How does systemd use /etc/init.d scripts?
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 Smoorenburg rc most definitely did not ignore LSB headers, which insserv used to calculate static orderings, for starters.) The Freedesktop documentation, such as that "Incompatibilities" page, is in fact wrong, on these and other points. (The HOME 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, named systemd-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 Smoorenburg rc 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:
[Unit] SourcePath=/etc/init.d/wibble [Service] ExecStart=/etc/init.d/wibble start ExecStop=/etc/init.d/wibble stop
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 native Wanted-By relationship to systemd's multi-user.target. Run levels are "obsolete" in the systemd world, and you can forget about them.
Further reading
- systemd-sysv-generator. systemd manual pages. Freedesktop.org.
- "Environment variables in spawned processes".
systemd.exec. systemd manual pages. Freedesktop.org. - https://unix.stackexchange.com/a/394191/5132
- https://unix.stackexchange.com/a/204075/5132
- https://unix.stackexchange.com/a/196014/5132
- https://unix.stackexchange.com/a/332797/5132
Systemd is backward compatible with SysV init scripts. According to LSB 3.1, the init script must have informational Comment Conventions, defining when the script has to start/stop and what is required for the script to start/stop. This is an example:
### BEGIN INIT INFO
# Provides: my-service
# Required-Start: $local_fs $network $remote_fs
# Required-Stop: $local_fs $network $remote_fs
# Default-Start: 2 3 4 5
# Default-Stop: 0 1 6
# Short-Description: start and stop service my-service
# Description: my-service blah blah ...
### END INIT INFO
This is a commented section which is ignored by SysV. On the other hand, systemd reads that dependency information and runs those scripts depending on that.
But there is one point, where systemd and SysV differ in terms of init scripts. SysV executes the scripts in sequential order based on their number in the filename. Systemd doesn't. If dependencies are met, systemd runs the scripts immediately, without honoring the numbering of the script names. Some of them will most probably fail because of the ordering. There are a lots of other incompatibilities that should be considered.
If there are init scripts and .service files for the same service, systemd will execute both, as soon as the dependencies are met (in case of the init script, those defined in the LSB header).