How to override systemd unit file settings?

All we need is an easy explanation of the problem, so here it is.

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)

How to solve :

I know you bored from this bug, So we are here to help you! Take a deep breath and look at the explanation of your problem. We have many solutions to this problem, But we recommend you to use the first method because it is tested & true method that will 100% work for you.

Method 1

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

Method 2

You may override a systemd unit file using

systemctl edit mysqld.service

Any statements made in the override file will take priority.

Note: Use and implement method 1 because this method fully tested our system.
Thank you 🙂

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