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

Local variables and recursion.

#!/bin/bash
# recursion-demo.sh
# Demonstration of recursion.
RECURSIONS=9   # How many times to recurse.
r_count=0      # Must be global. Why?
recurse ()
{
  var="$1"
  while [ "$var" -ge 0 ]
  do
    echo "Recursion count = "$r_count"  +-+  \$var = "$var""
    (( var-- )); (( r_count++ ))
    recurse "$var"  #  Function calls itself (recurses)
  done              #+ until what condition is met?
}
recurse $RECURSIONS
exit $?

#!/bin/bash
# recursion-def.sh
# A script that defines "recursion" in a rather graphic way.
RECURSIONS=10
r_count=0
sp=" "
define_recursion ()
{
  ((r_count++))
  sp="$sp"" "
  echo -n "$sp"
  echo "\"The act of recurring ... \""   # Per 1913 Webster's dictionary.
  while [ $r_count -le $RECURSIONS ]
  do
    define_recursion
  done
}
echo
echo "Recursion: "
define_recursion
echo
exit $?

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

#!/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.

#!/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.