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




       Generators are small binaries that live in
       /usr/lib/systemd/user-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
       can dynamically generate unit files or create symbolic links to unit
       files to add additional dependencies, thus extending or overriding
       existing definitions. Their main purpose is to convert configuration
       files that are not native unit files dynamically into native unit

       Generators are loaded from a set of paths determined during
       compilation, 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

       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 runtime
       directories where generators can place their generated unit files or

        1. normal-dir

           argv[1] may be used to override unit files in /usr, but not those
           in /etc. This means that unit files placed in this directory take
           precedence over vendor unit configuration but not over native

       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)!). They can however rely on the
           most basic kernel functionality to be available, including mounted
           /sys, /proc, /dev.

       o   Units written by generators are removed when 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, 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

       o   If you are careful you can implement generators in shell scripts.
           We do recommend C code however, since generators delay 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


       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 Implementing Offline
       System Updates[1].

       Example 3. Debuging a generator

           dir=$(mktemp -d)
           SYSTEMD_LOG_LEVEL=debug /usr/lib/systemd/system-generators/systemd-fstab-generator \
                   "$dir" "$dir" "$dir"
           find $dir


       systemd(1), systemd-cryptsetup-generator(8), systemd-debug-
       generator(8), systemd-efi-boot-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),


        1. Implementing Offline System Updates

systemd 219 SYSTEMD.GENERATOR(7)

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