- 09 July 2017
- 2127 words
- 11 minutes read time
systemd: it’s the init system that (some?) love to hate.
Full disclosure: I find systemd a little overbearing, although by no means would consider myself militantly anti-systemd. It has obvious advantages, and although I’m at philosophical odds with it at some levels, I see no reason why everybody shouldn’t understand it a bit better - especially now that most people will need to deal with it on their favorite distros.
This post is a sort of formalized set of operational notes I’ve made frequent use of in my experience with systemd. I hope this post serves as another tool in your operations/sysadmin utility belt; when you need to get solutions quickly but are working with a system that relies on different tools than your normal sysv toolbox.
Note: The paths and examples here will be drawn from Arch Linux, as it’s the distro I have easiest access to during the writing of this post. Most of it should carry over into other distributions.
A systemd Primer
If you’re unfamiliar with The Beast:
- systemd replaces the bash scripts you’re normally used to throwing in
/etc/init.d/*(or simple upstart/etc. configs)
- in addition to system services, systemd manages the following as well:
- runlevels (analagous to systemd “targets”)
- several types of common sockets
- logging (through
I have to do these things often with my normal init scripts, so how does one accomplish them in systemd?
Service Parameter Overrides
Remember how well-behaved init scripts can be opened up to change their behavior? What if you’d like to override how a unit file operates?
For example, say we have the following systemd unit file (actual file pulled from the Arch Linux nginx package):
[Unit] Description=A high performance web server and a reverse proxy server After=network.target [Service] Type=forking PIDFile=/run/nginx.pid PrivateDevices=yes SyslogLevel=err ExecStart=/usr/bin/nginx -g 'pid /run/nginx.pid; error_log stderr;' ExecReload=/usr/bin/kill -HUP $MAINPID KillSignal=SIGQUIT KillMode=mixed [Install] WantedBy=multi-user.target
What if you want to re-nice the daemon so that nginx gets priority under high load?
Typically this could be done under the classic init system by either munging with the init script or maybe using the auto nice daemon. However, systemd exposes multiple settings to control these types of values. See “Nice=” on that page’s documentation.
So, how does one add settings to an existing unit file? It’d be bad practice to edit the unit file shipped with the package in
/usr/lib/systemd/system, we want to keep custom configuration where it belongs:
Add a file like this:
This lets systemd know that you’re overriding the unit file with a custom setting for the service - the rest of the options will be left alone, you’re just appending the new value.
This generally works for most settings in unit files, although overriding some options requires special treatment (for example,
ExecStart= requires an empty option before defining a new one to avoid stringing together multiple stanzas).
As an added bonus, if you’ve already got sufficient privileges and your
EDITOR environment variable setup, systemd can actually create the necessary directories and files for you - try using:
systemd will open your
EDITOR and let you edit an override file, dropping into the appropriate path in
/etc once you write out and close the buffer.
Command-line sugar, but useful in some situations.
Viewing Boot Services
One very valid point of comparison with traditional init scripts is the predictability of startup services.
Whereas you may use a command like
chkconfig to view which services will start at the default boot runlevel, there’s a lot of different types of units systemd manages, and finding a command analagous to
chkconfig isn’t immediately evident.
The key here is to leverage targets.
Target units are (very roughly) comparable to runlevels; as systemd will usually attempt to reach one overarching target at boot-time (oftentimes
Typically (as seen with the previous
nginx.service example), services ask to be initialized by your distribution’s main target entrypoint (in the
[Install] section), and that gives us a window into what to expect at boot time.
The following command asks, “which units does
multi-user.target ask to be started in order for itself to be considered up?”
The resulting output looks something like this:
multi-user.target ● ├─fstrim.timer ● ├─gmetad.service ● ├─gmond.service ● ├─basic.target ● │ ├─-.mount ● │ ├─snapper-cleanup.timer ● │ ├─snapper-timeline.timer ● │ ├─tmp.mount ● │ ├─paths.target ● │ ├─slices.target ● │ │ ├─-.slice ● │ │ └─system.slice ● │ ├─sockets.target ● │ │ ├─cockpit.socket ● │ │ └─uuidd.socket ● │ └─timers.target ● │ ├─logrotate.timer ● │ ├─shadow.timer ● │ └─systemd-tmpfiles-clean.timer ● └─zfs.target ● ├─zfs-import-cache.service ● ├─zfs-mount.service ● ├─zfs-share.service ● └─zfs-zed.service
(I’ve trimmed out a great deal of these for brevity).
Note that in addition to visibility into plain old persistent daemons (like ganglia’s
gmond), the command also lists the dependency
sockets.target and its associated dependent sockets, so we have a pretty good picture of not only the services that will come online at boot, but any sockets systemd will open that could potentially start other services via socket activation.
Tricks with Timers
One systemd replacement that I wholeheartedly find more appealing than the traditional tooling is timers - output is captured more easily, environment variables are more predictable, and execution is more closely tracked. There are a few traits that timers expose that are particularly useful to be aware of.
