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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/*
/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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