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- All right, let's talk
about systemd timers.

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So a timer is what is used to start

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a corresponding service at a specific time

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or on occurrence of a specific event.

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The timer and service need
to have the same filename

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just as we have seen for the sockets.

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And in in the timer you can
use OnCalendar to specify

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when the timer should be
started in a cron like style.

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And you can also use
OnBootSec or OnUnitActiveSec

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to run a timer based on other events.

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When using timers the timer is

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enabled or started and not the service.

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And for more information
have a look at the main page

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systemd.time

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Oh and in case you wonder how
significant are these timers?

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Well, these timers are
replacing good old cron jobs.

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On modern distributions and you will

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see that on CentOS for instance.

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Cron jobs are not the standard anymore.

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Systemd timers are the
standard for anything

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that needs to be scheduled in the future.

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Let's check it out.

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(typing)

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So let me start using systemctl
list unit files -t timer.

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I like this optional list unit files

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because list unit files is showing

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all the unit files not
only the active units.

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As an alternative, you can use list units

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if you only want to get information

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about stuff that is currently running.

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Now here for instance,
we have logrotate timer.

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That's something that
previously was started

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by using a cron job and now
it has become a systemd timer.

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So what this is doing well
systemctl cat logrotate.

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The timer is showing the timer
section OnCalendar is daily.

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AccuracySec is one hour
and Persistent is true.

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Which means we wanna
learn it every single day.

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More or less at the same time.

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AccuracySec is one hour.

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That means within a one hour difference

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of the previous time it was started.

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If you want to know what exactly the

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meaning is of all these options, use man.

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Man on systemd.timer which is showing

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what you can do inside the timer.

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But there's also man systemd.time

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which is more specifically
about the time and date

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specification that you can
use in your systemd timer.

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Good, let's check out another one.

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And that will be the fstrim.

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So systemctl cat on fstrim.timer

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is running fstrim on a weekly basis.

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Now, what is fstrim?

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Fstrim is about optimization
of storage on SSD devices.

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So if you're on SSD it
makes sense to run this.

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And you do that by using
Systemctl enable - - fstrim.timer

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and that is going to make sure

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that the corresponding fstrim.service

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is going to be started on a weekly basis.

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And that's all there is to
say about systemd timers.

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The replacement of cron in
model Linux distributions.