Before we start, let’s clarify some things.
systemd’s Timers are units. Units are the simplest form of systemd files. Units are describing “when and if” a unit service “should or must” run, based on real or relative time.
A real time example is similar to a cron job entry. You can find out all the configuration settings/values for OnCalendar here.
A relative time example is more close to something like:
“Run this unit service, ten minutes after boot, before this service and also that services must have already been started cause I am depend on them.”
List of systemd-timers
To view the declared timers on a systemd, run the below command:
$ systemctl list-timers
we can see all timers, even the in-active ones, with:
# systemctl list-timers --all
one simple example
ok, let’s start with an example.
I’ll use the /usr/local/bin directory to store my custom scripts, as this directory is in the PATH enviroment variable and I can run these scripts from anywhere.
Our systemd unit files, must be under the /etc/systemd/system/ directory.
Part One: The Script
As an example, the script will mount some volumes after boot time.
The basic script contents the below lines:
# cat /usr/local/bin/mount.volumes.sh
#!/bin/sh /usr/bin/mount /mnt/backup /usr/bin/mount /var/lib/docker
and make it executable:
# chmod +x /usr/local/bin/mount.volumes.sh
You can run this script, once or twice to see if everything goes as planned.
Part Two: The Service
Now it’s time to create a systemd service unit:
# vim /etc/systemd/system/mount.volumes.service
[Unit] Description=Mount Backup & Docker Volume Service [Service] Type=simple ExecStart=/usr/local/bin/mount.volumes.sh [Install] WantedBy=multi-user.target
Part Three: The Timer
Now it is time to create the systemd timer unit:
# vim /etc/systemd/system/mount.volumes.timer
We have to decide when we want to service to run.
eg. Every day, but 45sec after boot
[Unit] Description=Mount Backup & Docker Volume @ reboot [Timer] OnBootSec=45sec OnUnitActiveSec=1day Unit=mount.volumes.service [Install] WantedBy=multi-user.target
Time to wait after booting before we run first time: OnBootSec
Time between running each consecutive time: OnUnitActiveSec
Part Four: Enable Service
Be aware, we havent finished yet!
Check that systemd can identify these files:
# systemctl list-unit-files | egrep mount.volumes
mount.volumes.service disabled mount.volumes.timer disabled
We can run the systemd service by hand:
# systemctl start mount.volumes.servicee
and see the ouput/results via journalct:
# journalctl -f
Part Five: Enable Timer
finally we need to start & enable (so that runs after reboot) the timer:
# systemctl start mount.volumes.timer
# systemctl enable mount.volumes.timer
Created symlink /etc/systemd/system/multi-user.target.wants/mount.volumes.timer → /etc/systemd/system/mount.volumes.timer.
# systemctl list-timers | egrep mount.volume
Sat 2018-01-27 09:51:01 EET 23h left Fri 2018-01-26 09:51:01 EET 1min 16s ago mount.volumes.timer mount.volumes.service
# systemctl list-unit-files | egrep mount.volume
systemctl list-unit-files | egrep mount.volume mount.volumes.service disabled mount.volumes.timer enabled
To all the systemd haters, I KNOW, its one line on crontab !