Ubuntu Manpage: systemd.generator - systemd unit generators

Provided by: systemd_249.11-0ubuntu3.17_amd64

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 placed in /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.

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

• 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:

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.

EXAMPLES