How is a systemd service unit activated by a timer unit

systemd

Given a unit file foo.service

# /etc/systemd/system/foo.service
[Service]
ExecStart=/opt/bin/foo --prod

And a unit file foo.timer

# /etc/systemd/system/foo.timer
[Timer]
OnCalendar=*-*-* 12:00:00

[Install]
WantedBy=timers.target

What happens at 12:00 every day if foo.service is off? (it will probably be started)
What happens at 12:00 every day if foo.service is already on? (will it be restarted?)

From reading man systemd.timer

For each timer file, a matching unit file must exist, describing the
unit to activate when the timer elapses. By default, a service by the
same name as the timer (except for the suffix) is activated. Example: a
timer file foo.timer activates a matching service foo.service. The unit
to activate may be controlled by Unit= (see below).

What does it mean to activate foo.service? Does it mean restart or verify that it's already started?

Best Answer

Normally the systemd service of a timer is a oneshot or simple (default) start-up type, which means the service runs a command and exits afterwards.
During the execution of the command given with ExecStart, the unit has the state active (running) and on exit something like inactive (dead).

When using timers you do not activate the service with systemctl enable --now foo.service but rather enable the corresponding timer unit systemctl enbale --now foo.timer.
What the timer does is more or less a systemctl start foo.service at the time given.

What happens at 12:00 every day if foo.service is off? (it will probably be started)

The foo.service will be started but is not enabled (auto-start at boot).

What happens at 12:00 every day if foo.service is already on? (will it be restarted?)

If the service is in an active (running) state nothing will happen. systemd won't try to start a service if that is already running.

What does it mean to activate foo.service? Does it mean restart or verify that it's already started?

To activate a service means to start it, e.g. systemctl start foo.service.

Related Solutions

How to set environment variable in systemd service

Times change and so do best practices.

The current best way to do this is to run systemctl edit myservice, which will create an override file for you or let you edit an existing one.

In normal installations this will create a directory /etc/systemd/system/myservice.service.d, and inside that directory create a file whose name ends in .conf (typically, override.conf), and in this file you can add to or override any part of the unit shipped by the distribution.

For instance, in a file /etc/systemd/system/myservice.service.d/myenv.conf:

[Service]
Environment="SECRET=pGNqduRFkB4K9C2vijOmUDa2kPtUhArN"
Environment="ANOTHER_SECRET=JP8YLOc2bsNlrGuD6LVTq7L36obpjzxd"

Also note that if the directory exists and is empty, your service will be disabled! If you don't intend to put something in the directory, ensure that it does not exist.

For reference, the old way was:

The recommended way to do this is to create a file /etc/sysconfig/myservice which contains your variables, and then load them with EnvironmentFile.

For complete details, see Fedora's documentation on how to write a systemd script.

Find the location of a systemd unit file service

For units that are defined in actual, static files, this can be seen in systemctl status:

$ systemctl status halt-local.service
● halt-local.service - /usr/sbin/halt.local Compatibility
   Loaded: loaded (/lib/systemd/system/halt-local.service; static)
   Active: inactive (dead)

But there are units that are not defined by files, e.g. with systemd-cron installed. These have no useful location listed with status:

$ systemctl status cron-jojo-0.timer
● cron-jojo-0.timer - [Cron] "*/10 * * * * ..."
   Loaded: loaded (/var/spool/cron/crontabs/jojo)
   Active: active (waiting) since Mon 2015-05-18 14:53:01 UTC; 9min ago

In either case, though, the FragmentPath field is educating:

$ systemctl show -p FragmentPath cron-daily.service
FragmentPath=/lib/systemd/system/cron-daily.service
$ systemctl show -p FragmentPath cron-jojo-0.service
FragmentPath=/run/systemd/generator/cron-jojo-0.service
$ systemctl show -p FragmentPath halt-local.service
FragmentPath=/lib/systemd/system/halt-local.service

Best Answer

Related Solutions

How to set environment variable in systemd service

Find the location of a systemd unit file service

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