112] Otherwise known as tautology.

[113] Otherwise known as a metaphor.

[114] Otherwise known as a recursive function.

[115] Too many levels of recursion may crash a script with a segfault.


#  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.