systemd.generator man page

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/*
/usr/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 /usr/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

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

Referenced By

dbus-daemon(1), systemctl(1), systemd(1), systemd-analyze(1), systemd-cryptsetup-generator(8), systemd-debug-generator(8), systemd.directives(7), systemd-environment-d-generator(8), systemd.environment-generator(7), systemd-fstab-generator(8), systemd-getty-generator(8), systemd-gpt-auto-generator(8), systemd.index(7), systemd.offline-updates(7), systemd-rc-local-generator(8), systemd-system-update-generator(8), systemd-sysv-generator(8), systemd-veritysetup-generator(8).

systemd 238 systemd.generator