I just switched to debian jessie, and most things run okay, including my graphical display manager wdm
.
The thing is, I just don't understand how this works. Obviously my /etc/init.d/wdm
script is called, because when I put an early exit
in there, wdm is not started. But when I alternatively rename the /etc/rc3.d
directory (my default runlevel used to be 3), then wdm is still started.
I could not find out how systemd finds this script and I do not understand what it does to all the other init.d scripts.
- When and how does systemd run init.d scrips?
- In the long run, should I get rid of all init.d scripts?
Best Answer
chaos' answer is what some documentation says. But it's not what systemd actually does. (It's not what van Smoorenburg
rc
did, either. The van Smoorenburgrc
most definitely did not ignore LSB headers, whichinsserv
used to calculate static orderings, for starters.) The Freedesktop documentation, such as that "Incompatibilities" page, is in fact wrong, on these and other points. (TheHOME
environment variable in fact is often set, for example. This went wholly undocumented anywhere for a long time. It's now documented in the manual, at least, but that Freedesktop WWW page still hasn't been corrected.)The native service format for systemd is the service unit. systemd's service management proper operates solely in terms of those, which it reads from one of nine directories where (system-wide)
.service
files can live./etc/systemd/system
,/run/systemd/system
,/usr/local/lib/systemd/system
, and/usr/lib/systemd/system
are four of those directories.The compatibility with van Smoorenburg
rc
scripts is achieved with a conversion program, namedsystemd-sysv-generator
. This program is listed in the/usr/lib/systemd/system-generators/
directory and is thus run automatically by systemd early in the bootstrap process at every boot, and again every time that systemd is instructed to re-load its configuration later on.This program is a generator, a type of ancillary utility whose job is to create service unit files on the fly, in a tmpfs where three more of those nine directories (which are intended to be used only by generators) are located.
systemd-sysv-generator
generates the service units that run the van Smoorenburgrc
scripts from/etc/init.d
, if it doesn't find a native systemd service unit by that name already existing in the other six locations.systemd service management only knows about service units. These automatically (re-)generated service units are written to invoke the van Smoorenburg
rc
scripts. They have, amongst other things:Received wisdom is that the van Smoorenburg
rc
scripts must have an LSB header, and are run in parallel without honouring the priorities imposed by the/etc/rc?.d/
system. This is incorrect on all points.In fact, they don't need to have an LSB header, and if they do not
systemd-sysv-generator
can recognize the more limited old RedHat comment headers (description:
,pidfile:
, and so forth). Moreover, in the absence of an LSB header it will fall back to the contents of the/etc/rc?.d
symbolic link farms, reading the priorities encoded into the link names and constructing a before/after ordering from them, serializing the services. Not only are LSB headers not a requirement, and not only do they themselves encode before/after orderings that serialize things to an extent, the fallback behaviour in their complete absence is actually significantly non-parallelized operation.The reason that
/etc/rc3.d
didn't appear to matter is that you probably had that script enabled via another/etc/rc?.d/
directory.systemd-sysv-generator
translates being listed in any of/etc/rc2.d/
,/etc/rc3.d/
, and/etc/rc4.d/
into a nativeWanted-By
relationship to systemd'smulti-user.target
. Run levels are "obsolete" in the systemd world, and you can forget about them.Further reading
systemd.exec
. systemd manual pages. Freedesktop.org.