systemd.generator

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/*
       /usr/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
       /usr/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. Their main
       purpose is to convert configuration that is not native into dynamically
       generated unit files.

       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,
           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.

       •   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.

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

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)



systemd 246                                                 SYSTEMD.GENERATOR(7)