systemd.environment-generator - systemd environment file





          Generators are small executables that live in
          /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

     Page 1                     systemd 247          (printed 5/22/22)


          file generators described in systemd.generator(7), with the
          following differences:

          +o   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.

          +o   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.

     Page 2                     systemd 247          (printed 5/22/22)



              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), systemctl(1)

     Page 3                     systemd 247          (printed 5/22/22)