Providing Bash substitution as a function argument

Question

I have a following loop that works well:

function uppercase_all_letters() {
  for filename in *;
    do echo "${filename^^}";
  done;
}

How do I provide "${filename^^}" as a parameter to the function?

I tried it the usual well (SUBSTITUTION="${filename^^}") but then Bash reports:

./test.sh: line 26: ${filename^^}: bad substitution

Answer 1

To do it literally, you'd probably have to resort to eval:

function subst_all_filenames() {
    subst="$1"
    for filename in *
    do
        eval 'echo ${filename'"$subst"'}'
    done
}

subst_all_filenames "^^"
subst_all_filenames ",,"
subst_all_filenames ""

Alternatively, support only a known list of substitutions:

function subst_all_filenames() {
    case "$1" in
      ^^) for filename in *; do echo "${filename^^}"; done ;;
      ,,) for filename in *; do echo "${filename,,}"; done ;;
       *) printf "%s\n" * ;;
    esac
}

Sometimes such a helper function also isn't even necessary in the first place. It's quite normal to have such loops all over the place in a shell script.