Post Syndicated from Lennart Poettering original https://0pointer.net/blog/projects/three-levels-of-off.html

It has been a while since the last
installment of
my systemd for
Administrators series,
but now with the release of Fedora 15 based on systemd looming, here’s
a new episode:

The Three Levels of “Off”

In systemd,
there are three levels of turning off a service (or other unit). Let’s
have a look which those are:

1. You can stop a service. That simply terminates the
   running instance of the service and does little else. If due to some
   form of activation (such as manual activation, socket activation, bus
   activation, activation by system boot or activation by hardware plug)
   the service is requested again afterwards it will be started. Stopping
   a service is hence a very simple, temporary and superficial
   operation. Here’s an example how to do this for the NTP service:

   $ systemctl stop ntpd.service

   This is roughly equivalent to the following traditional command which is available on most SysV inspired systems:

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   $ service ntpd stop

   In fact, on Fedora 15, if you execute the latter command it will be transparently converted to the former.

2. You can disable a service. This unhooks a service from
   its activation triggers. That means, that depending on your service it
   will no longer be activated on boot, by socket or bus activation or by
   hardware plug (or any other trigger that applies to it). However, you
   can still start it manually if you wish. If there is already a started
   instance disabling a service will not have the effect of stopping
   it. Here’s an example how to disable a service:

   $ systemctl disable ntpd.service

   On traditional Fedora systems, this is roughly equivalent to the following command:

   $ chkconfig ntpd off

   And here too, on Fedora 15, the latter command will be
   transparently converted to the former, if necessary.

   Often you want to combine stopping and disabling a service, to
   get rid of the current instance and make sure it is not started again (except when manually triggered):

   $ systemctl disable ntpd.service
   $ systemctl stop ntpd.service

   Commands like this are for example used during package
   deinstallation of systemd services on Fedora.

   Disabling a service is a permanent change; until you undo it it
   will be kept, even across reboots.

3. You can mask a service. This is like disabling a service,
   but on steroids. It not only makes sure that service is not started
   automatically anymore, but even ensures that a service cannot even be
   started manually anymore. This is a bit of a hidden feature in
   systemd, since it is not commonly useful and might be confusing the
   user. But here’s how you do it:

   $ ln -s /dev/null /etc/systemd/system/ntpd.service
   $ systemctl daemon-reload

   By symlinking a service file to /dev/null you tell
   systemd to never start the service in question and completely block
   its execution. Unit files stored in /etc/systemd/system
   override those from /lib/systemd/system that carry the same
   name. The former directory is administrator territory, the latter
   terroritory of your package manager. By installing your symlink in
   /etc/systemd/system/ntpd.service you hence make sure that
   systemd will never read the upstream shipped service file
   /lib/systemd/system/ntpd.service.

   systemd will recognize units symlinked to /dev/null and
   show them as masked. If you try to start such a service manually (via
   systemctl start for example) this will fail with an
   error.

   A similar trick on SysV systems does not (officially) exist. However,
   there are a few unofficial hacks, such as editing the init script and
   placing an exit 0 at the top, or removing its execution
   bit. However, these solutions have various drawbacks, for example they
   interfere with the package manager.

   Masking a service is a permanent change, much like disabling a service.

Now that we learned how to turn off services on three levels,
there’s only one question left: how do we turn them on again? Well,
it’s quite symmetric. use systemctl start to undo
systemctl stop. Use systemctl enable to undo
systemctl disable and use rm to undo
ln.

And that’s all for now. Thank you for your attention!