Document environment variables (#3833)

src/ansiblelint/cli.py

@@ -16,7 +16,7 @@
     Options,
     log_entries,
 )
-from ansiblelint.constants import CUSTOM_RULESDIR_ENVVAR, DEFAULT_RULESDIR, RC
+from ansiblelint.constants import CUSTOM_RULESDIR_ENVVAR, DEFAULT_RULESDIR, EPILOG, RC
 from ansiblelint.file_utils import (
     Lintable,
     abspath,
@@ -240,7 +240,11 @@ def merge_fix_list_config(
 
 def get_cli_parser() -> argparse.ArgumentParser:
     """Initialize an argument parser."""
-    parser = argparse.ArgumentParser()
+    parser = argparse.ArgumentParser(
+        epilog=EPILOG,
+        # Avoid rewrapping description and epilog
+        formatter_class=argparse.RawTextHelpFormatter,
+    )
 
     listing_group = parser.add_mutually_exclusive_group()
     listing_group.add_argument(

src/ansiblelint/constants.py

@@ -7,6 +7,17 @@
 CUSTOM_RULESDIR_ENVVAR = "ANSIBLE_LINT_CUSTOM_RULESDIR"
 RULE_DOC_URL = "https://ansible-lint.readthedocs.io/rules/"
 
+ENV_VARS_HELP = {
+    CUSTOM_RULESDIR_ENVVAR: "Used for adding another folder into the lookup path for new rules.",
+    "ANSIBLE_LINT_IGNORE_FILE": "Define it to override the name of the default ignore file `.ansible-lint-ignore`",
+    "ANSIBLE_LINT_WRITE_TMP": "Tells linter to dump fixes into different temp files instead of overriding original. Used internally for testing.",
+}
+
+EPILOG = (
+    "The following environment variables are also recognized but there is no guarantee that they will work in future versions:\n\n"
+    + "\n".join(f"{key}: {value}\n" for key, value in ENV_VARS_HELP.items())
+)
+
 
 # Not using an IntEnum because only starting with py3.11 it will evaluate it
 # as int.