userList.sh

function userList () {
    # Help string
    helpText="Usage: basename [-h]|[--help] [--username] [--homedir] [--uid] [--gid] [--ugid]
     ----------------------------------------------------------------;
               Help: -h or --help, shows this help page.
           username: --username, fetches a list of usernames which are associated with a current user account.
            homedir: --homedir, fetches a space delimited list of usernames to home directores.
                uid: --uid, fetches a space delimited list of usernames to user ids.
                gid: --gid, fetches a space delimited list of usernames to group ids.
               ugid: --ugid, fetches a space delimited list of usernames to user ids and group ids.
     ----------------------------------------------------------------\n\n";

    if [[ $( echo "${#} < 1" | bc ) != 0 ]]; then
        echo -ne "${helpText}";
        return
    fi
    # Parameter Regexes
    # Note: Regexes structured with regard for parameter-case
    # in order to ensure smooth interchangability between grep and sed
    # without regard to version (of sed) or system. Per-tool case
    # sensitivity flags may have otherwise resulted in more brittle code.

    helpShort="^\-[Hh]$";
    helpLong="^\-\-[Hh][Ee][Ll][Pp]$";
    userName="^\-\-[Uu][Ss][Ee][Rr][Nn][Aa][Mm][Ee]$"
    homeDir="^\-\-[Hh][Oo][Mm][Ee][Dd][Ii][Rr]$"
    uid="^\-\-[Uu][Ii][Dd]$"
    gid="^\-\-[Gg][Ii][Dd]$"
    ugid="^\-\-[Uu][Gg][Ii][Dd]$"

    # Out of order parameter processing loop (script may be invoked with params given in any order).
    paramList=("${@}");
    for param in "${paramList[@]}";
    do
        # In general: Detect presence of parameter i.e. ( $(echo ${param} | grep -Po) != "" )
        # then handle the particulars using sed for access if the param is a key:value pair.

        # Handle help flag detecting and display usage info / help message.
        if [[ "$(echo "${param}" | \grep -Po '('${helpShort}')' )" != "" ]]; then
            echo -ne "${helpText}";
            return
        elif [[ "$(echo "${param}" | \grep -Po '('${helpLong}')' )" != "" ]]; then
            echo -ne "${helpText}";
            return
        fi

        # Handle username
        if [[ "$(echo "${param}" | \grep -Po '('${userName}')' )" != "" ]]; then
            \awk '{FS=":"};{print $1}' /etc/passwd;
        #Handle homedir
        elif [[ "$(echo "${param}" | \grep -Po '('${homeDir}')' )" != "" ]]; then
            \awk '{FS=":"};{print $1" "$6}' /etc/passwd;
        # Handle uid
        elif [[ "$(echo "${param}" | \grep -Po '('${uid}')' )" != "" ]]; then
            \awk '{FS=":"};{print $1" "$3}' /etc/passwd;
        # Handle gid
        elif [[ "$(echo "${param}" | \grep -Po '('${gid}')' )" != "" ]]; then
            \awk '{FS=":"};{print $1" "$3}' /etc/passwd;
        # Handle user & group id
        elif [[ "$(echo "${param}" | \grep -Po '('${ugid}')' )" != "" ]]; then
            \awk '{FS=":"};{print $1" "$3" "$4}' /etc/passwd;
        fi
    done;
}