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Undo is a command in many computer programs. It erases the last change done to the document reverting it to an older state. In some more advanced programs such as graphic processing, undo will negate the last command done to the file being edited.
The opposite of undo is redo. The redo command reverses the undo or advances the buffer to a more current state.
In most Windows applications, the Undo command is activated by pressing the Ctrl+Z or Alt+Backspace keybindings. In all Macintosh applications, the Undo command is activated by pressing Command-Z. The common command for Redo on Microsoft Windows systems is Ctrl+Y or Ctrl+Shift+Z. The common command for Redo on Apple Macintosh systems is Command-Shift-Z.
History
The ability to undo an operation on a computer was independently invented multiple times, as a response to how people used computers.
The File Retrieval and Editing System, developed starting in 1968 at Brown University, is reported to be the first computer-based system to have had an "undo" feature.
Warren Teitelman developed a Programmer's Assistant as part of BBN-LISP with an Undo function, by 1971.
The Xerox PARC Bravo text editor had an Undo command in 1974. Behavioral Issues in the Use of Interactive Systems, a 1976 research report by Lance A. Miller and John C. Thomas of IBM, noted that "It would be quite useful to permit users to 'take back' at least the immediately preceding command (by issuing some special 'undo' command)." The programmers at the Xerox PARC research center assigned the keyboard shortcut Ctrl-Z to the undo command, which became a crucial feature of text editors and word processors in the personal computer era.
Multi-level undo commands were introduced in the 1980s, allowing the users to take back a series of actions, not just the most recent one. AtariWriter, a word-processing application introduced in 1982, featured undo. NewWord, another word-processing program released by NewStar in 1984, had an unerase command. IBM's VisiWord also had an undelete command.
Undo models
Undo models can be categorized as linear or non-linear:
When multiple users can edit the same document simultaneously, a multi-user undo is needed. Global multi-user undo reverts the latest action made to the document, regardless of who performed the edit. Local multi-user undo only reverts actions done by the local user, which requires a non-linear undo implementation.
The number of previous actions that can be undone varies by program, version, and hardware or software capabilities. For example, the default undo/redo stack size in Adobe Photoshop is 20 but can be changed by the user. Furthermore, earlier versions of Microsoft Paint only allowed up to three edits to be undone; the version introduced in Windows 7 increased this limit to 50.
Simplistic, single-edit undo features sometimes do away with "redo" by treating the undo command itself as an action that can be undone. This is known as the flip undo model, because the user can flip between two program states using the undo command. This was the standard model prior to the widespread adoption of multiple-level undo in the early 1990s.
Redo models
Redo reverts the effects of the undo action. The simplest form of redo is flip-undo, in which using undo after undoing redoes the undone action. In this case, the program flips back and forth between two states when the undo button is pressed.
In a more typical redo model, there are separate undo and redo buttons. The redo can be used for each undo action performed. Making a new edit usually clears the redo list. If a branching redo model is used, the new edit branches the action history.
Undo implementation
The Command pattern is a software design pattern which can be used to implement Multi-level Undo. The Memento pattern is useful for capturing the state of the program before user actions. This can be used for state reversal needed in some undo implementations.