SYSTEMD.GENERATOR(7) systemd.generator SYSTEMD.GENERATOR(7)
systemd.generator - systemd unit generators
/path/to/generator normal-dir early-dir late-dir
Generators are small executables that live in
/lib/systemd/system-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
may dynamically generate unit files (regular ones, instances as well as
templates) and unit file .d/ drop-ins, or create symbolic links to unit
files to add additional dependencies or instantiate existing templates,
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, 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
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
Generators are invoked with three arguments: paths to runtime
directories where generators can place their generated unit files or
argv may be used to override unit files in /usr, but not those
in /run or 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.
argv may be used to override unit files in /usr, in /run and in
/etc. This means that unit files placed in this directory take
precedence over all configuration, both vendor and
argv 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 over the generated
ones placed in this directory.
o 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
o 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, including a mounted /sys, /proc, /dev, /usr.
o 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.
o Generators should only be used to generate unit files and symlinks
to them, 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.
o Since syslog(3) is not available (see above), log messages have to
be written to /dev/kmsg instead.
o 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.
o 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
o 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.
o Regarding overriding semantics: there are two rules we try to
follow when thinking about the overriding semantics:
1. User configuration should override vendor configuration. This
(mostly) means that stuff from /etc should override stuff from
2. 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, argv, or argv, your default choice
should probably be argv.
o 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
Example 1. systemd-fstab-generator
systemd-fstab-generator(8) converts /etc/fstab into native mount units.
It uses argv 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
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. For details about this logic, see systemd.offline-
Example 3. Debugging a generator
SYSTEMD_LOG_LEVEL=debug /lib/systemd/system-generators/systemd-fstab-generator \
"$dir" "$dir" "$dir"
systemd(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-system-update-generator(8), systemd-sysv-
generator(8), systemd.unit(5), systemctl(1), systemd.environment-
systemd 236 SYSTEMD.GENERATOR(7)