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25.3 Reverting

If you have made extensive changes to a file and then change your mind about them, you can get rid of them by reading in the previous version of the file with the revert-buffer command. See Reverting a Buffer in The GNU Emacs Manual.

Command: revert-buffer &optional ignore-auto noconfirm preserve-modes

This command replaces the buffer text with the text of the visited file on disk. This action undoes all changes since the file was visited or saved.

By default, if the latest auto-save file is more recent than the visited file, and the argument ignore-auto is nil, revert-buffer asks the user whether to use that auto-save instead. When you invoke this command interactively, ignore-auto is t if there is no numeric prefix argument; thus, the interactive default is not to check the auto-save file.

Normally, revert-buffer asks for confirmation before it changes the buffer; but if the argument noconfirm is non-nil, revert-buffer does not ask for confirmation.

Normally, this command reinitializes the buffer’s major and minor modes using normal-mode. But if preserve-modes is non-nil, the modes remain unchanged.

Reverting tries to preserve marker positions in the buffer by using the replacement feature of insert-file-contents. If the buffer contents and the file contents are identical before the revert operation, reverting preserves all the markers. If they are not identical, reverting does change the buffer; in that case, it preserves the markers in the unchanged text (if any) at the beginning and end of the buffer. Preserving any additional markers would be problematical.

Variable: revert-buffer-in-progress-p

revert-buffer binds this variable to a non-nil value while it is working.

You can customize how revert-buffer does its work by setting the variables described in the rest of this section.

User Option: revert-without-query

This variable holds a list of files that should be reverted without query. The value is a list of regular expressions. If the visited file name matches one of these regular expressions, and the file has changed on disk but the buffer is not modified, then revert-buffer reverts the file without asking the user for confirmation.

Some major modes customize revert-buffer by making buffer-local bindings for these variables:

Variable: revert-buffer-function

The value of this variable is the function to use to revert this buffer. It should be a function with two optional arguments to do the work of reverting. The two optional arguments, ignore-auto and noconfirm, are the arguments that revert-buffer received.

Modes such as Dired mode, in which the text being edited does not consist of a file’s contents but can be regenerated in some other fashion, can give this variable a buffer-local value that is a special function to regenerate the contents.

Variable: revert-buffer-insert-file-contents-function

The value of this variable specifies the function to use to insert the updated contents when reverting this buffer. The function receives two arguments: first the file name to use; second, t if the user has asked to read the auto-save file.

The reason for a mode to change this variable instead of revert-buffer-function is to avoid duplicating or replacing the rest of what revert-buffer does: asking for confirmation, clearing the undo list, deciding the proper major mode, and running the hooks listed below.

Variable: before-revert-hook

This normal hook is run by the default revert-buffer-function before inserting the modified contents. A custom revert-buffer-function may or may not run this hook.

Variable: after-revert-hook

This normal hook is run by the default revert-buffer-function after inserting the modified contents. A custom revert-buffer-function may or may not run this hook.

Variable: buffer-stale-function

The value of this variable specifies a function to call to check whether a buffer needs reverting. The default value only handles buffers that are visiting files, by checking their modification time. Buffers that are not visiting files require a custom function (see (emacs)Supporting additional buffers).

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