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/* /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 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-system-update-generator(8), systemd-sysv- generator(8), systemd.unit(5), systemctl(1)
Opportunity
Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.
Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.
Free Software
Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.
Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.
Free Books
The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.
Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.
Education
Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.
Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.
