How to pass an argument to a script by ref in bash

Question

Please notice that I'm talking about passing arguments by-ref to a script, not to a function. I already checked here and here and they discuss about passing arguments using functions.

What I want is to call a script passing an argument to it and having the argument becoming a variable in the shell with the value assigned inside the script (which is exactly what happens with the read RESPONSE command, where RESPONSE becomes a variable in the current shell with the value assigned by the read command).

I know that using functions like this (using local -n):

#!/bin/bash

function boo() {
  local -n ref=$1
  ref="new"
}

SOME_VAR="old"
echo $SOME_VAR
boo SOME_VAR
echo $SOME_VAR

or like this (using eval):

function booeval() {
  ref=$1
  eval $(echo $ref=\"new\")
}

SOME_VAR="old"
echo $SOME_VAR
booeval SOME_VAR
echo $SOME_VAR

work as expected. But local (or declare/typedef) doesn't work outside of a function and eval seems to run in a subshell that when the script is over the variable doesn't survive in the shell. I wanted to have something like this:

$ cat /tmp/test.sh
#!/bin/bash

ref=$1
eval $(echo "export $ref=\"new value\"")
$ /tmp/test.sh VAR
$ echo $VAR

$

Any ideas?

Answer 1

As you are no doubt aware, what is assigned in the subshell stays in the subshell.

As noted in the comments, one solution is to not have a subshell: If the script is sourced then assignments will be retained. However this requires that the script be carefully written to not have unwanted side-effects.

If invoking the script normally (as a subshell) you might be able to work around the problem with convoluted IPC to pass values around but it seems to me you could bypass complexity entirely by simply having the script output the value instead of attempting to assign it to anything:

var=$(script)

You can wrap this using your first approach, so that the variable name can be passed in:

assign_to(){
    local -n ref=$1
    shift
    ref=$("$@")
}

For example:

$ myref=myvar
$ assign_to $myref date +%Y
$ declare -p myvar
declare -- myvar="2023"
$

Answer 2

A 'by reference' is not supported by bash.

There are a number of ways around it, using f.e. STDIO or using a separate file.

The STDIO works simple if a single variable is used:

parent_shell_var=$(bash script)

and just echo the value you want exported up.

A file can be used in this way: Parent script:

#!/bin/bash
hop=0
echo $hop
bash child_script tempfile
. tempfile
rm tempfile
echo $hop

child_script:

#!/bin/bash
echo "hop=1" > "$1"

These are very simplistic examples substitutes for the by-ref passing of arguments. These work for separate scripts as well as for subscripts.

Answer 3

Thank you all for the effort in answering the question. I will end up going with the gdb option to set the environmental variable using the parameters passed as arguments. As long as I do sudo, it will work just fine.

This is not intended to be a solution to be used without a lot of caution as you would be calling internal bash functions and things could be messed up. But in general it will run just fine.

Checking some ways to unset read-only variables, I stumbled upon using gdb to call the unbind_variable() function directly. So, I applied the same logic and set a new variable from the script. It "kinda" works, but not smoothly:

$ cat /tmp/test.sh
#!/bin/bash

ref="$1"
value="Value assigned from inside the script"
bashpid=$(ps -oppid,pid,cmd | grep $0 | grep -v grep | cut -f1 -d' ')

sudo gdb -ex 'call (int) setenv("'"$ref"'","'"$value"'",1)' --pid=$bashpid --batch 2>/dev/null 1>&2

And when I call it:

$ declare -p VARPASSED
bash: declare: VARPASSED: not found
$ /tmp/test.sh VARPASSED
$ declare -p VARPASSED
declare -x VARPASSED="Value assigned from inside the script"

Now the variable passed is correctly assigned to the current shell. The problem is that gdb requires sudo to attach to the current shell, which is a little bit annoying. How did they do with the read command?

PS: please cut some slack on the way to get the bash pid, I know there are much more efficient ways to do it. That was just not my focus ;)