add support for auto-generated environment variables (jsonargparse)

docs/usage/general/environment.rst.inc

@@ -284,3 +284,29 @@ Please note:
 .. _INI: https://docs.python.org/3/library/logging.config.html#configuration-file-format
 
 .. _tempfile: https://docs.python.org/3/library/tempfile.html#tempfile.gettempdir
+
+
+Automatically generated Environment Variables (jsonargparse)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Borg uses jsonargparse_ with ``default_env=True``, which means that every
+command-line option can also be set via an environment variable.
+
+The environment variable name is derived from the program name (``borg``),
+the subcommand (if any), and the option name, all converted to uppercase
+with dashes replaced by underscores.
+
+For **top-level options** (not specific to a subcommand), the pattern is::
+
+    BORG_<OPTION>
+
+For example, ``--lock-wait`` can be set via ``BORG_LOCK_WAIT``.
+
+For **subcommand options**, the subcommand and option are separated by a
+double underscore::
+
+    BORG_<SUBCOMMAND>__<OPTION>
+
+For example, ``borg create --comment`` can be set via ``BORG_CREATE__COMMENT``.
+
+.. _jsonargparse: https://jsonargparse.readthedocs.io/

src/borg/archiver/__init__.py

@@ -251,6 +251,8 @@ class Archiver(
             prog=self.prog,
             description="Borg - Deduplicated Backups",
             default_config_files=[os.path.join(get_config_dir(), "default.yaml")],
+            default_env=True,
+            env_prefix="BORG",
         )
         parser.add_argument("--config", action="config")
         # paths and patterns must have an empty list as default everywhere