Shell script functions that can take arguments as either named or abbreviated values

while [ $# -gt 0 ]; do
    case "$1" in
        --from*|-f*)
            if [[ "$1" != *=* ]]; then shift; fi # Value is next arg if no `=`
            local FROM="${1#*=}"
        ;;
        --to*|-t*)
            if [[ "$1" != *=* ]]; then shift; fi # Value is next arg if no `=`
            local TO="${1#*=}"
        ;;
        --file*|-z*)
            if [[ "$1" != *=* ]]; then shift; fi # Value is next arg if no `=`
            local SINGLEFILE="${1#*=}"
        ;;
        --help|-h)
            doc_convert_usage
            return;
        ;;
        *)
            >&2 printf "Error: Invalid argument\n"
            return;
        ;;
    esac
    shift
done

What does this code do?

• First we are grabbing the positional parameters
• Then we loop through each parameters and pass them through a case statement
• Depending on what we pass in we will be able to check for a named argument or an abbreviated version using the pipe symbol --from*|-f*).
• In each case we check if an equals sign was used. This gives us the ability to pass in arguments as myfunction arg1=somevalue1 or as myfunction arg1 somevalue1.

We get the best of both worlds for passing in values to our shell scripts.

Example in context

Here is a full example I’ve used in the past to show the above code in context of a full function. This is the function I use to quickly convert word documents into pdfs through the command line.

doc_convert_usage() {
    echo "doc_convert -f docx -t pdf"
    echo "-f\t from\t default=docx|doc\t The file type you are wanting to convert"
    echo "-t\t to\t default=pdf\t\t The file type you are converting to"
    echo "-z\t file\t default=none\t\t Use to target a single"
}

doc_convert() {
    command -v soffice >/dev/null 2>&1 || { echo "Install LibreOffice and add soffice to your path. Aborting." >&2; return; }
    command -v greadlink >/dev/null 2>&1 || { echo "You need coreutils, instal with: brew install coreutils. Aborting." >&2; return; }

    while [ $# -gt 0 ]; do
        case "$1" in
            --from*|-f*)
                if [[ "$1" != *=* ]]; then shift; fi # Value is next arg if no `=`
                local FROM="${1#*=}"
            ;;
            --to*|-t*)
                if [[ "$1" != *=* ]]; then shift; fi # Value is next arg if no `=`
                local TO="${1#*=}"
            ;;
            --file*|-z*)
                if [[ "$1" != *=* ]]; then shift; fi # Value is next arg if no `=`
                local SINGLEFILE="${1#*=}"
            ;;
            --help|-h)
                doc_convert_usage
                return;
            ;;
            *)
                >&2 printf "Error: Invalid argument\n"
                return;
            ;;
        esac
        shift
    done

    if [ -z "${FROM}" ]; then
        local FROM="docx|doc"
    fi

    if [ -z "${TO}" ]; then
        local TO="pdf"
    fi

    echo "\n**************************\n"
    echo "You are about to convert \"${FROM}\" files to \"${TO}\":";

    local PERMISSONTOCHANGEFILES
    vared -p "Convert? (y/n): " -c PERMISSONTOCHANGEFILES

    if [[ "$PERMISSONTOCHANGEFILES" == "y" ]]; then
        if [ -z "${SINGLEFILE}" ]; then
            for DOCUMENT in $(find -E . -type f -iregex ".*\.(${FROM})$")
                do
                    local FILE=$(greadlink -f "${DOCUMENT}")
                    local FILEURL=$(dirname "${FILE}")

                    soffice --headless --convert-to "${TO}" "${DOCUMENT}" --outdir "${FILEURL}"
                done
        else
            soffice --headless --convert-to "${TO}" "${SINGLEFILE}"
        fi
    else
        echo "\nNothing has been converted\n"
    fi

    return;
}