SYSTEMD.GENERATOR(7)                         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 placed in
          /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.

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          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.

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     SYSTEMD.GENERATOR(7)                         SYSTEMD.GENERATOR(7)

     NOTES ABOUT WRITING GENERATORS
          +o   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.

          +o   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.

          +o   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.

          +o   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.

          +o   Since syslog(3) is not available (see above), log
              messages have to be written to /dev/kmsg instead.

          +o   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

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              physical filesystem. For example, in the common case of
              the generator looking at the kernel command line,
              SourcePath=/proc/cmdline should be used.

          +o   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.

          +o   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.

          +o   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].

          +o   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 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

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          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-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)

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