Allow expansion of env vars in options

scripts/doc_parser/docopt_ng/__init__.py

@@ -308,7 +308,7 @@ def parse(cls, option_description: str) -> _Option:
                 argcount = 1
         if argcount:
             matched = re.findall(r"\[default: (.*)\]", description, flags=re.I)
-            value = matched[0] if matched else None
+            value = expand_env_vars(matched[0]) if matched else None
         return cls(short, longer, argcount, value)
 
     def single_match(self, left: list[_LeafPattern]) -> _SingleMatch:
@@ -950,6 +950,27 @@ def show_log(message: str) -> None:
     print(f"{filename}:{line_number} - {message}", file=sys.stderr)
 
 
+# cli customization:
+def expand_env_vars(value: str) -> str:
+    """Expand `$VAR` and `${VAR}` in a docstring default against the environment.
+
+    Defaults come from the (trusted) docstring, so expanding them is safe. This is a pure
+    string substitution — it never invokes a shell — so command substitution (`$(...)`,
+    backticks) and bash-only forms (`${VAR:-default}`) are left untouched, and the expanded
+    result still flows through `bash_quote()` before `eval`. An unset variable expands to the
+    empty string (like an unquoted `$VAR` in the shell). User-supplied values are parsed
+    elsewhere and are never expanded.
+    """
+    import os
+
+    return re.sub(
+        r"\$\{([A-Za-z_][A-Za-z0-9_]*)\}|\$([A-Za-z_][A-Za-z0-9_]*)",
+        lambda m: os.environ.get(m.group(1) or m.group(2), ""),
+        value,
+        flags=re.ASCII,
+    )
+
+
 # cli customization:
 def bash_quote(value: str) -> str:
     """Return a single-quoted bash literal that is safe to `eval`.

tests/core/helpers/test_cli_parse_args.sh

@@ -242,6 +242,39 @@ pos2='CDE'"
   assertEquals "$expected" "$result"
 }
 
+test_parse_args_expands_env_vars_in_defaults() {
+  local help_text result
+
+  # `$VAR`/`${VAR}` in a `[default: ...]` are expanded against the environment (defaults come
+  # from the trusted docstring). An unset variable expands to the empty string; `$(...)` and
+  # backticks stay literal, and a user-supplied value is never expanded.
+  # shellcheck disable=SC2016  # single quotes intentional: `$VAR` must reach docopt literally
+  help_text='Usage:
+  anything cmd [--foo=<x>]
+
+Options:
+  --foo=<x>  Some parameter [default: $MYCLI_TEST_DEFAULT]'
+
+  result=$(MYCLI_TEST_DEFAULT='from env' parse_args "$help_text" 'cmd')
+  assertContains "env var in default is expanded" "$result" "export foo='from env'"
+
+  # shellcheck disable=SC2016  # single quotes intentional: value must reach docopt literally
+  result=$(MYCLI_TEST_DEFAULT='from env' parse_args "$help_text" 'cmd' '--foo=$MYCLI_TEST_DEFAULT')
+  assertContains "user-supplied value is not expanded" "$result" "export foo='\$MYCLI_TEST_DEFAULT'"
+
+  result=$(unset MYCLI_TEST_DEFAULT; parse_args "$help_text" 'cmd')
+  assertContains "unset var in default expands to empty" "$result" "export foo=''"
+
+  # shellcheck disable=SC2016  # single quotes intentional: `$(id)` must reach docopt literally
+  help_text='Usage:
+  anything cmd [--foo=<x>]
+
+Options:
+  --foo=<x>  Some parameter [default: $(id)]'
+  result=$(parse_args "$help_text" 'cmd')
+  assertContains "command substitution in default stays inert" "$result" "export foo='\$(id)'"
+}
+
 test_parse_help_does_not_evaluate_injected_code() {
   local result