in core/shelllib.py [0:0]
def _repl_env_var(match):
varname = match.group(1)
value = os.getenv(varname, None)
if value is None:
value = match.group(3) or ""
if varname in value:
raise ShellExpansionError(f"Environment variable is recursively defined: {varname}")
return _shell_expand_str(value)