Systemd has become the defacto new standard init for Linux-based systems. While not everyone has made the switch yet, pretty much all the major distros have made the decision to switch.
For most people this has not meant all that much yet, other than a lot of controversy. Systemd has built in SysV init system compatibility, and so it's possible to avoid dealing with it quite well.
But there is much to be gained from picking up some basics. Systemd is very poweful.
I'm not going to deal with the basics of interacting with systemd as that's well covered elsewhere. You can find a number of basic tips and tricks here.
Instead I want to talk about how to write systemd units.
A systemd unit is pretty much the "superclass" of a bunch of other systemd concepts.
Systemd lets you manage dependencies between services (your applications, daemons etc.), sockets, mountpoints and a number of other things. Each of these are a unit.
All units share a number of possible configuration options.
Systemd schedules the activation and deactivation of such units based on a dependency graph and various events.
Here is an example of a displaying an installed unit file (dbus.service) using systemctl:
$ systemctl cat dbus # /usr/lib64/systemd/system/dbus.service [Unit] Description=D-Bus System Message Bus Documentation=man:dbus-daemon(1) Requires=dbus.socket [Service] ExecStart=/usr/bin/dbus-daemon --system --address=systemd: --nofork --nopidfile --systemd-activation ExecReload=/usr/bin/dbus-send --print-reply --system --type=method_call --dest=org.freedesktop.DBus / org. OOMScoreAdjust=-900
A couple of observations:
[Unit] section is generic for all unit files.
[Service] is specific to service files.
The "Unit" section above is fairly standard and self-explanatory. "Requires" creates a dependency on the "dbus.socket" unit. Systemd provides a rich set of ways of indicating dependencies to allow it to parallelise startup as much as possible while also making it easy to ensure units are brought up and down in the pre-requisite order.
Systemd units can also specify conditions indicating whether or not the actual activation of the unit should be skipped. For example you may chose to only activate a unit if a given file or directory exists.
The first thing you may want to actually use is likely a systemd service. A service is any process that should be controlled by systemd.
This can be "oneshot" services: A process that's executed, runs and then exits and is not meant to be restarted other than when something explicitly asks for it (via dependencies, or user action). This might be for example a process that reads a config file and updates various files on the server on every boot.
Services can also be long-running processes, such as e.g. a web server.
Unlike a SysV-init, systemd can and typically will handle automatic restarts, and comes with a rich set of configuration options for determining how it should deal with errors, cleanup, notifications etc.
With the right setup it can also handle cleanup of child-processes etc. pretty much automatically.
Let's write a simple one-shot service first.
For the purposes of testing, if you have a systemd system handy (a VM works fine; recent versions of Debian can run Systemd, and CoreOS is another good alternative), you can put this in /etc/systemd/system/oneshot.service:
[Unit] Description=Our test oneshot service [Service] Type=oneshot ExecStartPre=/bin/echo "Starting" ExecStart=/bin/echo "Hello World" ExecStopPost=/bin/echo "Stopping"
Then try this:
$ systemctl start oneshot $ systemctl status oneshot ● oneshot.service - Our test oneshot service Loaded: loaded (/etc/systemd/system/oneshot.service; static) Active: inactive (dead) Oct 27 04:02:21 example.com echo: Starting Oct 27 04:02:21 example.com echo: Hello World Oct 27 04:02:21 example.com echo: Stopping
It should seem reasonably self-explanatory, I hope.
ExecStartPre command is executed before the
is executed after the service has been stopped.
ExecStart represents the command that
will start the service. As long as this command is running, the service is running.
There can be more than one
ExecStopPost, and their execution does
not influence when the service is deemed to be started or stopped respectively for
the purposes of systemd's dependency resolution (that is, e.g. a service that depends
on our example service starting will not be scheduled until the
has started executing)
In this case, since
Type=oneshot, the service instantly stops without any attempt to
Notice how I wrote
/bin/echo instead of
echo? systemd is picky about the syntax
for the "Exec*" commands. You can read more about the details of that on the
You could, if you wanted to, use this to package up stuff to execute on your server
systemctl start, but in itself it would not buy you anything over just putting
the commands in a script.
