systemd.generator — Systemd unit generators
/path/to/generator normal-dir early-dir late-dir
/run/systemd/system-generators/*
/etc/systemd/system-generators/*
/usr/local/lib/systemd/system-generators/*
/usr/lib/systemd/system-generators/*
/run/systemd/user-generators/*
/etc/systemd/user-generators/*
/usr/local/lib/systemd/user-generators/*
/usr/lib/systemd/user-generators/*
Generators are small binaries that live in
/usr/lib/systemd/user-generators/ and other directories
listed above.
systemd(1)
will execute those binaries very early at bootup and at
configuration reload time — before unit files are loaded.
Generators can dynamically generate unit files or create symbolic
links to unit files to add additional dependencies, thus extending
or overriding existing definitions. Their main purpose is to
convert configuration files that are not native unit files
dynamically into native unit files.
Generators are loaded from a set of paths determined during
compilation, listed above. System and user generators are loaded
from directories with names ending in
system-generators/ and
user-generators/, respectively. Generators
found in directories listed earlier override the ones with the
same name in directories lower in the list. A symlink to
/dev/null or an empty file can be used to
mask a generator, thereby preventing it from running. Please note
that the order of the two directories with the highest priority is
reversed with respect to the unit load path and generators in
/run overwrite those in
/etc.
After installing new generators or updating the configuration, systemctl daemon-reload may be executed. This will delete the previous configuration created by generators, re-run all generators, and cause systemd to reload units from disk. See systemctl(1) for more information.
Generators are invoked with three arguments: paths to runtime directories where generators can place their generated unit files or symlinks.
normal-dir
argv[1] may be used to override unit files in
/usr, but not those in
/etc. This means that unit files placed
in this directory take precedence over vendor unit
configuration but not over native user/administrator unit
configuration.
early-dir
argv[2] may be used to override unit files in
/usr and in
/etc. This means that unit files placed
in this directory take precedence over all configuration,
both vendor and user/administrator.
late-dir
argv[3] may be used to extend the unit file tree without overridding any other unit files. Any native configuration files supplied by the vendor or user/administrator take precedence over the generated ones placed in this directory.
All generators are executed in parallel. That means all executables are started at the very same time and need to be able to cope with this parallelism.
Generators are run very early at boot and cannot rely on
any external services. They may not talk to any other
process. That includes simple things such as logging to
syslog(3),
or systemd itself (this means: no
systemctl(1)!). They
can however rely on the most basic kernel functionality to
be available, including mounted /sys,
/proc, /dev.
Units written by generators are removed when configuration is reloaded. That means the lifetime of the generated units is closely bound to the reload cycles of systemd itself.
Generators should only be used to generate unit files, not any other kind of configuration. Due to the lifecycle logic mentioned above generators are not a good fit to generate dynamic configuration for other services. If you need to generate dynamic configuration for other services do so in normal services you order before the service in question.
Since
syslog(3)
is not available (see above) log messages have to be
written to /dev/kmsg instead.
It is a good idea to use the
SourcePath= directive in generated unit
files to specify the source configuration file you are
generating the unit from. This makes things more easily
understood by the user and also has the benefit that
systemd can warn the user about configuration files that
changed on disk but have not been read yet by systemd.
Generators may write out dynamic unit files or just hook
unit files into other units with the usual
.wants/ or
.requires/ symlinks. Often it is
nicer to simply instantiate a template unit file from
/usr with a generator instead of
writing out entirely dynamic unit files. Of course this
works only if a single parameter is to be used.
If you are careful you can implement generators in shell scripts. We do recommend C code however, since generators delay are executed synchronously and hence delay the entire boot if they are slow.
Regarding overriding semantics: there are two rules we try to follow when thinking about the overriding semantics:
User configuration should override vendor
configuration. This (mostly) means that stuff from
/etc should override stuff from
/usr.
Native configuration should override non-native configuration. This (mostly) means that stuff you generate should never override native unit files for the same purpose.
Of these two rules the first rule is probably the more important one and breaks the second one sometimes. Hence, when deciding whether to user argv[1], argv[2], or argv[3], your default choice should probably be argv[1].
Instead of heading off now and writing all kind of generators for legacy configuration file formats, please think twice! It's often a better idea to just deprecate old stuff instead of keeping it artificially alive.
Example 1. systemd-fstab-generator
systemd-fstab-generator(8)
converts /etc/fstab into native mount
units. It uses argv[1] as location to place the generated unit
files in order to allow the user to override
/etc/fstab with her own native unit files,
but also to ensure that /etc/fstab
overrides any vendor default from /usr.
After editing /etc/fstab, the user
should invoke systemctl daemon-reload. This
will re-run all generators and cause systemd
to reload units from disk. To actually mount new directories
added to fstab, systemctl start
/path/to/mountpoint or
systemctl start local-fs.target may be used.
Example 2. systemd-system-update-generator
systemd-system-update-generator(8)
temporarily redirects default.target to
system-update.target if a system update is
scheduled. Since this needs to override the default user
configuration for default.target it uses
argv[2]. For details about this logic, see
Implementing
Offline System Updates.
Example 3. Debuging a generator
dir=$(mktemp -d)
SYSTEMD_LOG_LEVEL=debug /usr/lib/systemd/system-generators/systemd-fstab-generator \
"$dir" "$dir" "$dir"
find $dirsystemd(1), systemd-cryptsetup-generator(8), systemd-debug-generator(8), systemd-efi-boot-generator(8), systemd-fstab-generator(8), fstab(5), systemd-getty-generator(8), systemd-gpt-auto-generator(8), systemd-hibernate-resume-generator(8), systemd-system-update-generator(8), systemd-sysv-generator(8), systemd.unit(5), systemctl(1)