[arch-general] Configuring enabled services

[Thomas Bächler]

Am 27.03.2014 09:41, schrieb Gesh:
> Basically, if I understood what happens correctly, the units under /etc/systemd/system/*.wants/ - or their targets, if they're symlinks - replace their corresponding units in the dependency graph.

Not exactly.

When you place a unit in foo.wants, then foo gets an additional Wants=
dependency. Basically, this is a way to extend units with new Wants= and
Requires= statements without modifying unit files. You don't *replace*
the dependency graph, but *extend* it.

> In addition, all unit files are installed to /usr/lib/systemd/system/,

Unit files can also be installed to /etc/systemd/system/, for example if
you want to use a modified version of a system-supplied unit file.

When you run start or enable on a unit name, system looks into those
directories, taking the first one it finds.

/run/systemd/generator.early
/etc/systemd/system
/run/systemd/system (*)
/run/systemd/generator (*)
/usr/lib/systemd/system
/run/systemd/generator.late

I am not 100% sure if I got the order of the ones marked with (*) right.

As you can see here, there are units installed by the package manager
(/usr/lib), units installed by the admin (/etc), temporary units
installed either by automatic tools or by the admin
(/run/systemd/system) and units installed by generators during systemd
startup or reload (/run/systemd/generator*).

Again, backing up the ones in /usr/lib does not make sense since those
are contained in packages and can be reinstalled. Backing up the ones in
/run makes no sense either since they only live until you reboot (or
reload the configuration, in the case of generator units). However,
backing up /etc/systemd/system entirely (not just the symlinks) make
lots of sense, since the admin is likely to have custom units in there.

> and the symlinks must have the same name as their targets, so only the symlinks' names need to be backed up.

Not necessarily. For example,
/etc/systemd/system/display-manager.service points to
/usr/lib/systemd/system/kdm.service. Most symlinks have the same name,
but that's only convention, not necesity.

> Therefore, only what's under /etc/systemd/system/ needs to be backed up, ignoring symlinks' targets.

I'd back up the symlinks including their targets. This makes it easier
to restore configuration.

> Besides that, the .wants directories in /usr/lib/systemd/system/ are managed by pacman, and represent upstream decisions to automatically start their units.

Indeed.


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