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What is the purpose of a timer in systemd?
Timers are systemd unit files whose name ends in .timer that control .service files or events. Timers can be used as an alternative to cron (read #As a cron replacement ). Timers have built-in support for calendar time events, monotonic time events, and can be run asynchronously.
When to use systemd timers instead of cron jobs?
Like cron jobs, systemd timers can trigger events—shell scripts and programs—at specified time intervals, such as once a day, on a specific day of the month (perhaps only if it is a Monday), or every 15 minutes during business hours from 8am to 6pm. Timers can also do some things that cron jobs cannot.
What is the fourth line of a systemd timer?
The fourth line contains the date and time the timer will be triggered next and an approximate time until the trigger occurs. The fifth line shows the name of the event or the service that is triggered by the timer. Some (but not all) systemd unit files have pointers to the relevant documentation.
When does systemd stop a service in a unit?
If you have Wants=a.service in a unit called b.service, when it starts, b.service will run a.service if it is not running already. Likewise, if you have a line that reads Conflicts= a.service in your b.service unit, as soon as b.service starts, systemd will stop a.service.
How to schedule tasks with systemd timers in Linux?
Depending on how the schedule is created, a timer can be: Systemd provides a list of keywords we can use in a timer unit to schedule the execution of a task a certain amount of time after a predefined event takes place. The keywords must be used in the [Timer] section of the timer unit. Let’s see them and explain their meaning:
Can a systemd timer be triggered by a cron event?
Systemd timer units can be triggered by events like boot and hardware changes. Can be easily disabled or enabled using systemctl. Timer units can use real time or monotonic time. Cron has two glaring benefits over systemd timers.
When to disable defaultdependencies option in systemd.timer?
Only timer units involved with early boot or late system shutdown should disable the DefaultDependencies= option. Timer units with at least one OnCalendar= directive acquire a pair of additional After= dependencies on time-set.target and time-sync.target, in order to avoid being started before the system clock has been correctly set.