#!/bin/bash

set -e

SYSTEMD_BACKUP_VERSIONS_DIR="/var/systemd/versions/"
LOG_FILE=$SYSTEMD_BACKUP_VERSIONS_DIR/log

function atomic_cp_to_root() {
        for f in $(find -type f); do
                cp --preserve=all $f $f.bak
                mkdir -p $(dirname /$f)
                mv $f.bak /$f
        done
}

function record() {
        case $1 in
                list)
                        versions=$(ls -t -I log ${SYSTEMD_BACKUP_VERSIONS_DIR} | tr "\n" " ")
                        echo "$(date "+%Y-%m-%d %H:%M:%S") [LIST]   $versions" >> $LOG_FILE
                        ;;
                switch)
                        echo "$(date "+%Y-%m-%d %H:%M:%S") [SWITCH] $2 -> $3" >> $LOG_FILE
                        ;;
                remove)
                        echo "$(date "+%Y-%m-%d %H:%M:%S") [REMOVE] $2" >> $LOG_FILE
                        ;;
        esac
}

echo -e "------!
Be careful to execute this tool, you MUST know what you are doing!
------!\n"

action="${1}"
case ${action} in
        list)
                if [ ${#} != 1 ]; then
                        echo "Too many arguments."
                        exit 1
                fi
                if [ ! -d ${SYSTEMD_BACKUP_VERSIONS_DIR} ] || [ "`ls -A $SYSTEMD_BACKUP_VERSIONS_DIR`" = "" ]; then
                        echo "No available versions."
                        exit 1
                fi
                echo "Versions:"
                ls -1t -I log ${SYSTEMD_BACKUP_VERSIONS_DIR}
                record list
                ;;
        switch-to)
                if [ ${#} -lt 2 ]; then
                        echo "Too few arguments."
                        exit 1
                fi
                if [ ${#} -gt 2 ]; then
                        echo "Too many arguments."
                        exit 1
                fi

                # make sure the user knows what he is doing
                echo "NOTE!!!"
                echo "After switching versions, \`rpm -q systemd\` will not report the right"
                echo "version of the installed systemd, use \`systemctl --version\` instead."
                echo ""
                read -r -p "Are You Sure To Continue? [Y/n] " input
                case $input in
                        [yY])
                                ;;
                        *)
                                exit 1
                                ;;
                esac

                version="${2}"
                if [ ! -d "${SYSTEMD_BACKUP_VERSIONS_DIR}${version}" ]; then
                        echo "No such systemd version installed: ${version}."
                        exit 1
                fi
                if [ `id -u` -ne 0 ]; then
                        echo "Need to be root."
                        exit 1
                fi
                from=$(systemctl --version | sed -n '1p' | awk -F "(" '{print $2}' | awk -F ")" '{print $1}')
                cd ${SYSTEMD_BACKUP_VERSIONS_DIR}${version}
                atomic_cp_to_root
                record switch $from $version
                /sbin/ldconfig &>/dev/null || :
                systemctl daemon-reexec &>/dev/null || :
                systemctl try-restart systemd-udevd.service &>/dev/null || :
                ;;
        remove)
                if [ ${#} -lt 2 ]; then
                        echo "Too few arguments."
                        exit 1
                fi
                if [ ${#} -gt 2 ]; then
                        echo "Too many arguments."
                        exit 1
                fi
                version="${2}"
                if [ ! -d "${SYSTEMD_BACKUP_VERSIONS_DIR}${version}" ]; then
                        echo "No such systemd version installed: ${version}."
                        exit 1
                fi
                rm -rf "${SYSTEMD_BACKUP_VERSIONS_DIR}${version}"
                record remove $version
                ;;
        *)
                echo "`basename ${0}`: Usage:"
                echo "    list                 List installed systemd versions"
                echo "    switch-to VERSION    Switch to the specified systemd version"
                echo "    remove    VERSION    Remove the backup files of the specified systemd version"
                exit 1
                ;;
esac