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 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 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)

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