Assuming you have all of your .avi video files in a single directory, navigate to that directory in a terminal and you can use the following single line of code to iterate through all of the .avi files and convert them to .mp4 files:

for i in .avi; do ffmpeg -i "$i" "${i%.}.mp4"; done

In case you’re interested, the code is a loop. The first part starts the loop (“for i in *.avi;”), telling the computer to look for every file with the file extension .avi. The second part tells the computer what to do with every file with that extension – convert it to a .mp4 file with the same name. The last piece indicates what to do when the loop is completed – done.

Of course, this code could also be used to convert any other file format into a different format by replacing the .avi or .mp4 file extensions in the appropriate places. For instance, to convert all the .avi files to .mkv, the code would look like this:

for i in .avi; do ffmpeg -i "$i" "${i%.}.mkv"; done