1. backup Android filesInstall Software Data Cale https://play.google.com/store/apps/details?id=com.lyy.softdatacable&hl=en. This app start a ftp server on your phone so that you can access your files with WiFi. After you start the app, it shows its IP address in the home screen, something like ftp://22.214.171.124:8888/
Next in your local machine, you can use lftp to mirror the files in your phone to your desktop machine. The following command will sync all newer files with in the root directory of your phone to the the n4 directory at your desktop
lftp ftp://126.96.36.199:8888/ -e "mirror --verbose --only-newer / n4"