Script creates udev and systemd configuration files for my auto-starting backup system. Beware: backup program is missing here!

backup-reinstall.sh

#!/usr/bin/env bash

##############################################################################
#
#  Manual configuration:
#
readonly        IS_TEST=false
readonly      CRYPTNAME=cryptback
readonly     SOURCE_DIR=/mnt/data
readonly     BACKUP_DIR=/mnt/backup
readonly BACKUP_DEVNAME=backup
readonly        PROFILE=esprimo



##############################################################################
#
#  Auto configuration and check of the prerequisites:
#
if ! timeout 1s xset q &>/dev/null  ||  [ "${EUID}" -ne 0 ]
then
	>&2 echo "[FATAL] Please run as root in your regular X user session."
	exit 1
fi

readonly ABS_APP_DIR=$( dirname $( readlink -f "$0" ))
readonly  BACKUP_CMD="${ABS_APP_DIR}/backup.sh"
readonly  CRYPT_UUID=$( grep -Po "^\s*${CRYPTNAME}\s+UUID=\K[^\s]+" /etc/crypttab )
readonly      X_USER=${USER}

if [ ! -f "${BACKUP_CMD}" ]
then
	>&2 echo "[FATAL] Missing file ${BACKUP_CMD}"
	exit 1
fi

if [ -z "${CRYPT_UUID}" ]
then
	>&2 echo "[FATAL] Missing UUID for ${CRYPT_UUID} in /etc/crypttab"
	exit 1
fi

if [ "${IS_TEST}" = true ]
then
	readonly PREFIX="${ABS_APP_DIR}/test"
else
	readonly PREFIX=
fi

mkdir -p "${PREFIX}/lib/systemd/system"
mkdir -p "${PREFIX}/etc/udev/rules.d"



##############################################################################
#
#  Start installation:
#
echo "[INFO] Installing to '${PREFIX}/' directory..."

###########
#
#  Config-files for our $BACKUP_CMD
#
cat > "${PREFIX}/etc/backup-${PROFILE}" << EOL
readonly X_USER=${X_USER}
readonly SOURCE_DIR="${SOURCE_DIR}"
readonly BACKUP_DIR="${BACKUP_DIR}"
readonly CRYPTNAME=${CRYPTNAME}
readonly LOGPATH="/var/log/backup-${PROFILE}.log"
readonly EXCLFILE_PATH="${PREFIX}/etc/backup-${PROFILE}.exclude"
readonly NOTIFY_ICOPATH="${ABS_APP_DIR}/icon.png"
EOL

cat > "${PREFIX}/etc/backup-${PROFILE}.exclude" << EOL
*.swp
*.spam
*.*~
EOL


###########
#
#  This udev rule creates /dev/$BACKUP_DEVNAME
#  (systemd recognizes devices tagged 'systemd') 
#  and executes $BACKUP_CMD by triggering our systemd unit.
#  Calling $BACKUP_CMD directly via RUN+="..." 
#  would cause errors (long running, crypt-deps, ...).
#
cat > "${PREFIX}/etc/udev/rules.d/999-backup-${PROFILE}.rules" << EOL
ACTION=="add",                          \\
     ENV{ID_FS_UUID}=="${CRYPT_UUID}",  \\
     TAG+="systemd",                    \\
     SYMLINK+="${BACKUP_DEVNAME}",      \\
     ENV{SYSTEMD_WANTS}="backup-${PROFILE}.service"
EOL


###########
#
#  Systemd-Unit:
#  Environment variables are required for 'notify-send' from
#  systemd as root user. Values stem from the regular X user.
#  Don't save to /etc/systemd/system because you won't be
#  able to run systemctl mask 
#  (which creates same-named symlinks in same dir)
#
cat > "${PREFIX}/lib/systemd/system/backup-${PROFILE}.service" << EOL
[Unit]
Description=Backup
After=dev-${BACKUP_DEVNAME}.device
Wants=dev-${BACKUP_DEVNAME}.device

[Service]
ExecStart=${BACKUP_CMD}
Environment=DISPLAY=${DISPLAY}
Environment=XAUTHORITY=${XAUTHORITY}
Environment=DBUS_SESSION_BUS_ADDRESS=${DBUS_SESSION_BUS_ADDRESS}
Type=oneshot

[Install]
WantedBy=user.target
EOL


###########
#
#  Reload everything:
#
if [ "${IS_TEST}" = false ]
then
	systemctl daemon-reload            &&
		udevadm control --reload   &&
		udevadm trigger

	systemctl unmask backup-${PROFILE}
fi