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1
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2 @node Undo, Minibuffer, Basic, Top
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3 @chapter Undoing Changes
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4 @cindex undo
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5 @cindex mistakes, correcting
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6
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7 Emacs allows you to undo all changes you make to the text of a buffer,
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8 up to a certain amount of change (8000 characters). Each buffer records
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9 changes individually, and the undo command always applies to the
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10 current buffer. Usually each editing command makes a separate entry
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11 in the undo records, but some commands such as @code{query-replace}
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12 make many entries, and very simple commands such as self-inserting
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13 characters are often grouped to make undoing less tedious.
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14
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15 @table @kbd
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16 @item C-x u
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17 Undo one batch of changes (usually, one command's worth) (@code{undo}).
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18 @item C-_
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19 The same.
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20 @end table
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21
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22 @kindex C-x u
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23 @kindex C-_
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24 @findex undo
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25 The command @kbd{C-x u} or @kbd{C-_} allows you to undo changes. The
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26 first time you give this command, it undoes the last change. Point
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27 moves to the text affected by the undo, so you can see what was undone.
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28
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29 Consecutive repetitions of the @kbd{C-_} or @kbd{C-x u} commands undo
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30 earlier and earlier changes, back to the limit of what has been
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31 recorded. If all recorded changes have already been undone, the undo
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32 command prints an error message and does nothing.
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33
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34 Any command other than an undo command breaks the sequence of undo
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35 commands. Starting at this moment, the previous undo commands are
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36 considered ordinary changes that can themselves be undone. Thus, you can
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37 redo changes you have undone by typing @kbd{C-f} or any other command
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38 that have no important effect, and then using more undo commands.
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39
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40 If you notice that a buffer has been modified accidentally, the
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41 easiest way to recover is to type @kbd{C-_} repeatedly until the stars
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42 disappear from the front of the mode line. When that happens, all the
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43 modifications you made have been canceled. If you do not remember
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44 whether you changed the buffer deliberately, type @kbd{C-_} once. When
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45 you see Emacs undo the last change you made, you probably remember why you
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46 made it. If the change was an accident, leave it undone. If it was
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47 deliberate, redo the change as described in the preceding paragraph.
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48
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49 Whenever an undo command makes the stars disappear from the mode line,
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50 the buffer contents is the same as it was when the file was last read in
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51 or saved.
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52
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53 Not all buffers record undo information. Buffers whose names start with
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54 spaces don't; these buffers are used internally by Emacs and its extensions
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55 to hold text that users don't normally look at or edit. Minibuffers,
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56 help buffers, and documentation buffers also don't record undo information.
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57
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58 Emacs can remember at most 8000 or so characters of deleted or
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59 modified text in any one buffer for reinsertion by the undo command.
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60 There is also a limit on the number of individual insert, delete, or
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61 change actions that Emacs can remember.
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62
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63 There are two keys to run the @code{undo} command, @kbd{C-x u} and
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64 @kbd{C-_}, because on some keyboards, it is not obvious how to type
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65 @kbd{C-_}. @kbd{C-x u} is an alternative you can type in the same
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66 fashion on any terminal.
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