GNU Emacs Manual - Comparison in Dired

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## File Comparison with Dired

Here are two Dired commands that compare specified files using `diff`.

`=`
  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.
`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 backup version of your choice. The backup file is the first file given to `diff`.

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