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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/*
       /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 that live in
       /lib/systemd/system-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
       may dynamically generate unit files (regular ones, instances as well as
       templates) and unit file .d/ drop-ins, or create symbolic links to unit
       files to add additional dependencies or instantiate existing templates,
       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 /run or 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, in /run 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 and symlinks
	   to them, 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 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-system-update-generator(8), systemd-sysv-
       generator(8), systemd.unit(5), systemctl(1), systemd.environment-
       generator(7)

systemd 236						  SYSTEMD.GENERATOR(7)
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