How to find out what codec is being used in a video

One way of doing this is by using VLC. Open the file in question, right click on it in the playlist and you'll see something like this:

I'm on OS X but the idea is the same.

Another way is to open a terminal, navigate to the directory containing the file and do:

file foo.avi

This doesn't provide a great deal of information but it is a start.

Another command line option with less "harsh" information:

ffprobe -show_streams "file.mp4"

Even with better output, it's still hard to a regular guy understand. If you need samples of several file formats to test it:

Video Format Examples

You can get information about Audio/Video Codecs and additional information by installing mediainfo. Both GUI (Graphical User Interface) and command-line utilities are available. Use these commands to install:

sudo apt-get update
sudo apt-get install mediainfo

If you want to install GUI,

sudo apt-get install mediainfo-gui

for reference visit mediainfo

The hard to find 'midenitfy' script. An ancient copy looks like:

#!/bin/sh
#
# This is a wrapper around the -identify functionality.
# It is supposed to escape the output properly, so it can be easily
# used in shellscripts by 'eval'ing the output of this script. 
#
# Written by Tobias Diedrich <[email protected]>
# Licensed under GNU GPL.

if [ -z "$1" ]; then
        echo "Usage: midentify <file> [<file> ...]"
        exit 1
fi

 mplayer -vo null -ao null -frames 0 -identify "$@" 2>/dev/null |
        sed -ne '/^ID_/ {
                           s/[]()|&;<>`'"'"'\\!$" []/\\&/g;p
                        }'