Provided by: systemd_252.5-2ubuntu3_amd64 bug


       systemd.generator - systemd unit generators


       /path/to/generator normal-dir [early-dir] [late-dir]




       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 and execution context parameters that are 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.

       systemd will call each generator 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,
       allowing generated configuration to extend or override existing definitions. For tests,
       generators may be called with just one argument; the generator should assume that all
       three paths are the same in that case.

       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


       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. If only one argument is provided, the
       generator should use the same directory as the the three output paths.

        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.


       The service manager sets a number of environment variables when invoking generator
       executables. They carry information about the execution context of the generator, in order
       to simplify conditionalizing generators to specific environments. The following
       environment variables are set:

           If the generator is invoked from the system service manager this variable is set to
           "system"; if invoked from the per-user service manager it is set to "user".

           If the generator is run as part of an initrd this is set to "1". If it is run from the
           regular host (i.e. after the transition from initrd to host) it is set to "0". This
           environment variable is only set for system generators.

           If this boot-up cycle is considered a "first boot", this is set to "1"; if it is a
           subsequent, regular boot it is set to "0". For details see the documentation of
           ConditionFirstBoot= in systemd.unit(5). This environment variable is only set for
           system generators.

           If the service manager is run in a virtualized environment, $SYSTEMD_VIRTUALIZATION is
           set to a pair of strings, separated by a colon. The first string is either "vm" or
           "container", categorizing the type of virtualization. The second string identifies the
           implementation of the virtualization technology. If no virtualization is detected this
           variable will not be set. This data is identical to what systemd-detect-virt(1)
           detects and reports, and uses the same vocabulary of virtualization implementation

           This variable is set to a short identifier of the reported architecture of the system.
           For details about defined values, see documentation of ConditionArchitecture= in


       •   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

       •   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

       •   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

           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.


       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 may be used.

       Example 2. systemd-system-update-generator

       systemd-system-update-generator(8) temporarily redirects to, if a system update is scheduled. Since this needs to override the
       default user configuration for, 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


       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)