But if you want to execute something that needs to be executed based on dependencies,
or that needs to pull in other dependencies, you now have a starting point. The missing
piece to make the above execute on boot is to add an
Basically the "Install" section lists directives you want systemd to add to another unit when you call "systemctl enable myservicefile" (instead of systemctl start myservicefile)
(NOTE: If you want to test changes to an existing service file, you need to do
in between to make systemd pick up the changes, unless you reboot or call
A long-running service is very similar to a oneshot command.
[Unit] Description=Foo [Service] ExecStart=/bin/sh -c 'while true; do echo "Running"; sleep 5; done' [Install] WantedBy=multi-user.target
You'll note the lack of
Type=onshot. And I've omitted the
though it's perfectly valid to include them for long-running services.
And after copying this to
/etc/systemd/system and running
systemctl enable foo and
systemctl start "foo,
systemctl status foo now gives (after waiting a few seconds
to show it runs through more than once):
● foo.service - Foo Loaded: loaded (/etc/systemd/system/foo.service; enabled) Active: active (running) since Tue 2015-10-27 04:24:43 GMT; 5s ago Main PID: 7818 (sh) CGroup: /system.slice/foo.service ├─7818 /bin/sh -c while true; do echo "Running"; sleep 5; done └─7923 sleep 5 Oct 27 04:24:43 example.com sh: Running Oct 27 04:24:48 example.com sh: Running
The biggest difference you'll note is that it now shows information about the processes that has been started (and compared to SysV init systems, notice how awesome it is to have this information so readily available). Systemd tries very hard to keep track of all processes started for your service, by using cgroups to isolate them so that it can clean up even quite misbehaved services.
If you now do
systemctl stop foo and
systemctl status foo again you'lll note it gives you the exit
ExecReload you can determine which commands systemd will execute to try to stop or
reload your process gracefully. In the absence of
ExecStop, systemd will send
SIGTERM and then
SIGKILL to your processes when asked to stop or restart them, so for many applications you can leave
out explicit stop instructions. If your app supports a graceful "reload" operation, then you should
ExecReload so people can reload it without looking up your specific mechanism.
But just the simple examples above does not give much over SysV init and derivatives, so let us do something more advanced.
Lets say you want have a service that needs another service to run before it will work. E.g. before starting your web server, you'll probably want the network to be up.
You can do this easily by specifying scheduling dependencies.
Let's say our "oneshot" service requires
foo.service to be running first:
[Unit] Description=Our test oneshot service After=foo.service Requires=foo.service [Service] Type=oneshot ExecStartPre=/bin/echo "Starting" ExecStart=/bin/echo "Hello World" ExecStopPost=/bin/echo "Stopping"
Notice how we've added both
After specifies an ordering dependency. In other words, which unit gets started first assuming
both get scheduled to start. But without other directives, it does not specify that
needs to be started.
Requires on the other hand specifies that
foo.service must be started when
is started, but does not explicitly require an order.
After, they could have been started at the same time. In this case it wouldn't have mattered,
but try to enforce ordering where there is a genuine dependency.
To test this, copy the new
oneshot.service file into place and don't forget to do
systemctl start oneshot followed by
systemctl status foo (I'm assuming you still have
it in place from earlier).
You should see that "foo" has been started.
See the man-page for more on the dependencies.
