Ubuntu Manpage: systemd.generator - Systemd unit generators

name
synopsis
description
writing generators
examples
see also
notes

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

        1. Implementing Offline System Updates
           http://www.freedesktop.org/wiki/Software/systemd/SystemUpdates