Here are two Dired commands that compare specified files using diff .
diff program (dired-diff ). The file at the mark is the first argument of diff , and the file at point is the second argument.
dired-backup-diff ). If the current file is itself a backup, compare it with the file it is a backup of; this way, you can compare a file with any backup version of your choice.
The backup file is the first file given to diff .