19.11 Useless Whitespace

It is easy to leave unnecessary spaces at the end of a line, or empty lines at the end of a file, without realizing it. In most cases, this trailing whitespace has no effect, but there are special circumstances where it matters. It can also be a nuisance that the line has “changed,” when the change is just spaces added or removed at the end.

You can make trailing whitespace at the end of a line visible on the screen by setting the buffer-local variable show-trailing-whitespace to t. Then Emacs displays trailing whitespace in the face trailing-whitespace.

This feature does not apply when point is at the end of the line containing the whitespace. Strictly speaking, that is “trailing whitespace” nonetheless, but displaying it specially in that case looks ugly while you are typing in new text. In this special case, the location of point is enough to show you that the spaces are present.

To delete all trailing whitespace within the current buffer's accessible portion (see section Narrowing), type M-x delete-trailing-whitespace <RET>. (This command does not remove the form-feed characters.)

Emacs can indicate unused lines at the end of the window with a small image in the left fringe (see section Window Fringes). The image appears for window lines that do not correspond to any buffer text. Blank lines at the end of the buffer then stand out because they do not have this image in the fringe.

To enable this feature, set the buffer-local variable indicate-empty-lines to a non-nil value. The default value of this variable is controlled by the variable default-indicate-empty-lines; by setting that variable, you can enable or disable this feature for all new buffers. (This feature currently doesn't work on text-only terminals.)