SYSTEMD

SYSTEM Daemon Starts and monitors system and user services.

Overview

Command	Description
`systemctl status <serivename>.service`	Status of service (active, inactive, enabled, disabled, ...)
`systemctl list-timers <servicename>.timer`	Status of service timer (time since last run, next planned start, ...)
`/etc/systemd/system/`	Directory of self created services

Create timed service

Create service entry

sudo nano /etc/systemd/system/<servicename>.service

[Unit]
Description=<what the service / called script does>
# Wants=network-online.target
# After=network-online.target

[Service]
Type=oneshot|simple
# RemainAfterExit=no
User=<user for script>
Group=<group of files?>
Environment=HOME=<home directory>
ExecStart=<script with absolute path>
# ExecStart=/usr/bin/flock --nonblock /tmp/<lock filename>.lock <script with absolute path>
# ExecStop=

# [Install]
# WantedBy=multi-user.target|timers.target

wants/after network-online.target mean that this service needs the network to be online
Type oneshot is for short running tasks like backup scripts, simple for permanently running things like server and databases
additional line RemainAfterExit=yes would be for things like mount which result is active even if the service has exited - then also an additional lne for ExecStop would be necessary to unmount the service
Environment could also be directory of python (inside a virtual environment)
flock create lock file to prevent different backups running in parallel
nonblock means, that additional starts doesn't wait but stop executing
Restart=on-failure means restart if service crashed
RestartSec=5 delay for automatic restart
Install WantedBy tells the service what to do when systemctl enable is called

Test (only needed, if not started by timer (see below))

sudo systemctl daemon-reload
sudo systemctl start <servicename>.service
sudo systemctl status <servicename>.service

Create timer

sudo nano /etc/system/system/<servicename>.timer

[Unit]
Description=<name and how often as title>

[Timer]
OnCalendar=*-*-* 03:15:00
Persistent=false
Unit=<servicename>.service

[Install]
WantedBy=timers.target

OnCalendar sets time for daily start
Persistent=true would mean, that if Raspi would be offline during start time, the run will be catched up after the next start

Enable / activate Timer

sudo systemctl daemon-reload
sudo systemctl enable --now <servicetimer>.timer
systemctl status <servicetimer>.timer

--now starts the timer after enabling

List all timers

systemctl list-timers or systemctl list-timers <servicename>.timer