It’s time to see an example on timers in systemd.
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 !