Last and Next Run
Because systemd tracks when timers execute, their last and next execution time can be easily discovered:
You’ll see something like this:
NEXT LEFT LAST PASSED UNIT Fri 2017-07-07 21:00:00 EDT 34min left Fri 2017-07-07 20:00:00 EDT 25min ago snapper-timeline.timer Fri 2017-07-07 22:56:43 EDT 2h 30min left Thu 2017-07-06 22:56:43 EDT 21h ago snapper-cleanup.timer Fri 2017-07-07 23:01:44 EDT 2h 36min left Thu 2017-07-06 23:01:44 EDT 21h ago systemd-tmpfiles-clean.timer Sat 2017-07-08 00:00:00 EDT 3h 34min left Fri 2017-07-07 00:00:00 EDT 20h ago shadow.timer Sat 2017-07-08 03:00:00 EDT 6h left Fri 2017-07-07 03:00:00 EDT 17h ago logrotate.timer Sat 2017-07-08 03:00:00 EDT 6h left Fri 2017-07-07 03:00:00 EDT 17h ago snazzer.timer Sat 2017-07-08 03:30:00 EDT 7h left Fri 2017-07-07 03:30:00 EDT 16h ago man-db.timer Mon 2017-07-10 03:00:00 EDT 2 days left Mon 2017-07-03 03:00:00 EDT 4 days ago fstrim.timer Tue 2017-08-01 05:00:00 EDT 3 weeks 3 days left Sat 2017-07-01 05:00:00 EDT 6 days ago reflector.timer
You can pretty easily see, for example, that logrotate will run in about six hours, and that my filesystem got trimmed four days ago on July third.
If you’ve ever written a script to run periodically, you may have questioned what happens if the machine is off when the execution time passes.
Timers have a convenient option called
Persistent= that can record when the timer was fired, and if the interval lapsed when the timer wasn’t able to fire (if, for example, the host was offline), systemd will trigger the timer once it’s able to.
One use case that I have for this is a job to renew Let’s Encrypt certificates. I occasionally bring down my servers for kernel updates, and with a persistent timer, I never worry about an expiring certificate as those timers will trigger once they’re able to if the host happened to be down during that time.
This is a profoundly userful feature that I’ve been using more and more often. If you’ve ever poked around your environment in an ssh login on a systemd-based system, you’ll notice that you’re in a session systemd has spawned for your user. This is a little slice of the system that gives your user some dedicated tools, including your own session to run user units in.
Note that, by default, most systems only spawn user sessions on login and kill them on logout. You can ensure users get more permanent sessions at boot that persist after logout with a command:
$ loginctl enable-linger tylerjl
At this point you can do most of the things you can do with normal systemd units, but entirely within userspace and without the need to prepend
sudo to most
As an example, I keep Kodi backups on a ZFS dataset that I prune occasionally. My normal, non-root user has read/write permission on these backups, so why not do this as an unprivileged user (and dodge a possibly destructive
I have the following file at
[Unit] Description=regularly clean kodi backups [Timer] OnCalendar=Thursday *-*-* 02:00:00 [Install] WantedBy=default.target
And an associated service at
[Unit] Description=clean up old kodi backups [Service] ExecStart=/usr/bin/find /srv/storage/backups/kodi -type f -name '*.zip' -mtime +40 -delete
Two important items of note:
- The units live at a specific path that the user session will recognize.
- I’ve found that the
defaulttarget is brought up with the user session, providing the necessary hook to get units started at boot for users. Your distribution may vary.
All that’s left is to enable and start the timer, with the
--user flag to connect to the user’s instance, not the system one:
$ systemctl --user enable --now kodi-backup-cleanup.timer
And my timer will fire the
find command as my user on the specified schedule.
What’s more, you get all the associated timer goodness in the forementioned sections (like
list-timer, but with
Path unit are a somewhat lesser used type of unit that can make a system very event-driven and add dependencies in a more natural way. Simply, they provide a mechanism to start other services if files or directories change in some way. A good illustration here is some of the automation I have around my Let’s Encrypt certificate renewals.
As mentioned previously, I have a timer and service that regularly renews my certificates.
Suppose my certificate bundle ends up at
I have a couple of services that consume this certificate; for example,
nginx reads the cert to serve up HTTPS.
My certificate renewal script is a nice, standalone command that just does one thing, and it would be nice to provide a decoupled way to trigger a
reload for nginx if the bundle gets updated.
In addition, it would be convenient for any other applications that consume the certificate (and I have a few) to watch for updates without needing to update my script all the time.
With that in mind, consider the following path unit,
[Unit] Description=watches the cert bundle [Path] PathModified=/etc/pki/certs/blog_tjll_net-bundle.pem Unit=reload-nginx.service [Install] WantedBy=multi-user.target
started, this unit expresses “when the cert file gets updated, start
service file looks like this:
[Unit] Description=reload nginx [Service] Type=oneshot ExecStart=/usr/bin/systemctl reload nginx
Pretty straightforward. The entire flow seems overcomplicated at first, but it’s actually very convenient as any updates to the certificate file outside of my script (which, because I can write buggy scripts, happens sometimes) will always trigger a reload, making the entire certificate update process agnostic to the method that actually drops the certificate there.
Path units can activate any unit that isn’t another path and can monitor most inotify-related events, so there’s a great deal of flexibility in how they can be used.
Aside from those broader topics, there’s a few little niceties that I’ve been using that may be useful in day-to-day operations with systemd:
- Start and enable a unit with one command with the
systemctl enable --now foo.service
- Want to quickly find problems? List failed units that need attention with
systemctl --state=failed(I use this one a lot)
catoperation got some shade thrown at it for re-inventing unix commands, but it actually shows a unit file’s contents in addition to any override files that may be in effect for it (for example
systemctl cat nginx.service)
- I haven’t dove into
journalctlin this post, but I use filtering fairly regularly. For example, to live-tail logs for both nginx and ssh, try
journalctl -f _SYSTEMD_UNIT=nginx.service + _SYSTEMD_UNIT=sshd.service. The plus is a logical or.
If you want more tips like this, I’d refer you to my post over at SysAdvent that dives more deeply into topics like journald and dbus.