#!/usr/bin/env bash

# CONSTANTS

BASEDIR=$(dirname "$0")
COMMAND=show

# FUNCTIONS
function usage {
    echo "usage: $(basename "$0") [--set NEW_FQDN]"
    echo "       either returns the fully qualified name when pass with no argument"
    echo "       or define the new fully qualified name when pass --set {NEW_FQDN}"
}

function parse_options {
    while [[ $# -gt 0 ]]; do
        case "$1" in
        '--set')
            COMMAND=define
            shift 1
            [[ -z ${1:-} || $1 =~ ^- ]] && usage && exit 1
            NEW_FQDN=$1
            ;;
        --help | -h)
            usage && exit 0
            ;;
        *)
            echo >&2 "Unknown option: $1" && usage && exit 2
            ;;
        esac

        shift 1 # Move to the next argument
    done
}

function show {
    hostname -f
}

function define {
    if [[ $NEW_FQDN == *.* ]]; then
        name=$(echo $NEW_FQDN | cut -d. -f1)
        domain=${NEW_FQDN#${name}.}
        if [[ -n $domain ]]; then
            define_fqdn $name $domain
        else
            define_single $name
        fi
    else
        define_single $NEW_FQDN
    fi
}

function define_single {
    local single=$1
    local previous=$(hostname -f)
    for i in $(grep $previous /etc/hosts | cut -f1); do
        append_or_replace ^$i.* "$i\t$single" /etc/hosts >/dev/null
    done
    hostnamectl hostname $single
}

function define_fqdn {
    local name=$1
    local domain=$2
    local previous=$(hostname -f)

    for i in $(grep $previous /etc/hosts | cut -f1); do
        append_or_replace ^$i.* "$i\t$name.$domain $name" /etc/hosts >/dev/null
    done
    hostnamectl hostname $name
}

function perform_command {
    case $COMMAND in
    show)
        show
        ;;
    define)
        [[ $(id -u) != 0 ]] && echo 'require root privilege, please prepend the command with `sudo`' && exit 10
        define
        ;;
    esac
}

# MAIN

set -Eue
parse_options $*
perform_command