When you edit text and make changes to a vi file, you might occasionally wish that you had not changed something. vi's undo commands enable you to back up one operation and continue on from there.
If you make a mistake in vi or if you just change your mind after an operation is completed, you can undo your last command by pressing u immediately after the command. You do not need to press Esc after you type u. By pressing u a second time you undo the undo.
Type U to undo all changes you've made to a line. This command works only if you haven't moved the cursor off the line. You do not need to press Esc after you type U.