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.
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