You may have thought "hmm, but foo.service isn't needed anymore". If
foo.service is only ever needed
by services managed by systemd on this system, you may want to have it automatically shut down when
no dependencies requires it to be running. You can do that like this:
[Unit] Description=Foo StopWhenUnneeded=true [Service] ExecStart=/bin/sh -c 'while true; do echo "Running"; sleep 5; done'
Copy this into place in
systemctl stop foo to make sure the
previous instance was stopped, and
systemctl daemon-reload to load the modified service file.
systemctl start oneshot again. If you do
systemctl status foo it should be stopped, but
there should be a single new "Running" line in the log output, proving that it was started, then
stopped again when there were no longer any dependencies requiring it.
Systemd supports a wide range of mechanisms to determine how to restart failed services.
The most basic is to add a
Restart=something line to the
"something" can be a number of things, but the most common for you are likely to be:
no: Don't restart.
on-failure: Restart the service if it crashed, but not if it stopped gracefully.
on-success: Restart the service if it stopped gracefully, but not if it crashed.
always: "Always" (with caveats, see below) restart
When restarting the service you may want to add a delay to prevent overloading the
system if there's a real problem. You can do this with
"time-value" can be e.g.
One gotcha is that systemd will enforce this interval even if you manually run
systemctl restart servicename. While this can be useful to enforce e.g. safe restart
intervals for some processes, don't overdo it.
You may also want to consider defining
TimeoutStartSec= which specifies how long
systemd should wait for the startup to complete before it considers the startup to have
failed and tries to restart theunit.
If this is set too low, your service may end up just restarting repeatedly.
(Note that I specifically said it will wait for the startup to complete; for the units we have seen so far, this is until ExecStart has started execution (but not until it has exited), however systemd also has a mechanism to let the starting process notify systemd when it has fully initialized, which can be very useful if you have a process that takes a while before it is usefully accessible - this makes for better dependencies. We won't be looking into that here)
Also relevant is
StartLimitInterval=. If a process restarts too often within a short
time interval, systemd will mark the unit as failed and stop trying to restart it. If
this is causing problems, either change the interval, or if you specify
StartLimitInterval=0, systemd will never stop trying to restart the unit.
Be careful: if your process consumes a lot of resources when starting, and you set
StartLimitInterval to 0 and sets
RestartSec too low, systemd will happily do what
you've told it and proceed to put your system under a lot of load.
If your unit fails, systemd provides two mechanisms (at least...) to take corrective action; for example by notifying you:
OnFailure= in the
[Unit] section lets you specify a unit to activate if the current
For services, a
FailureAction= lets you tell systemd to reboot or shut down your
system if a service fails.
OnFailure is perhaps the most likely one to be something you might use.
oneshot.service and add something to get run on failure:
[Unit] Description=Our test oneshot service OnFailure=failure.service [Service] Type=oneshot ExecStart=/bin/exit 1
[Unit] Description=Our failure handler [Service] Type=oneshot ExecStart=/bin/echo "You failed"
After copying these into place, and issuing a "systemctl daemon-reload", you can do this:
$ systemctl status failure ● failure.service - Our failure handler Loaded: loaded (/etc/systemd/system/failure.service; static) Active: inactive (dead) $ systemctl start oneshot Job for oneshot.service failed. See 'systemctl status oneshot.service' and 'journalctl -xn' for details. $ systemctl status failure ● failure.service - Our failure handler Loaded: loaded (/etc/systemd/system/failure.service; static) Active: inactive (dead) since Tue 2015-10-27 05:11:31 GMT; 3s ago Process: 19420 ExecStart=/bin/echo You failed (code=exited, status=0/SUCCESS) Main PID: 19420 (code=exited, status=0/SUCCESS) Oct 27 05:11:31 example.com echo: You failed
You can trigger e-mails; log more details; Wake someone up with a page; reboot the system. Anything you want. And since this is handled by systemd, it will execute as long as systemd is still running and able to start it, regardless of how broken your service is, so it works even in e.g. cases where your service gets killed without being able to attempt to handle errors.
