Triggering regular tasks with Systemd

Alarm Clock

Article from Issue 214/2018
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Systemd can start timers that automatically perform tasks at specified times. The configuration files are known as timer units.

You might want to use your Linux system to automatically create a backup every evening and rotate the log files at regular intervals. In most distributions, time-controlled tasks are handled by the Cron daemon. But Systemd is an interesting alternative to Cron. Systemd controls the startup process of most distributions, and it can also trigger time-controlled and recurring tasks.

Service Providers

The first task is to tell Systemd which task to perform. To do this, you create a configuration file, the Service Unit. Listing 1 shows an example.

Listing 1

Service Unit

[Unit]
Description=Create a backup of the system
[Service]
ExecStart=/usr/bin/backup.sh /mnt

A service unit is a text file divided into several sections. The [Service] section is required. ExecStart= is followed by the command to be executed by the system. In Listing 1, Systemd would simply run a script that backs up the system to the /mnt directory. The [Unit] section adds some metadata. In the simplest case, Description= is followed by a description of the task.

Service Units usually tell Systemd which services to boot when the system starts. (See the article on Systemd units elsewhere in this issue.) Systemd also supports additional sections and settings. However, since the system just needs to schedule the task, these settings are not (absolutely) necessary. In particular, you can leave out the complete [Install] section.

Save the newly created service unit to /etc/systemd/system. The filename corresponds to the (internal) name of the service unit. It must be unique among all service units and end with .service, as in backup.service. Systemd can also start existing service units or service units supplied by the distribution on a time-controlled basis. In this case, simply make a note of the filename of the service file.

Tick-Tock

To avoid burning the cake to a crisp, most hobby bakers set a kitchen timer. In a similar way, you need to set a separate timer for a task you wish to assign to Systemd.

First, create a new text file in the /etc/system/system subdirectory. The text file should have the same filename as the service unit you created earlier, but it ends with .timer. In the example, the file would be named backup.timer. In Systemd speak, the file with the .timer extension is known as the timer unit. In the timer unit, you describe when the timer should "go off," at which point, Systemd will start the backup.

The structure of a timer unit is very similar to that of a service unit. As the example from Listing 2 shows, it typically consists of three sections: [Unit] is followed by general information about the timer. In Listing 2, this information would include a Description= that serves mainly as a reminder for the user. Make a note on why the timer exists and what actions it triggers.

Listing 2

Timer Unit

[Unit]
Description=Create a daily backup of the system
[Timer]
OnCalendar=*-*-* 18:15:00
Persistent=true
RandomizedDelaySec=2h
[Install]
ZWantedBy=timers.target

Current Events

In the next section, [Timer], you tell Systemd when to start the task. Make a note of this time after OnCalendar= in the notation weekday year-month-day hour:minutes: seconds. The setting OnCalendar=Fr 2018-11-30 12:00:00 tells Systemd to create the backup on Friday, November 30, 2018 at noon precisely. You can omit unnecessary information, such as the day of the week or the seconds.

Normally, you will not want Systemd to run the task once only, but repeat it. To set up a repeating event, you can simply list the corresponding days, dates, and times separated by commas. In the example from the first line of Listing 3, Systemd starts the backup November 30, 2018 at 1AM and 12 Noon.

Listing 3

Date and Time

OnCalendar=2018-11-30 01,12:00:00
OnCalendar=2018-01..12-01 01,12:00:00
OnCalendar= 2018-*-01 01,12:00:00

You can also abbreviate the number ranges with two dots .., which means that you do not have to list all the months, for example. The entry from the second line of Listing 3, tells Systemd to take action on the first day of each month. If the statement applies to all months, you can also use the wildcard * (line three).

The *-*-* entry from Listing 2 tells Systemd to run the backup every day at 18:15 in every month and every year.

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