генераторы модулей systemd (systemd unit generators)
Имя (Name)
systemd.generator - systemd unit generators
Синопсис (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/*
Описание (Description)
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 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.
Примеры (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 /usr/lib/systemd/system-generators/systemd-fstab-generator \
"$dir" "$dir" "$dir"
find $dir
Смотри также (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)
