37.10 File Comparison with Dired

Here are two Dired commands that compare specified files using diff. They show the output in a buffer using Diff mode (see Comparing Files).

=

Compare the current file (the file at point) with another file (the file at the mark) using the diff program (dired-diff). The file at the mark is the first argument of diff, and the file at point is the second argument. This refers to the ordinary Emacs mark, not Dired marks; use C-<SPC> (set-mark-command) to set the mark at the first file's line (see Setting Mark).

M-=

Compare the current file with its latest backup file (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 one of its backups.

The backup file is the first file given to diff.