NAME | SYNOPSIS | DESCRIPTION | 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 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         top

       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         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 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=$(mktemp -d)
           SYSTEMD_LOG_LEVEL=debug /usr/lib/systemd/system-generators/systemd-fstab-generator \
                   "$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-system-update-generator(8), systemd-sysv-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 2017-09-15.  If you dis‐
       cover 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 234                                             SYSTEMD.GENERATOR(7)

Pages that refer to this page: systemctl(1)systemd(1)systemd.unit(5)systemd.directives(7)systemd.environment-generator(7)systemd.index(7)systemd.offline-updates(7)systemd-debug-generator(8)systemd-environment-d-generator(8)systemd-fstab-generator(8)systemd-getty-generator(8)systemd-gpt-auto-generator(8)systemd-system-update-generator(8)systemd-sysv-generator(8)