How to concatenate to string variable from inside function in bash script?

Question

I want to be able to call a function, and keep all the modifications that the function will perform on the string.

For example: I tried this script

#!/bin/bash

change_string() {
  var=$1
  var+=",Hello2"
  echo "Function: $var"
}

myString="Hello1"
change_string $myString

echo "Main: $myString"

This will print out:

Function: Hello1,Hello2
Main: Hello1

Is there any way to change the $myString inside function call, but keep those changes in main? I think I probably missed something in bash related to pass-by-reference?

Answer 1

With Bash 4.3+ (namerefs!)

Modern versions of bash provide "nameref" support, which allows a variable to refer to another.

#!/usr/bin/env bash

case $BASH_VERSION in
  [123].*|4.[012].*) echo "This needs bash 4.3 or newer" >&2; exit 1;;
esac

# local variable is prefixed because this will fail if we're passed a variable name we use
# internally; prefixing the local names makes such collisions unlikely.
change_string() {

  # make change_string__var an alias for the variable named in our argument
  declare -n change_string__var=$1

  # append to that variable
  change_string__var+=",Hello2"

  # ...and log the new value, 'cuz that's what the original code did.
  echo "Function: $change_string__var"
}

myString="Hello1"
change_string myString  ## pass variable **name**, not variable value
echo "Main: $myString"

---

With Bash 3.x+ (indirect evaluation + indirect assignment)

Alternately, as a less-newfangled approach, one can use ${!var} to do an indirect reference, and printf -v to do an indirect assignment.

#!/usr/bin/env bash

change_string() {

  # Store the variable name in a regular local variable
  local change_string__var=$1

  # Use ${!var} to get the value of the variable thus named
  local change_string__val=${!change_string__var}

  # Use ''printf -v varname ...'' to assign a new value
  printf -v "$change_string__var" %s "${change_string__val},Hello2"
}

myString="Hello1"
change_string myString
echo "Main: $myString"