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Let's talk about systemd-timers. A systemd-timer is an alternative for a cronjob. And cronjobs

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in the past have always been used to start some tasks at a specific moment. Currently,

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timers and cronjobs are used side by side, but systemd-timers are slowly replacing cronjobs.

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For Red Hat Linux, for instance, you will notice that there is no more default cronjobs.

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It's all done by timers if it needs to be done periodically. You can use systemctl listunits

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minus t-timer for a list of timers that are currently active. Timers have a matching service

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file, and the service file typically has the same name. That is how the timer finds the

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matching file. For instance, fstrim.timer works with fstrim.service. And to use timers,

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you enable the timer. You don't enable the service. Because you don't want the service

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to be running at all time, you want the timer to be running and to trigger the service when

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the time is there. So systemctl enable fstrim.timer is what you would be doing. You can consult

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man systemd.timer for more details about the timer unit type, and also systemd.time

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for more details about the time settings that you can use. Let me show you. So systemctl

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listunit files minus t-timer. I'm doing listunit files here, because listunit files normally

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gives a little bit more information. And what do we see? Do we have anything interesting?

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Well, we can see fstrim.timer. We can analyze what is going on in fstrim.timer. So let's

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use systemctl status fstrim.timer, where we can see the unit file that it has loaded.

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It's enabled. The preset is enabled, so it was enabled by default. And it's active and

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currently waiting. Then we can see a trigger, three days left. And the trigger, so it will

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trigger fstrim.service. So systemctl status on fstrim.service is giving me what? It's

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only giving me active, inactive, and triggered by fstrim.timer. And the green dot is indicating

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that fstrim.timer is indeed listening. So if we check the contents of the timer, fstrim.timer

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in systemctl cat, then we see the timer section. And in the timer section, you define how you

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want it to be started. So on calendar is weekly. You should do this on a weekly basis. Accuracy

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sec is one hour, which means more or less within an hour time frame of the last time

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it started. Persistent is true, which means we keep the information about the timer activity

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across reboots. And randomized delay seconds, every time we give a 100 minutes randomization.

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If you want to know more about the way so you can run a timer, I can really recommend

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to have a look at man systemd.time. Timer is for the unit. Time is for the way how you

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can define it. And you get information about timestamps and even about calendar events

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right here. So it's very precise in when exactly you want to run it. And if we have a look

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at systemd.timer, well, you get some more information about the options. I like table

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one here. In table one, we see some interesting options that allow you to run the timer really

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in a conditional way. So on active sec means that you are going to run it specific moment

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after the timer has been activated. On boot sec means that you are going to run it after

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booting. On unit active sec, that's a very nice one that is going to monitor another

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unit and is going to run a couple of seconds, minutes, hours, or whatever after that unit

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has been activated. So lots of options with your systemd.timer. You can still run crontabs

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if you want to. I mean, I can still run crontab minus e student. Sorry, crontab minus e for

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the editor minus u student and put something like slash star slash 10 for every 10 minutes.

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If you want to get longer greetings from students, that will still work, but timer has become the

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default. So on rel 10, you don't find any crontabs anymore. That will show if you use

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ls on etc crontab star dot star. As you can see, it's nearly empty. Timers are the standard nowadays.