Provided by: systemd_239-7ubuntu10_amd64 bug

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. Their main purpose is to
       convert configuration that is not native into dynamically generated unit files.

       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, 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 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.

       ·   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 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 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-rc-local-generator(8), systemd-system-
       update-generator(8), systemd-sysv-generator(8), systemd.unit(5), systemctl(1),
       systemd.environment-generator(7)