In bash, should I unset a local variable inside a function?

Question

Should I unset my local variables at the end of a function in a bash script? As an example, the following function:

square()
{
  local var=$1
  (( var = var * var ))
  echo $var
  ## should I unset $var here??
}

Just curious about best practices, thanks!

Answer 1

If you weren't using the local command, then you would want to unset the variable before leaving the function to avoid polluting the global namespace.

square () {
    var=$1    # var is global, and could be used after square returns
    (( var = var * var ))
    echo $var
    unset var  # Remove it from the global namespace
}

The problem with that approach is that square doesn't know if it actually created var in the first place. It may have overwritten and ultimately unset a global variable that was in use before square was called.

Using local, you are guaranteed to create a new variable that is only visible inside the function. If there were a global var, it's value is ignored for the duration of the function. When the function exits, the local var is discarded, and the global var (if any) can be used as before.

$ var=3
$ echo $var
3
$ square 9
81
$ echo $var
3

Answer 2

If that comment ## should I unset $var here?? is the end of your subroutine, you would not need to unset it, because it would go out of scope right after that anyway.