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NAME | SYNOPSIS | DESCRIPTION | OUTPUT DIRECTORIES | ENVIRONMENT | NOTES ABOUT WRITING GENERATORS | EXAMPLES | SEE ALSO | NOTES | COLOPHON |
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SYSTEMD.GENERATOR(7) systemd.generator SYSTEMD.GENERATOR(7)
systemd.generator - systemd unit generators
/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/*
Generators are small executables placed in
/usr/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.
Generators executed by the system manager are invoked in a sandbox
with a private writable /tmp/ directory and where most of the file
system is read-only except for the generator output directories.
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 [1]. 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.
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 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.
Note: generators must not write to other locations or otherwise
make changes to system state. Generator output is supposed to last
only until the next daemon-reload or daemon-reexec; if the
generator is replaced or masked, its effects should vanish.
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".
Added in version 251.
$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.
Added in version 251.
$SYSTEMD_SOFT_REBOOTS_COUNT
If the system has soft-rebooted, this variable is set to the
count of soft-reboots. This environment variable is only set
for system generators.
Added in version 257.
$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.
Added in version 251.
$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.
Added in version 251.
$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).
Added in version 251.
$CREDENTIALS_DIRECTORY, $ENCRYPTED_CREDENTIALS_DIRECTORY
If set, refers to the directory system credentials have been
placed in. Credentials passed into the system in plaintext
form will be placed in $CREDENTIALS_DIRECTORY, and those
passed in in encrypted form will be placed in
$ENCRYPTED_CREDENTIALS_DIRECTORY. Use the systemd-creds(1)
command to automatically decrypt/authenticate credentials
passed in, if needed. Specifically, use the systemd-creds
--system cat command.
Added in version 254.
$SYSTEMD_CONFIDENTIAL_VIRTUALIZATION
If the service manager is run in a confidential virtualized
environment, $SYSTEMD_CONFIDENTIAL_VIRTUALIZATION is set to a
string that identifies the confidential virtualization
hardware technology. If no confidential 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 confidential virtualization technology
identifiers.
Added in version 254.
• 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.
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 /usr/lib/systemd/system-generators/systemd-fstab-generator \
"$dir" "$dir" "$dir"
find $dir
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)
1. 💣💥🧨💥💥💣 Please note that those configuration files must
be available at all times. If /usr/local/ is a separate
partition, it may not be available during early boot, and must
not be used for configuration.
This page is part of the systemd (systemd system and service
manager) project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have a
bug report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
This page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2025-02-02. (At that
time, the date of the most recent commit that was found in the
repository was 2025-02-02.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
systemd 258~devel SYSTEMD.GENERATOR(7)
Pages that refer to this page: systemctl(1), systemd(1), systemd-analyze(1), systemd.unit(5), systemd.directives(7), systemd.environment-generator(7), systemd.index(7), systemd.offline-updates(7), systemd-bless-boot-generator(8), systemd-cryptsetup-generator(8), systemd-debug-generator(8), systemd-environment-d-generator(8), systemd-fstab-generator(8), systemd-getty-generator(8), systemd-gpt-auto-generator(8), systemd-import-generator(8), systemd-integritysetup-generator(8), systemd-network-generator.service(8), systemd-rc-local-generator(8), systemd-run-generator(8), systemd-ssh-generator(8), systemd-system-update-generator(8), systemd-sysv-generator(8), systemd-tpm2-generator(8), systemd-veritysetup-generator(8), systemd-xdg-autostart-generator(8)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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