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Systemd Timers for Scheduling Tasks
Systemd has provided timers for a while and it is worth taking a look at this feature as an alternative to cron. This article will show you how to use timers with systemd to run a task after the system has booted and on a recurring basis after that. This is not a comprehensive discussion of systemd, only an introduction to this one feature.
Cron vs anacron vs systemd: a quick review
Cron can schedule a task to be run at a granularity ranging from minutes to months or more. It is relatively simple to set up, requiring a single configuration file. Although the configuration line is somewhat esoteric. It can also be used by general users.
Cron, however, fails if your system happens to not be running when the appropriate execution time occurs.
Anacron, overcomes the “system not running” issue. It insures that the task will be executed when your system is again active. While it was intended to be used by administrators, some systems give general users access.
However, the anacron frequency of execution can be no less than daily.
Both cron and anacron have issues with consistency in execution context. Care must be taken that the environment in effect when the task runs is exactly that used when testing. The same shell, environment variables and paths must be provided. This means that testing and debugging can sometimes be difficult.
Systemd timers offer the best of both cron and anacron. Allows scheduling down to minute granularity. Assures that the task will be executed when the system is again running even if it was off during the expected execution time. Is available to all users. You can test and debug the execution in the environment it will run in.
However, the configuration is more involved, requiring at least two configuration files.
If your cron and anacron configuration is serving you well then there may not be a reason to change. But systemd is at least worth investigating since it may simplify any current cron /anacron work-arounds.
Configuration
Systemd timer executed functions require, at a minimum, two files. These are the “timer unit” and the “service unit”. Actions consisting of more than a simple command, you will also need a “job” file or script to perform the necessary functions.
The timer unit file defines the schedule while the service unit file defines the task(s) performed. More details on the .timer unit is available in “man systemd.timer”. Details on the service unit are available in “man systemd.service”.
There are several locations where unit files exist (listed in the man page). Perhaps the easiest location for the general user, however, is “~/.config/systemd/user”. Note that “user” here, is the literal string “user”.
Demo
This demo is a simple example creating a user scheduled job rather than a system schedule job (which would run as root). It prints a message, date, and time to a file.
- Start by creating a shell script that will perform the task. Create this in your local “bin” directory, for example, in “~/bin/schedule-test.sh”
To create the file:
touch ~/bin/schedule-test.sh
Then add the following content to the file you just created.
#!/bin/sh
echo "This is only a test: $(date)" >> "$HOME/schedule-test-output.txt"
Remember to make your shell script executable.
- Create the .service unit that will call the script above. Create the directory and file in: “~/.config/systemd/user/schedule-test.service”:
[Unit]
Description=A job to test the systemd scheduler
[Service]
Type=simple
ExecStart=/home/<user>/bin/schedule-test.sh
[Install]
WantedBy=default.target
Note that <user> should be your @HOME address but the “user” in the path name for the unit file is literally the string “user”.
The ExecStart line should provide an absolute address with no variables. An exception to this is that for user units you may substitute “%h” for $HOME. In other words you can use:
ExecStart=%h/bin/schedule-test.sh
This is only for user unit file use. It is not good for system units since “%h” will always return “/root” when run in the system environment. Other substitutions are found in “man systemd.unit” under the heading “SPECIFIERS”. As it is outside the scope of this article, that’s all we need to know about SPECIFIERS for now.
3. Create a .timer unit file which actually schedules the .service unit you just created. Create it in the same location as the .service file “~/.config/systemd/user/schedule-test.timer”. Note that the file names differ only in their extensions, that is ,”.service” versus “.timer”
[Unit]
Description=Schedule a message every 1 minute
RefuseManualStart=no # Allow manual starts
RefuseManualStop=no # Allow manual stops
[Timer]
#Execute job if it missed a run due to machine being off
Persistent=true
#Run 120 seconds after boot for the first time
OnBootSec=120
#Run every 1 minute thereafter
OnUnitActiveSec=60
#File describing job to execute
Unit=schedule-test.service
[Install]
WantedBy=timers.target
Note that the .timer file has used “OnUnitActiveSec” to specify the schedule. Much more flexible is the “OnCalendar” option. For example:
# run on the minute of every minute every hour of every day
OnCalendar=*-*-* *:*:00
# run on the hour of every hour of every day
OnCalendar=*-*-* *:00:00
# run every day
OnCalendar=*-*-* 00:00:00
# run 11:12:13 of the first or fifth day of any month of the year
# 2012, but only if that day is a Thursday or Friday
OnCalendar=Thu,Fri 2012-*-1,5 11:12:13
More information on “OnCalendar” is available here.
4. All the pieces are in place but you should test to make certain everything works. First, enable the user service:
$ systemctl --user enable schedule-test.service
This should result in output similar to this:
Created symlink /home/<user>/.config/systemd/user/default.target.wants/schedule-test.service → /home/<user>/.config/systemd/user/schedule-test.service.
Now do a test run of the job:
$ systemctl --user start schedule-test.service
Check your output file ( $HOME/schedule-test-output.txt ) to insure that your script is performing correctly. There should be a single entry since we have not started the timer yet. Debug as necessary. Don’t forget to enable the service again if you needed to change your .service file as opposed to the shell script it invokes.
5. Once the job works correctly, schedule it for real by enabling and starting the user timer for your service:
$ systemctl --user enable schedule-test.timer
$ systemctl --user start schedule-test.timer
Note that you have already started and enabled the service in step 4, above, so it is only necessary to enable and start the timer for it.
The “enable” command will result in output similar to this:
Created symlink /home/<user>/.config/systemd/user/timers.target.wants/schedule-test.timer → /home/<user>/.config/systemd/user/schedule-test.timer.
and the “start” will simply return you to a CLI prompt.
Other operations
You can check and monitor the service. The first command below is particularly useful if you receive an error from the service unit:
$ systemctl --user status schedule-test
$ systemctl --user list-unit-files
Manually stop the service:
$ systemctl --user stop schedule-test.service
Permanently stop and disable the timer and the service, reload the daemon config and reset any failure notifications:
$ systemctl --user stop schedule-test.timer
$ systemctl --user disable schedule-test.timer
$ systemctl --user stop schedule-test.service
$ systemctl --user disable schedule-test.service
$ systemctl --user daemon-reload
$ systemctl --user reset-failed
Summary
This article will jump-start you with systemd timers, however, there is much more to systemd than covered here. This article should provide you with a foundation on which to build. You can explore more about it starting in the Fedora Magazine systemd series .
References – Further reading:
- man systemd.timer
- man systemd.service
- Use systemd timers instead of cronjobs
- Understanding and administering systemd
- Also, https://opensource.com/ has a good systemd cheatsheet
via: https://fedoramagazine.org/systemd-timers-for-scheduling-tasks/
作者：Richard England 选题：lujun9972 译者：译者ID 校对：校对者ID