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How to do this varies across the different Linux distributives, but you usually need to find a program that has a dark screen with some bright characters on it as an icon. It is usually labelled as something like "Terminal" or "Console". On a few systems, you can open it by pressing Ctrl+Alt+T. You can convert video-to-video, video-to-audio, and audio-to-audio using the following formats:   Video - MP4, MOV, WEBM, FLV, AIFF, and AVI are all common video formats which are supported by FFmpeg.  Audio - MP3, WAV, WMA, M4A, AAC, and OGG are all common audio formats which are supported by FFmpeg. If you approximately know where it is, you can navigate into the directory using cd dir_name for changing into a directory and ls for listing the contents of the current directory. The conversion command follows the format ffmpeg -i source_file target_file. Be prepared to wait a long time, especially if you're converting a long and/or high-quality video.  For example, to convert an MP4 video called "Kitties" into an OGG video called "Cats", you would type ffmpeg -i Kitties.mp4 Cats.ogg. If you want to convert video to audio, add the -vn option to tell FFmpeg that you don't need a video. Try also adding the option -acodec copy to avoid audio quality loss, although that may not always be possible. For example, to convert an MP4 video called "Kitties" into a WAV audio file called "Kitty-sounds", you would type ffmpeg -i Kitties.mp4 -vn -acodec copy Kitty-sounds.wav. By default, FFmpeg outputs a lot of text during the conversion. If you don't want to see it, you can add the -loglevel panic option to tell FFmpeg to only print serious errors and questions. Be sure to keep your file name exactly as it appears (for example, if your file name uses a capital letter, enter it using the capital letter in the command line). The Linux terminal is case-sensitive, so it won't open your file otherwise. If your file name uses spaces, you can place the video's name and extension in quotes. For example, YouTube Drama.mp4 would become "YouTube Drama.mp4" (placing underscores doesn't work if they aren't in the title). To open the new file from the terminal, try mpv file_name, vlc file_name or totem file_name. These are some common video players for Linux, although it is possible that you don't have any of these installed. If you know how to open any other video player from the terminal, you can also use that. You can also open the file manager, navigate to the directory where the original and now also the converted file is, and double-click on the converted file. This should open it with the default video player of the system.
Open the terminal. Figure out a target format. Locate the file that you want to convert. Enter the conversion command. View your converted file.