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SYSTEMD.GENERATOR(7) | systemd.generator | SYSTEMD.GENERATOR(7) |
NAME¶
systemd.generator - Systemd unit generatorsSYNOPSIS¶
/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 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, 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.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 overriding 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¶
•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, including a
mounted /sys, /proc, /dev, /usr.
•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, 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.
•Since syslog(3) is not available (see
above), log messages have to be written to /dev/kmsg instead.
•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.
•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 user 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 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. Debugging a generatordir=$(mktemp -d) SYSTEMD_LOG_LEVEL=debug /lib/systemd/system-generators/systemd-fstab-generator \ "$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-system-update-generator(8), systemd-sysv-generator(8), systemd.unit(5), systemctl(1)NOTES¶
- 1.
- Implementing Offline System Updates
systemd 230 |