Passing arguments by reference

Question

I want to ask if it is possible to pass arguments to a script function by reference:

i.e. to do something that would look like this in C++:

Answer 1

Use a helper function upvar:

# Assign variable one scope above the caller.
# Usage: local "$1" && upvar $1 value [value ...]
# Param: $1  Variable name to assign value to
# Param: $*  Value(s) to assign.  If multiple values, an array is
#            assigned, otherwise a single value is assigned.
# NOTE: For assigning multiple variables, use 'upvars'.  Do NOT
#       use multiple 'upvar' calls, since one 'upvar' call might
#       reassign a variable to be used by another 'upvar' call.
# See: http://fvue.nl/wiki/Bash:_Passing_variables_by_reference
upvar() {
    if unset -v "$1"; then           # Unset & validate varname
        if (( $# == 2 )); then
            eval $1=\"\$2\"          # Return single value
        else
            eval $1=\(\"\${@:2}\"\)  # Return array
         fi
    fi
}

And use it like this from within Newfun():

local "$1" && upvar $1 new

For returning multiple variables, use another helper function upvars. This allows passing multiple variables within one call, thus avoiding possible conflicts if one upvar call changes a variable used in another subsequent upvar call.

See: http://www.fvue.nl/wiki/Bash:_Passing_variables_by_reference for helper function upvars and more information.

The problem with:

eval $1=new

is that it's not safe if $1 happens to contain a command:

set -- 'ls /;true'
eval $1=new  # Oops

It would be better to use printf -v:

printf -v "$1" %s new

But printf -v cannot assign arrays.

Moreover, both eval and printf won't work if the variable happens to be declared local:

g() { local b; eval $1=bar; }  # WRONG
g b                            # Conflicts with `local b'
echo $b                        # b is empty unexpected

The conflict stays there even if local b is unset:

g() { local b; unset b; eval $1=bar; }  # WRONG
g b                                     # Still conflicts with `local b'
echo $b                                 # b is empty unexpected