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EXTRA-CONFIGS.md

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Extra Configs and Features

DROP-IN FILES

The systemd service descriptors do already have the feature of drop-in configurations. So when there is a docker container with these files:

/usr/lib/systemd/system/some.service
/usr/lib/systemd/system/some.service.d/extra.conf
/etc/systemd/system/some.service.d/override.conf

then ALL these files will be parsed. Therefore some *.rpm program may install the some.service file but you can always override the settings with your override.conf.

In general the new lines will be added to the existing config entries. So when you have

# some.service
[Service]
Environment=A=1 B=2
ExecStartPre=/usr/bin/somecheck

# some.service.d/extra.conf
[Service]
Environment=C=2
ExecStartPre=/usr/bin/morechecks

then both of these commands are being executed and the environment contains A, B and C variables for all commands.

There is a feature where you can also replace the original settings completely by giving an option with an empty argument:

# some.service
[Service]
Environment=A=1 B=2
ExecStart=/usr/bin/startup $A $B

# some.service.d/override.conf
[Service]
Environment=C=2
ExecStart=
ExecStart=/usr/bin/startup $A $B $C

In the latter case, only one ExecStart command will be executed having three arguments.

All these are standard features of systemd. In general they are meant to be used by configuration programs that can add or change some environment variables materializing the extra settings as an extra.conf instead of changing the original service descriptor file. However the way to append/replace a setting is completely generic - it is part of the parser and not of the interpreter for the service definitions. Thus it works also for the Exec-parts.

STANDARD DEFINITION

https://www.freedesktop.org/software/systemd/man/systemd.unit.html

Along with a unit file foo.service, a "drop-in" directory foo.service.d/ may exist. All files with the suffix ".conf" from this directory will be parsed after the unit file itself is parsed. This is useful to alter or add configuration settings for a unit, without having to modify unit files. Drop-in files must contain appropriate section headers. For instantiated units, this logic will first look for the instance ".d/" subdirectory (e.g. "foo@bar.service.d/") and read its ".conf" files, followed by the template ".d/" subdirectory (e.g. "foo@.service.d/") and the ".conf" files there. Moreover for units names containing dashes ("-"), the set of directories generated by truncating the unit name after all dashes is searched too. Specifically, for a unit name foo-bar-baz.service not only the regular drop-in directory foo-bar-baz.service.d/ is searched but also both foo-bar-.service.d/ and foo-.service.d/. This is useful for defining common drop-ins for a set of related units, whose names begin with a common prefix. This scheme is particularly useful for mount, automount and slice units, whose systematic naming structure is built around dashes as component separators. Note that equally named drop-in files further down the prefix hierarchy override those further up, i.e. foo-bar-.service.d/10-override.conf overrides foo-.service.d/10-override.conf.

In addition to /etc/systemd/system, the drop-in ".d/" directories for system services can be placed in /usr/lib/systemd/system or /run/systemd/system directories. Drop-in files in /etc take precedence over those in /run which in turn take precedence over those in /usr/lib. Drop-in files under any of these directories take precedence over unit files wherever located. Multiple drop-in files with different names are applied in lexicographic order, regardless of which of the directories they reside in.

Note that the last paragraph is a bit misleading as "predence" is defined for *.conf files of the same name. An "extra.conf" in the /etc area will mask an "extra.conf" in /usr/lib. Otherwise the order of conf-files is just lexicographic irrespective of the location, so that administrators will often choose to prefix the conf-files by number to ensure proper inclusion order, i.e. using some "01-extra.conf" to "99-override.conf" file names.

As for the systemctl replacement script the feature of a shortened foo-.service.d is not implemented. (The systemctl script does also ignore many of the standard definitions about template service files). Therefore overrides must be specific for each service and not a group of them.

EXAMPLE

/etc/systemd/system/some.service
/etc/systemd/system/some.service.d/extra.conf
/etc/systemd/system/some.service.d/zen.conf
/usr/lib/system/some.service.d/addon.conf
/usr/lib/system/some.service.d/override.conf

The parsing order is this

some.service
addon.conf
extra.conf
override.conf
zen.conf