systemd.generator(7)



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/*
       /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
       /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. Generators
       may dynamically generate unit files (regular ones, instances as well as
       templates) and unit file .d/ drop-ins, or create symbolic links to unit
       files to add additional dependencies or instantiate existing templates,
       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 /run or 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, in /run 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
       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, including a mounted /sys, /proc, /dev, /usr.

       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 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.

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

       o   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.

       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 user 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 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), systemd.environment-
       generator(7)

systemd 236                                               SYSTEMD.GENERATOR(7)

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