man > 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/*
       /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
       Generators are small executables placed in /usr/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
       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. If only one argument is provided, the generator should use the
       same directory as 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/systemd/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/systemd/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/systemd/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.

       Note: generators must not write to other locations or otherwise make changes to system state.
       Generator output is supposed to last only until the next daemon-reload or daemon-reexec; if
       the generator is replaced or masked, its effects should vanish.

ENVIRONMENT
       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:

       $SYSTEMD_SCOPE
           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".

           Added in version 251.

       $SYSTEMD_IN_INITRD
           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.

           Added in version 251.

       $SYSTEMD_FIRST_BOOT
           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.

           Added in version 251.

       $SYSTEMD_VIRTUALIZATION
           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 identifiers.

           Added in version 251.

       $SYSTEMD_ARCHITECTURE
           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
           systemd.unit(5).

           Added in version 251.

       $CREDENTIALS_DIRECTORY, $ENCRYPTED_CREDENTIALS_DIRECTORY
           If set, refers to the directory system credentials have been placed in. Credentials
           passed into the system in plaintext form will be placed in $CREDENTIALS_DIRECTORY, and
           those passed in in encrypted form will be placed in $ENCRYPTED_CREDENTIALS_DIRECTORY. Use
           the systemd-creds(1) command to automatically decrypt/authenticate credentials passed in,
           if needed. Specifically, use the systemd-creds --system cat command.

           Added in version 254.

       $SYSTEMD_CONFIDENTIAL_VIRTUALIZATION
           If the service manager is run in a confidential virtualized environment,
           $SYSTEMD_CONFIDENTIAL_VIRTUALIZATION is set to a string that identifies the confidential
           virtualization hardware technology. If no confidential 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 confidential virtualization technology
           identifiers.

           Added in version 254.

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=$(mktemp -d)
           SYSTEMD_LOG_LEVEL=debug /usr/lib/systemd/system-generators/systemd-fstab-generator \
                   "$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 255                                                                     SYSTEMD.GENERATOR(7)