systemd.environment-generator - systemd environment file generators





       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. 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

       ·   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.

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



           # set the default value

           # add a directory if it exists
           if [[ -d /opt/foo/share ]]; then

           # write our output
           echo XDG_DATA_DIRS=$XDG_DATA_DIRS

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


           #!/usr/bin/env python3


           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')
               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, we is OK too.
               new = rearrange_bin_sbin(path)
               if new != path:

       Example 3. Debugging a generator

           SYSTEMD_LOG_LEVEL=debug VAR_A=something VAR_B="something else" \

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

systemd 243                                   SYSTEMD.ENVIRONMENT-GENERATOR(7)