How to override systemd unit file settings?

Question

I have installed MySQL on my Arch Linux server. I moved the data directory to a place under /home, where my RAID volume is mounted. I noticed that mysqld will not start in this configuration by default since the systemd unit contains the setting ProtectHome=true.

I want to override just this setting. I don't want to re-specify the ExecStart or similar commands, in case they change when the package is upgraded.

I tried making a simple file at /etc/systemd/system called mysqld.service and added only these lines:

[Service]
ProtectHome=false

This doesn't work as it looks like the service in /etc replaces, not overrides, the system service.

Is there a way to override settings in systemd unit files this way without directly modifying the files in /usr/lib/systemd/system? (which is what I have done for now as a temporary fix, although that will end up reverted if the package is updated)

A related question is https://unix.stackexchange.com/q/265281/5132 . — JdeBP, Dec 13 '19 at 18:24

score 111 · Answer 1 · answered Sep 10 '18 at 17:49

111

systemctl edit will create a drop-in file where you can override most of the settings, but these files have some specifics worth mentioning:

Note that for drop-in files, if one wants to remove entries from a setting that is parsed as a list (and is not a dependency), such as AssertPathExists= (or e.g. ExecStart= in service units), one needs to first clear the list before re-adding all entries except the one that is to be removed.
#/etc/systemd/system/httpd.service.d/local.conf
[Unit]
AssertPathExists=
AssertPathExists=/srv/www
Dependencies (After=, etc.) cannot be reset to an empty list, so dependencies can only be added in drop-ins. If you want to remove dependencies, you have to override the entire unit.

To override the entire unit, use systemctl edit --full, this will make a copy in /etc if there is none yet and let you edit it.

See also Systemd delete overrides

answered Sep 10 '18 at 17:49

user

1,429

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+1 for the tip about clearing ExecStart – JoeNahmias Aug 20 '19 at 02:36
+1 Do you know where this is documented? I mean specifically the altering/clearing/adding-to behaviour of drop-ins? – AndreasT Apr 09 '20 at 12:15
2

@AndreasT: this is a direct quote from the documentation linked to in the post ("drop-in file"). Or have I misunderstood your question? – user Apr 09 '20 at 12:23
As an addition: You can also override systemd sockets the same way: sudo systemctl edit exampled.socket. – rugk May 11 '21 at 12:33

score 34 · Accepted Answer · edited Oct 17 '17 at 10:39

34

You may override a systemd unit file using

systemctl edit mysqld.service

Any statements made in the override file will take priority.

edited Oct 17 '17 at 10:39

Jeff Schaller

67,283
35
116
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answered Oct 17 '17 at 04:17

jdwolf

5,017

1

if only it wasn't interactive only ;( – Hvisage Nov 07 '21 at 12:37
1

@Hvisage it's possible to just create a folder and a file instead of running systemctl edit command. For example: mkdir /etc/systemd/system/coturn.service.d/ && echo -e "[Service]\nRestart=always" > /etc/systemd/system/coturn.service.d/overrides.conf – Filip Kwiatkowski Jan 24 '22 at 14:36
3

@Hvisage If writing the override file non-interactively, you may subsequently need to run sudo systemctl daemon-reload. – Asclepius Feb 18 '22 at 18:54

How to override systemd unit file settings?

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