WriteOnFile.swift

func getDocumentsDirectory() -> URL {
    let paths = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)
    return paths[0]
}

func writeOnFile(str: String) {
    let filename = getDocumentsDirectory().appendingPathComponent("output.txt")

    do {
        if FileManager.default.fileExists(atPath: filename.path), let data = "\(str)\n".data(using: .utf8) {
            do {
                let fileHandle = try FileHandle(forWritingTo: filename)
                fileHandle.seekToEndOfFile()
                fileHandle.write(data)
                fileHandle.closeFile()
            } catch {
                print(error)
            }
        } else {
            try str.write(to: filename, atomically: false, encoding: String.Encoding.utf8)
        }
    } catch {
        // failed to write file – bad permissions, bad filename, missing permissions, or more likely it can't be converted to the encoding
    }
}