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 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 and execution context parameters
that are 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.
systemd will call each generator 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, allowing generated configuration to extend or override
existing definitions. For tests, generators may be called with just one
argument; the generator should assume that all three paths are the same
in that case.
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. If only one argument is provided, the generator
should use the same directory as the the three output paths.
1. normal-dir
In normal use this is /run/systemd/generator in case of the system
generators and $XDG_RUNTIME_DIR/systemd/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/systemd/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/systemd/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.
ENVIRONMENT
The service manager sets a number of environment variables when
invoking generator executables. They carry information about the
execution context of the generator, in order to simplify
conditionalizing generators to specific environments. The following
environment variables are set:
$SYSTEMD_SCOPE
If the generator is invoked from the system service manager this
variable is set to "system"; if invoked from the per-user service
manager it is set to "user".
$SYSTEMD_IN_INITRD
If the generator is run as part of an initrd this is set to "1". If
it is run from the regular host (i.e. after the transition from
initrd to host) it is set to "0". This environment variable is only
set for system generators.
$SYSTEMD_FIRST_BOOT
If this boot-up cycle is considered a "first boot", this is set to
"1"; if it is a subsequent, regular boot it is set to "0". For
details see the documentation of ConditionFirstBoot= in
systemd.unit(5). This environment variable is only set for system
generators.
$SYSTEMD_VIRTUALIZATION
If the service manager is run in a virtualized environment,
$SYSTEMD_VIRTUALIZATION is set to a pair of strings, separated by a
colon. The first string is either "vm" or "container", categorizing
the type of virtualization. The second string identifies the
implementation of the virtualization technology. If no
virtualization is detected this variable will not be set. This data
is identical to what systemd-detect-virt(1) detects and reports,
and uses the same vocabulary of virtualization implementation
identifiers.
$SYSTEMD_ARCHITECTURE
This variable is set to a short identifier of the reported
architecture of the system. For details about defined values, see
documentation of ConditionArchitecture= in systemd.unit(5).
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
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-xdg-autostart-
generator(8), systemd.unit(5), systemctl(1), systemd.environment-
generator(7)
systemd 252 SYSTEMD.GENERATOR(7)
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