61 lines
No EOL
2.3 KiB
Markdown
61 lines
No EOL
2.3 KiB
Markdown
---
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obj: concept
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---
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# Systemd Timers
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Timers are _systemd_ unit files with a suffix of _.timer_. Timers are like other unit configuration files and are loaded from the same paths but include a `[Timer]` section which defines when and how the timer activates. Timers are defined as one of two types:
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- **Realtime timers** (a.k.a. wallclock timers) activate on a calendar event, the same way that cronjobs do. The option `OnCalendar=` is used to define them.
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- **Monotonic timers** activate after a time span relative to a varying starting point. They stop if the computer is temporarily suspended or shut down. There are number of different monotonic timers but all have the form: `On_Type_Sec=`. Common monotonic timers include `OnBootSec` and `OnUnitActiveSec`.
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For each _.timer_ file, a matching _.service_ file exists (e.g. `foo.timer` and `foo.service`). The _.timer_ file activates and controls the _.service_ file. The _.service_ does not require an `[Install]` section as it is the _timer_ units that are enabled. If necessary, it is possible to control a differently-named unit using the `Unit=` option in the timer's `[Timer]` section.
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List timers:
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```shell
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systemctl list-timers
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```
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## Examples
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### Monotonic timer
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A timer which will start 15 minutes after boot and again every week while the system is running.
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`/etc/systemd/system/foo.timer`
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```
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[Unit]
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Description=Run foo weekly and on boot
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[Timer]
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OnBootSec=15min
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OnUnitActiveSec=1w
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[Install]
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WantedBy=timers.target
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```
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### Realtime timer
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A timer which starts once a week (at 12:00am on Monday). When activated, it triggers the service immediately if it missed the last start time (option `Persistent=true`), for example due to the system being powered off:
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`/etc/systemd/system/foo.timer`
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```
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[Unit]
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Description=Run foo weekly
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[Timer]
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OnCalendar=weekly
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Persistent=true
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[Install]
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WantedBy=timers.target
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```
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When more specific dates and times are required, `OnCalendar` events uses the following format:
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`DayOfWeek Year-Month-Day Hour:Minute:Second`
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An asterisk may be used to specify any value and commas may be used to list possible values. Two values separated by `..` indicate a contiguous range.
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In the below example the service is run the first four days of each month at 12:00 PM, but _only_ if that day is a Monday or a Tuesday.
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`OnCalendar=Mon,Tue *-*-01..04 12:00:00` |