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SYSTEMD.GENERATOR(7) systemd.generator SYSTEMD.GENERATOR(7)

NAME
systemd.generator - systemd unit generators

SYNOPSIS
/path/to/generator normal-dir early-dir late-dir

/run/systemd/system-generators/*
/etc/systemd/system-generators/*
/usr/local/lib/systemd/system-generators/*
/lib/systemd/system-generators/*

/run/systemd/user-generators/*
/etc/systemd/user-generators/*
/usr/local/lib/systemd/user-generators/*
/usr/lib/systemd/user-generators/*

DESCRIPTION
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. 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.

OUTPUT DIRECTORIES
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.

NOTES ABOUT WRITING GENERATORS
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
this parallelism.

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 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.

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
used.

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
/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].

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
artificially alive.

EXAMPLES
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=$(mktemp -d)
SYSTEMD_LOG_LEVEL=debug /lib/systemd/system-generators/systemd-fstab-generator \
"$dir" "$dir" "$dir"
find $dir

SEE ALSO
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.unit(5), systemctl(1),
systemd.environment-generator(7)

systemd 245 SYSTEMD.GENERATOR(7)