18.9 Comparing Files

The command M-x diff prompts for two file names, using the minibuffer, and displays the differences between the two files in a buffer named *diff*. This works by running the diff program, using options taken from the variable diff-switches. The value of diff-switches should be a string; the default is "-u" to specify a unified context diff. See Diff in Comparing and Merging Files, for more information about the diff program.

The output of the diff command is shown using a major mode called Diff mode. See Diff Mode.

The command M-x diff-backup compares a specified file with its most recent backup. If you specify the name of a backup file, diff-backup compares it with the source file that it is a backup of. In all other respects, this behaves like M-x diff.

The command M-x diff-buffer-with-file compares a specified buffer with its corresponding file. This shows you what changes you would make to the file if you save the buffer.

The command M-x compare-windows compares the text in the current window with that in the window that was the selected window before you selected the current one. (For more information about windows in Emacs, Windows.) Comparison starts at point in each window, after pushing each initial point value on the mark ring in its respective buffer. Then it moves point forward in each window, one character at a time, until it reaches characters that don’t match. Then the command exits.

If point in the two windows is followed by non-matching text when the command starts, M-x compare-windows tries heuristically to advance up to matching text in the two windows, and then exits. So if you use M-x compare-windows repeatedly, each time it either skips one matching range or finds the start of another.

With a numeric argument, compare-windows ignores changes in whitespace. If the variable compare-ignore-case is non-nil, the comparison ignores differences in case as well. If the variable compare-ignore-whitespace is non-nil, compare-windows normally ignores changes in whitespace, and a prefix argument turns that off.

You can use M-x smerge-mode to turn on Smerge mode, a minor mode for editing output from the diff3 program. This is typically the result of a failed merge from a version control system update outside VC, due to conflicting changes to a file. Smerge mode provides commands to resolve conflicts by selecting specific changes.

See Emerge, for the Emerge facility, which provides a powerful interface for merging files.