systemd.generator(7) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | OUTPUT DIRECTORIES | NOTES ABOUT WRITING GENERATORS | EXAMPLES | SEE ALSO | COLOPHON

SYSTEMD.GENERATOR(7)          systemd.generator         SYSTEMD.GENERATOR(7)

NAME         top

       systemd.generator - systemd unit generators

SYNOPSIS         top

       /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         top

       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         top

       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         top

       ·   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         top

       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         top

       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)

COLOPHON         top

       This page is part of the systemd (systemd system and service manager)
       project.  Information about the project can be found at 
       ⟨http://www.freedesktop.org/wiki/Software/systemd⟩.  If you have a bug
       report for this manual page, see
       ⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.  This
       page was obtained from the project's upstream Git repository
       ⟨https://github.com/systemd/systemd.git⟩ on 2020-09-18.  (At that
       time, the date of the most recent commit that was found in the repos‐
       itory was 2020-09-18.)  If you discover any rendering problems in
       this HTML version of the page, or you believe there is a better or
       more up-to-date source for the page, or you have corrections or
       improvements to the information in this COLOPHON (which is not part
       of the original manual page), send a mail to man-pages@man7.org

systemd 246                                             SYSTEMD.GENERATOR(7)

Pages that refer to this page: init(1)systemctl(1)systemd(1)systemd-analyze(1)systemd.unit(5)30-systemd-environment-d-generator(7)systemd.directives(7)systemd.environment-generator(7)systemd.index(7)systemd.offline-updates(7)30-systemd-environment-d-generator(8)systemd-bless-boot-generator(8)systemd-cryptsetup-generator(8)systemd-debug-generator(8)systemd-environment-d-generator(8)systemd-fstab-generator(8)systemd-getty-generator(8)systemd-gpt-auto-generator(8)systemd-rc-local-generator(8)systemd-run-generator(8)systemd-system-update-generator(8)systemd-sysv-generator(8)systemd-veritysetup-generator(8)systemd-xdg-autostart-generator(8)