NAME

       systemd.environment-generator - systemd environment file generators

SYNOPSIS

       /usr/lib/systemd/system-environment-generators/some-generator

       /usr/lib/systemd/user-environment-generators/some-generator

           /run/systemd/system-environment-generators/*
           /etc/systemd/system-environment-generators/*
           /usr/local/lib/systemd/system-environment-generators/*
           /usr/lib/systemd/system-environment-generators/*

           /run/systemd/user-environment-generators/*
           /etc/systemd/user-environment-generators/*
           /usr/local/lib/systemd/user-environment-generators/*
           /usr/lib/systemd/user-environment-generators/*

DESCRIPTION

       Generators are small executables that live in
       /usr/lib/systemd/system-environment-generators/ and other directories listed above.
       systemd(1) will execute those binaries very early at the startup of each manager and at
       configuration reload time, before running the generators described in systemd.generator(7)
       and before starting any units. Environment generators can override the environment that
       the manager exports to services and other processes.

       Generators are loaded from a set of paths determined during compilation, as listed above.
       System and user environment generators are loaded from directories with names ending in
       system-environment-generators/ and user-environment-generators/, respectively. Generators
       found in directories listed earlier override the ones with the same name in directories
       lower in the list [1]. 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 re-run all generators, updating environment configuration. It will
       be used for any services that are started subsequently.

       Environment file generators are executed similarly to unit file generators described in
       systemd.generator(7), with the following differences:

       •   Generators are executed sequentially in the alphanumerical order of the final
           component of their name. The output of each generator output is immediately parsed and
           used to update the environment for generators that run after that. Thus, later
           generators can use and/or modify the output of earlier generators.

       •   Generators are run by every manager instance, their output can be different for each
           user.

       It is recommended to use numerical prefixes for generator names to simplify ordering.

EXAMPLES

       Example 1. A simple generator that extends an environment variable if a directory exists
       in the file system

           # 50-xdg-data-dirs.sh

           #!/bin/sh
           # SPDX-License-Identifier: MIT-0

           # set the default value
           XDG_DATA_DIRS="${XDG_DATA_DIRS:-/usr/local/share/:/usr/share}"

           # add a directory if it exists
           if [ -d /opt/foo/share ]; then
               XDG_DATA_DIRS="/opt/foo/share:${XDG_DATA_DIRS}"
           fi

           # write our output
           echo "XDG_DATA_DIRS=${XDG_DATA_DIRS}"

       Example 2. A more complicated generator which reads existing configuration and mutates one
       variable

           # 90-rearrange-path.py

           #!/usr/bin/env python3
           # SPDX-License-Identifier: MIT-0

           """

           Proof-of-concept systemd environment generator that makes sure that bin dirs
           are always after matching sbin dirs in the path.
           (Changes /sbin:/bin:/foo/bar to /bin:/sbin:/foo/bar.)

           This generator shows how to override the configuration possibly created by
           earlier generators. It would be easier to write in bash, but let's have it
           in Python just to prove that we can, and to serve as a template for more
           interesting generators.

           """

           import os
           import pathlib

           def rearrange_bin_sbin(path):
               """Make sure any pair of .../bin, .../sbin directories is in this order

               >>> rearrange_bin_sbin('/bin:/sbin:/usr/sbin:/usr/bin')
               '/bin:/sbin:/usr/bin:/usr/sbin'
               """
               items = [pathlib.Path(p) for p in path.split(':')]
               for i in range(len(items)):
                   if 'sbin' in items[i].parts:
                       ind = items[i].parts.index('sbin')
                       bin = pathlib.Path(*items[i].parts[:ind], 'bin', *items[i].parts[ind+1:])
                       if bin in items[i+1:]:
                           j = i + 1 + items[i+1:].index(bin)
                           items[i], items[j] = items[j], items[i]
               return ':'.join(p.as_posix() for p in items)

           if __name__ == '__main__':
               path = os.environ['PATH'] # This should be always set.
                                         # If it's not, we'll just crash, which is OK too.
               new = rearrange_bin_sbin(path)
               if new != path:
                   print('PATH={}'.format(new))

       Example 3. Debugging a generator

           SYSTEMD_LOG_LEVEL=debug VAR_A=something VAR_B="something else" \
           /usr/lib/systemd/system-environment-generators/path-to-generator

SEE ALSO

       systemd-environment-d-generator(8), systemd.generator(7), systemd(1), systemctl(1)

NOTES

        1. 💣💥🧨💥💥💣 Please note that those configuration files must be available at all times.
           If /usr/local/ is a separate partition, it may not be available during early boot, and
           must not be used for configuration.