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NAME | SYNOPSIS | DESCRIPTION | OUTPUT DIRECTORIES | NOTES ABOUT WRITING GENERATORS | EXAMPLES | SEE ALSO | COLOPHON |
SYSTEMD.GENERATOR(7) systemd.generator SYSTEMD.GENERATOR(7)
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 that live in
/usr/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.
Their main purpose is to convert configuration that is not native
into dynamically generated unit files.
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.
1. 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.
2. 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.
3. 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, including a mounted /sys, /proc,
/dev, /usr.
· 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 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.
· 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:
1. User configuration should override vendor configuration. This
(mostly) means that stuff from /etc should override stuff
from /usr.
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 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 $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-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)
This page is part of the systemd (systemd system and service manager)
project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have a bug
report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩. This
page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2020-08-13. (At that
time, the date of the most recent commit that was found in the repos‐
itory was 2020-08-11.) If you discover any rendering problems in
this HTML version of the page, or you believe there is a better or
more up-to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not part
of the original manual page), send a mail to man-pages@man7.org
systemd 246 SYSTEMD.GENERATOR(7)
Pages that refer to this page: init(1) , systemctl(1) , systemd(1) , systemd-analyze(1) , systemd.unit(5) , 30-systemd-environment-d-generator(7) , systemd.directives(7) , systemd.environment-generator(7) , systemd.index(7) , systemd.offline-updates(7) , 30-systemd-environment-d-generator(8) , systemd-bless-boot-generator(8) , systemd-cryptsetup-generator(8) , systemd-debug-generator(8) , systemd-environment-d-generator(8) , systemd-fstab-generator(8) , systemd-getty-generator(8) , systemd-gpt-auto-generator(8) , systemd-rc-local-generator(8) , systemd-run-generator(8) , systemd-system-update-generator(8) , systemd-sysv-generator(8) , systemd-veritysetup-generator(8) , systemd-xdg-autostart-generator(8)