BS Backspace

Move the cursor one column to the left. Nothing is erased: the character under the cursor stays where it is until something is written over it, which is how a shell’s line editor rubs out a character, with BS, a space and another BS.

At the left margin, or the first column when there are no margins, the cursor does not move. xterm’s reverse wraparound mode (CSI ? 45 h) makes it move to the end of the previous line instead; that mode is off by default and not covered here.

If a wrap is pending, because the last thing written filled the last column, the wrap is cancelled and the cursor moves left from the last column. xterm handles BS as a one-column CursorBack, which clears it.

Validation

BS-1: Overwrite the previous character

ABC
\b
X

|ABX__|
cursor 1,4

BS-2: Nothing happens in the first column

\b\b
X

|X____|
cursor 1,2

BS-3: Rubbing out a character

ABC
\b\s\b    # the line editor's erase

|AB___|
cursor 1,3

BS-4: From a pending wrap

ABCDE     # fills the line; a wrap is pending
\b
X

|ABCXE|
cursor 1,5

BS-5: Stops at the left margin

\e[?69h   # allow left and right margins
\e[3;5s   # margins at columns 3 and 5
\e[1;3H   # on the left margin
\b
X

|__X__|

libghostty-vt implements BS as a one-column CUB, and has the same difference: the cursor moves past the left margin into column 2. The VT510 manual says that at the left margin “no action occurs”, and esctest2’s test_BS_StopsAtLeftMargin expects the cursor to stay put, as xterm does.