Erasing Basic Basic Files

3.4: Undoing Changes

You can undo all the recent changes in the buffer text, up to a certain point. Each buffer records changes individually, and the undo command always applies to the current buffer. Usually each editing command makes a separate entry in the undo records, but some commands such as query-replace make many entries, and very simple commands such as self-inserting characters are often grouped to make undoing less tedious.

The command C-x u or C-_ is how you undo. The first time you give this command, it undoes the last change. Point moves back to where it was before the command that made the change.

Consecutive repetitions of C-_ or C-x u undo earlier and earlier changes, back to the limit of the undo information available. If all recorded changes have already been undone, the undo command prints an error message and does nothing.

Any command other than an undo command breaks the sequence of undo commands. Starting from that moment, the previous undo commands become ordinary changes that you can undo. Thus, to redo changes you have undone, type C-f or any other command that will harmlessly break the sequence of undoing, then type more undo commands.

Ordinary undo applies to all changes made in the current buffer. You can also perform selective undo, limited to the current region. To do this, specify the region you want, then run the undo command with a prefix argument (the value does not matter): C-u C-x u or C-u C-_. This undoes the most recent change in the region. To undo further changes in the same region, repeat the undo command (no prefix argument is needed). In Transient Mark mode, any use of undo when there is an active region performs selective undo; you do not need a prefix argument.

If you notice that a buffer has been modified accidentally, the easiest way to recover is to type C-_ repeatedly until the stars disappear from the front of the mode line. At this time, all the modifications you made have been canceled. Whenever an undo command makes the stars disappear from the mode line, it means that the buffer contents are the same as they were when the file was last read in or saved.

If you do not remember whether you changed the buffer deliberately, type C-_ once. When you see the last change you made undone, you will see whether it was an intentional change. If it was an accident, leave it undone. If it was deliberate, redo the change as described above.

Not all buffers record undo information. Buffers whose names start with spaces don't; these buffers are used internally by Emacs and its extensions to hold text that users don't normally look at or edit.

You cannot undo mere cursor motion; only changes in the buffer contents save undo information. However, some cursor motion commands set the mark, so if you use these commands from time to time, you can move back to the neighborhoods you have moved through by popping the mark ring (see Mark Ring).

When the undo information for a buffer becomes too large, Emacs discards the oldest undo information from time to time (during garbage collection). You can specify how much undo information to keep by setting two variables: undo-limit and undo-strong-limit. Their values are expressed in units of bytes of space.

The variable undo-limit sets a soft limit: Emacs keeps undo data for enough commands to reach this size, and perhaps exceed it, but does not keep data for any earlier commands beyond that. Its default value is 20000. The variable undo-strong-limit sets a stricter limit: the command which pushes the size past this amount is itself forgotten. Its default value is 30000.

Regardless of the values of those variables, the most recent change is never discarded, so there is no danger that garbage collection occurring right after an unintentional large change might prevent you from undoing it.

The reason the undo command has two keys, C-x u and C-_, set up to run it is that it is worthy of a single-character key, but on some keyboards it is not obvious how to type C-_. C-x u is an alternative you can type straightforwardly on any terminal.

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