Why 'getopts' within a function fails to work?

Question

function readArgs() {
    while getopts "i:o:p:s:l:m" OPTION; do
        case "$OPTION" in
            i)
                input="$OPTARG"
                ;;
            o)
                output="$OPTARG"
                ;;
            ...
        esac
    done
}

readArgs

if [[ -z "$input" ]]; then
    echo "Not set!"
fi

This is always giving me Not set! but if I comment out the lines function readArgs() {, } and readArgs, it works. Why?

Also,

input="$OPTARG"
echo "$input"
;;

does not work.

Answer 1

getopts is parsing the arguments to the readArgs function, and you're not giving that function any arguments.

Try with:

readArgs "$@"

Answer 2

getopts relies on the OPTIND variable being initialized to 1. Either do

readArgs() { OPTIND=1; ...

or

readArgs() { local OPTIND; ...