systemd.generator(7) - man - phpMan

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 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. 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, as well as mounted /sys/, /proc/, /dev/,
           /usr/ and /run/ file systems.

       •   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, .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.

       •   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=$(mktemp -d)
           SYSTEMD_LOG_LEVEL=debug /lib/systemd/system-generators/systemd-fstab-generator \
                   "$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 249                                                                     SYSTEMD.GENERATOR(7)