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 247                                     SYSTEMD.ENVIRONMENT-GENERATOR(7)