Linux: Batch Convert .avi files to .mp4/.mkv

I’ve been trying to clean up my video library since building my latest NAS. In the process, I found a number of .avi files, which is an older file format that isn’t widely used these days. While every time a file is converted it loses some of its quality, I was willing to risk the slight quality reduction to convert the remaining few files I had to convert into more modern file formats.

I initially tried converting the files using HandBrake. But given the number I needed to convert, I decided pretty quickly that I needed a faster method for this. Enter stackoverflow.

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

Or if you wanted to convert a bunch of .mp4 files into .mkv files, you could do this:

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

BONUS:

If you have .avi files in a number of subfolders, you’ll want to use this script:

find . -exec ffmpeg -i {} {}.mp4 \;

To use it, navigate in a terminal to the top-level folder, then execute this command. It will search through all the subfolders, find all the files in those subfolders, and convert them all to .mp4 files.

Of course, if you have a mixture of file types in the folders, you’ll want a variation of this command that searches for just the files of a certain type. To do that, use this command:

find . -name *.avi -exec ffmpeg -i {} {}.mp4 \;

This command will find all the files with the extension .avi and convert them all to .mp4 files using ffmpeg.

And, if you’re feeling particularly adventurous, you could search for multiple file types and convert them all:

find . -name *.avi -or -name *.mkv -exec ffmpeg -i {} {}.mp4 \;

This code would find every file with the extension .avi or .mkv and convert it to .mp4 using ffmpeg.

NOTE: This command uses the default conversion settings of ffmpeg. If you want more fine-tuned conversions, you can always check the options available in converting video files using ffmpeg.

BONUS 2

If you want to specify the codec to use in converting the files, that is a little more complicated. For instance, if I want to use H265 instead of H264 as my codec, I could use the following code to convert all of my .avi files in a folder into .mkv files with H265 encoding:

for i in *.avi; do ffmpeg -i "$i" -c:v libx265 -crf 26 -preset fast -c:a aac -b:a 128k "${i%.*}.mkv"; done

The default setting in ffmpeg for audio is to pass it through. Thus, if you wanted to just convert the video to a new codec but leave the audio, you could use the following command:

for i in *.avi; do ffmpeg -i "$i" -c:v libx265 -crf 26 -preset fast "${i%.*}.mkv"; done

This will convert the video to H265 but retain whatever audio was included with the video file (the default is to take the audio with the highest number of channels).

Additional information on the various settings for H.265 is available here. Some quick notes: the number after “-crf” is basically an indicator of quality, but is inverted. Lower numbers improve the quality; higher numbers reduce the quality. Depending on what I’m encoding, I vary this from 24 (higher quality) to 32 (lower quality). This will affect the resulting file size. If time is not a concern, you can also change the variable after “-preset.” The options are ultrafast, superfast, veryfast, faster, fast, medium, slow, slower, and veryslow. Slower encodes will result in better quality output but it will take substantially longer to do the encode.

If you run into the problem where you are trying to do bulk conversions but the names of the videos you are converting have spaces in them, you may get the error: “find: paths must precede expression.” The solution is to put the pattern in single quotes, like this:

find . -name '*.mkv' -exec ffmpeg -i {} -c:v libx265 -crf 26 -preset fast {}-new.mkv \;