let stdout = Pipe() process.standardOutput = stdout let stderr = Pipe() process.standardError = stderr try process.run() process.waitUntilExit() if let data = try stderr.fileHandleForReading.readToEnd(), let error = String(data: data, encoding: .utf8) { print(error) } ...

まあでもこんな感じでしょ。Foundation.Processでもまあいいんじゃないかなあ。