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/*
/usr/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 binaries that live in
/usr/lib/systemd/user-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
can dynamically generate unit files or create symbolic links to unit
files to add additional dependencies, 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, 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.
WRITING GENERATORS
Generators are invoked with three arguments: paths to runtime
directories where generators can place their generated unit files or
symlinks.
1. normal-dir
argv[1] may be used to override unit files in /usr, but not those
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.
2. early-dir
argv[2] may be used to override unit files in /usr and in /etc.
This means that unit files placed in this directory take precedence
over all configuration, both vendor and user/administrator.
3. late-dir
argv[3] may be used to extend the unit file tree without
overridding 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.
Notes
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)!). They can however rely on the
most basic kernel functionality to be available, including mounted
/sys, /proc, /dev.
o Units written by generators are removed when 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, 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
used.
o If you are careful you can implement generators in shell scripts.
We do recommend C code however, since generators delay 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 user 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's 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 her 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 Implementing Offline
System Updates[1].
Example 3. Debuging a generator
dir=$(mktemp -d)
SYSTEMD_LOG_LEVEL=debug /usr/lib/systemd/system-generators/systemd-fstab-generator \
"$dir" "$dir" "$dir"
find $dir
SEE ALSO
systemd(1), systemd-cryptsetup-generator(8), systemd-debug-
generator(8), systemd-efi-boot-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)
NOTES
1. Implementing Offline System Updates
http://www.freedesktop.org/wiki/Software/systemd/SystemUpdates
systemd 219 SYSTEMD.GENERATOR(7)
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