Here's a neat trick: Systemd supports "template" units. These have names like
[email protected]. If you try
[email protected], systemd first looks for
[email protected]. But if it doesn't exist, it will
[email protected]. This is great for spinning up units with different arguments. In this case,
let us quickly parameterise
failure.service so that it can tell us what failed. First lets update
[Unit] Description=Our test oneshot service OnFailure=[email protected] [Service] Type=oneshot ExecStart=/bin/exit 1
[Unit] Description=Our failure handler [Service] Type=oneshot ExecStart=/bin/echo "Service '%i' failed"
Reload as usual, and try
systemctl start oneshot followed by
systemctl status [email protected]:
systemctl status [email protected] ● [email protected] - Our failure handler Loaded: loaded (/etc/systemd/system/[email protected]; static; vendor preset: disabled) Active: inactive (dead) since Tue 2016-01-12 02:57:54 UTC; 1min 6s ago Process: 9053 ExecStart=/bin/echo Service '%i' failed (code=exited, status=0/SUCCESS) Main PID: 9053 (code=exited, status=0/SUCCESS) Jan 12 02:57:54 example.com systemd: Starting Our failure handler... Jan 12 02:57:54 example.com echo: Service 'oneshot' failed Jan 12 02:57:54 example.com systemd: Started Our failure handler.
(There's nothing specific to failure handling about this, you can use it for anything. '%i' in
Exec* commands will be replaced with the part after '@' in the unit name - use it as
The above is just a tiny little sliver of what systemd provides. The real power comes once you start combining the above with conditions, timers, paths, mounts, sockets and more. This allows you to do fun things like wait for changes to paths, wait for a specific time, wait until a specific drive has been mounted, or until someone tries to connect to a socket, and much more.
But if you don't feel like deciphering those man-pages, here's a little (contrived) example: A poor mans message queue.
[Unit] Description=Our test oneshot service [Service] Type=oneshot ExecStart=/bin/bash -c 'F=/tmp/oneshot-queue/$(ls /tmp/oneshot-queue/ | head -n1) && cat $F && rm $F
ExecStart just picks the first filename in
/tmp/oneshot-queue, cat's it so its content
ends up in the journal, and then removes the file. It's just a crude example.
Now create a
oneshot.path that will monitor a path for us:
[Unit] Description=Triggers oneshot when a message arrives in /tmp/oneshot-queue [Path] DirectoryNotEmpty=/tmp/oneshot-queue MakeDirectory=true Unit=oneshot.service
What this does is tell systemd to monitor
/tmp/oneshot-queue (creating it if it doesn't exist), and as long as there are files in there, keep restarting
Copy them in place in
/etc/systemd/system and issue
systemctl daemon-reload as before, and you can test it like this:
$ systemctl start oneshot.path $ systemctl status oneshot.path ● oneshot.path - Triggers oneshot when a message arrives in /tmp/oneshot-queue Loaded: loaded (/etc/systemd/system/oneshot.path; enabled) Active: active (waiting) since Tue 2015-10-27 05:34:38 GMT; 4s ago $ echo "hi there" >/tmp/oneshot-queue/x $ systemctl status oneshot ● oneshot.service - Our test oneshot service Loaded: loaded (/etc/systemd/system/oneshot.service; static) Active: inactive (dead) since Tue 2015-10-27 05:34:52 GMT; 4s ago Process: 25414 ExecStart=/bin/bash -c F=/tmp/oneshot-queue/$(ls /tmp/oneshot-queue/ | head -n1) && cat $F && rm $F (code=exited, status=0/SUCCESS) Main PID: 25414 (code=exited, status=0/SUCCESS) Oct 27 05:34:52 example.com bash: hi there
Systemd paths uses inotify and can check for the presence of a specific path or pattern, or writes or
changes to a file or directory too. E.g. imagine you have config files that depends on having the right nameserver details - you can use systemd paths to watch
/etc/resolv.conf for changes and trigger a script to update your config accordingly.
That's it for this time. Please comment if you'd like examples of more advanced systemd usage.