====== Ubuntu - Systemd - Timers ======

To have a systemd service not start automatically at system boot time, but instead to run at a specific time.

  * Create a systemd timer for the systemd service. 

----

===== Example =====

Create a systemd service (/etc/systemd/system/iris.service) for the Iris eye protection software.

<file bash /etc/systemd/system/iris.service>
[Unit]
Description=Iris Eye Protection Software 
After=lightdm.service

[Service]
  Type=oneshot
  Environment=DISPLAY=:0
  User=your_username
  ExecStart=/bin/bash -c 'DISPLAY=:0 xhost 127.0.0.1 && /opt/iris/Iris.sh'
  KillSignal=SIGINT
  Restart=on-failure
  RestartSec=2

[Install]
  WantedBy=multi-user.target
</file>

----

===== To have it automatically start at boot time =====

<code bash>
sudo systemctl enable iris.service
</code>

----

===== To start only at night =====

Create a systemd timer.

<code bash>
sudo vi /etc/systemd/system/iris.timer
</code>

...add the following lines in this file:

<file bash /etc/systemd/system/iris.timer>
[Unit]
Description=Iris Night Mode Timer
Requires=iris.service

[Timer]
Unit=iris.service
OnCalendar=*-*-* 18:00:00

[Install]
WantedBy=timers.target
</file>

<WRAP info>
**NOTE:** This systemd timer will start the iris.service at 18:00:00 every day.
</WRAP>

----

===== Enable this systemd timer =====

<code bash>
sudo systemctl enable iris.timer
</code>

----

===== Check the status of this timer =====

<code bash>
sudo systemctl status iris.timer
</code>

returns:

<code bash>
● iris.timer - Iris Night Mode Timer
     Loaded: loaded (/etc/systemd/system/iris.timer; enabled; vendor preset: enabled)
     Active: active (waiting) since Sat 2023-06-03 17:32:12 +00; 24min ago
    Trigger: Sat 2023-06-03 18:00:00 +00; 22h left
   Triggers: ● iris.service
</code>