When executing external commands via subprocess, don't rely solely on return codes for error detection. Some tools may return 0 even when they fail. Instead, check multiple error indicators including return codes, stderr output, and stdout content to ensure robust error handling. Use higher-level subprocess methods like `subprocess.run()` or `subprocess.check_output()` when possible, as they provide better built-in error handling. When using `subprocess.Popen()`, always examine both the return code and error output: ```python # Instead of only checking return code: if self.hdc_process.returncode != 0: logging.warning(f"HDC reverse port forwarding failed: {stderr.decode()}") # Check both return code AND error message content: stdout, stderr = self.hdc_process.communicate() if self.hdc_process.returncode != 0 or "error" in stderr.decode().lower(): logging.warning(f"HDC reverse port forwarding failed: {stderr.decode()}") return False ``` This approach is especially important when working with external tools that may not follow standard Unix conventions for exit codes.