Ubuntu Manpage: systemd.generator - Systemd unit generators

Provided by: systemd_229-4ubuntu21.31_amd64

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 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 files.

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.

WRITING GENERATORS

Generators are invoked with three arguments: paths to runtime directories where generators
can place their generated unit files or symlinks.

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 user/administrator unit configuration.

2. early-dir

argv[2] may be used to override unit files in /usr and in /etc. This means that unit
files placed in this directory take precedence over all configuration, both vendor and
user/administrator.

3. late-dir

argv[3] 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 over the generated ones placed in this directory.

Notes
• 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, 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 user 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 Implementing Offline System Updates[1].

       Example 3. Debugging a generator

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

NOTES