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 executables placed in /usr/lib/systemd/system-generators/ and other
directories listed above.
systemd(1) will execute
these binaries very early at bootup and at configuration reload time — before unit files are
loaded. Their main purpose is to convert configuration that is not native to the service manager into
dynamically generated unit files, symlinks or unit file drop-ins, so that they can extend the unit file
hierarchy the service manager subsequently loads and operates on.
Each generator is called with three directory paths that are to be used for
generator output. In these three directories, generators may dynamically generate
unit files (regular ones, instances, as well as templates), unit file
.d/ drop-ins, and create symbolic links to unit files to add
additional dependencies, create aliases, or instantiate existing templates. Those
directories are included in the unit load path of
systemd(1),
allowing generated configuration to extend or override existing
definitions.
Directory paths for generator output differ by priority:
…/generator.early has priority higher than the admin
configuration in /etc/, while
…/generator has lower priority than
/etc/ but higher than vendor configuration in
/usr/, and …/generator.late has priority
lower than all other configuration. See the next section and the discussion of
unit load paths and unit overriding in
systemd.unit(5).
Generators are loaded from a set of paths determined during
compilation, as 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 directories where generators can place their generated unit files or symlinks. By default those paths are runtime directories that are included in the search path of systemd, but a generator may be called with different paths for debugging purposes.
normal-dir
In normal use this is /run/systemd/generator in
case of the system generators and
$XDG_RUNTIME_DIR/generator in case of the user
generators. Unit files placed in this directory take precedence over vendor
unit configuration but not over native user/administrator unit configuration.
early-dir
In normal use this is /run/systemd/generator.early
in case of the system generators and
$XDG_RUNTIME_DIR/generator.early in case of the user
generators. Unit files placed in this directory override unit files in
/usr/, /run/ and
/etc/. This means that unit files placed in this
directory take precedence over all normal configuration, both vendor and
user/administrator.
late-dir
In normal use this is /run/systemd/generator.late
in case of the system generators and
$XDG_RUNTIME_DIR/generator.late in case of the user
generators. This directory may be used to extend the unit file tree without
overriding any other unit files. Any native configuration files supplied by
the vendor or user/administrator take precedence.
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))!
Non-essential file systems like /var/ and /home/ are
mounted after generators have run. Generators can however rely on the most basic kernel functionality
to be available, as well as mounted /sys/, /proc/,
/dev/, /usr/ and /run/ file systems.
Units written by generators are removed when the 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, .d/*.conf drop-ins
for them and symlinks to them, not any other kind of non-unit related 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.
Note that using the StandardInputData=/StandardInputText=
settings of service unit files (see
systemd.exec(5)), it
is possible to make arbitrary input data (including daemon-specific configuration) part of the unit
definitions, which often might be sufficient to embed data or configuration for other programs into
unit files in a native fashion.
Since
syslog(3)
is not available (see above), log messages have to be written to
/dev/kmsg instead.
The generator should always include its own name in a comment at the top of the generated file, so that the user can easily figure out which component created or amended a particular unit.
The SourcePath= directive should be used in generated files to specify the
source configuration file they are generated 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. The SourcePath= value does not have
to be a file in a physical filesystem. For example, in the common case of the generator looking at
the kernel command line, SourcePath=/proc/cmdline should be used.
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 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 use 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 is 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 their 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
systemd.offline-updates(7).
Example 3. Debugging 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-fstab-generator(8), fstab(5), systemd-getty-generator(8), systemd-gpt-auto-generator(8), systemd-hibernate-resume-generator(8), systemd-rc-local-generator(8), systemd-system-update-generator(8), systemd-sysv-generator(8), systemd-xdg-autostart-generator(8), systemd.unit(5), systemctl(1), systemd.environment-generator(7)