Why does pressing escape go back one character?

Question

In vim, I've always been curious why pressing the esc key causes the cursor to go back by one character (usually). For example if I've typed:

This is |

With the cursor being the |, after I have finished typing and press esc, the line will look like:

This is|

And to re-type from where I left off I would have to press a instead of i. Why is this the expected behavior in vim?

Answer 1

I found the following answer in this question on the Unix & Linux Stack Exchange:

In insert mode, the cursor is between characters, or before the first or after the last character. In normal mode, the cursor is over a character (newlines are not characters for this purpose). This is somewhat unusual: most editors always put the cursor between characters, and have most commands act on the character after (not, strictly speaking, under) the cursor. This is perhaps partly due to the fact that before GUIs, text terminals always showed the cursor on a character (underline or block, perhaps blinking). This abstraction fails in insert mode because that requires one more position (posts vs fences).

Switching between modes has to move the cursor by a half-character, so to speak. The i command moves left, to put the cursor before the character it was over. The a command moves right. Going out of insert mode (by pressing Esc) moves the cursor left if possible (if it's at the beginning of the line, it's moved right instead).

I suppose the Esc behavior sort of makes sense. Often, you're typing at the end of the line, and there Esc can only go left. So the general behavior is the most common behavior.

Think of the character under the cursor as the last interesting character, and of the insert command as a. You can repeat a Esc without moving the cursor, except that you'll be bumped one position right if you start at the beginning of a non-empty line.

Credits to the original author.

If you would like to edit this behavior, you could follow the advice from @ib. in this answer:

Although I would not recommend changing the default cursor mechanics, one way of achieving the behavior in question is to use the following Insert-mode mapping.

:inoremap `^

Here the Esc key is overloaded in Insert mode to additionally run the `^ command which moves the cursor to the position where it had been the last time Insert mode was left. Since in this mapping it is executed immediately after leaving Insert mode with Esc, the cursor is left one character to the right as compared to its position with default behavior.

Unlike some other workarounds, this one does not require Vim to be compiled with the +ex_extra feature.