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Removing blank lines in a buffer

How to remove blank lines and leave just one blank line in a buffer.

By Mickey Petersen

Updated for emacs 28

This is a frequent question so I figured I’d mention the solution here:

You want to remove all empty (blank) lines from a buffer. How do you do it? Well, it’s super easy.

Mark what you want to change (or use C-x h to mark the whole buffer) and run this:

M-x flush-lines RET ^$ RET

And you’re done. So what does that mean? Well, M-x flush-lines will flush (remove) lines that match a regular expression, and ^$ contain the meta-characters ^ for beginning of string and $ for end of string. Ergo, if the two meta-characters are next to eachother, it must be a blank line.

We can also generalize it further and remove lines that may have whitespace (only!) characters:

M-x flush-lines RET ^\s-*$ RET

In this case \s- is the syntax class (type C-h s to see your buffer’s syntax table) for whitespace characters. The * meta-character, in case you are not a regexp person, means zero or more of the preceding character.

Update – Pete Wilson asks: “How do you collapse multiple lines into one blank line?”.

That’s a bit harder, mostly because flush-lines only works well on whole, single lines. For multi-line processing you have two choices: you can abuse regexp, or you can use a macro. It’s fairly easy to do it with regexp in this case, but for more complex data-scrubbing I would use a macro; nevertheless, I will do it both ways*.

*I’m pretty sure my macro/regexp examples are general enough to work in all cases; but let me know if they aren’t

For the regexp approach I will use C-M-% (query-replace-regexp) and because I have to use a literal newline character I will use Emacs’s quoted-insert command, bound to C-q. So to insert a newline, you would type C-q C-j.

The ^J represents the literal newline or line feed character (see ASCII Control Characters on Wikipedia for more information).

So the text we want to search for looks like this:

Search For: ^^J\{2,\}
Replace With: ^J

So how does it work? Well, we tell Emacs to search for any two or more newlines that are at the beginning of a string – where each line is considered a string by Emacs – and because we search for two or more we skip the ones that only have a single newline. So if there are 10 newlines in a row, we replace them all in one fell swoop with a single newline. You can omit the replacement newline to remove them altogether!

In the early 80's, i wrote a filter in C called 'onespc' (V7 Unix had 14 char filenames, i tended to omit vowels). 'onespc' by default would read stdin, compress groups of blank lines (with optional white space) to a single blank line, and write to stdout. I'd use it from the command line, sometimes in scripts that changed many files. I'd use it from Emacs on the whole buffer or regions. 'onespc' has a bunch of options to do similar things, like remove all blank lines.

Emacs was sluggish starting up until the 386/33 or 486/25. This may be ancient history, but it was over a decade for me. I haven't come up with a good way to use Emacs to edit hundreds of files. My current plan is to learn elisp. That should fix everything. Lisp isn't easy to learn. But i've used Lisp and scheme in the past.

In the late 80's, i wanted something easier in scripts than

for i in *.txt; do

onespc $i > x

mv x $i

done

It's not a good solution, since x might exist as a file. So, i wrote a version of 'into'. The syntax is

for i in *.txt; do

onespc $i | into $i

done

So, 'into' copies stdin to a temp named file, which it determines does not exist in advance. On EOF, it renames it to the argument, deleting the existing file if it can.

I was about to publish all these utilities, but then someone publish the 'getopt' functions for command line. I liked my least ambigous command line parser better. I never liked the '--' standard. I stalled.

Who knows? Maybe Emacs has a simple file mapper. I'd like to be able to do this easily:

for i in `find . $HOME/some/other/place -type f -name \*.txt`; do

onespc $i | into $i

done

But 'find' and my filter set (including all of the *ix filters, including 'sed' and 'perl') is a pretty powerful set of flexibility to allow.

Despite emacs's internal docs, 'info', 'man', and google, i can't always find docs for what i want to do just at this moment.

— Stephen · reply

Herewith two functions to do what you've described in this article:

(defun flush-blank-lines ()

"Removes all blank lines from buffer or region"

(interactive)

(save-excursion

(let (min max)

(if (equal (region-active-p) nil)

(mark-whole-buffer))

(setq min (region-beginning) max (region-end))

(flush-lines "^ *$" min max t)

)

)

)

(defun collapse-blank-lines ()

"Collapse multiple blank lines from buffer or region into a single blank line"

(interactive)

(save-excursion

(let (min max)

(if (equal (region-active-p) nil)

(mark-whole-buffer))

(setq min (region-beginning) max (region-end))

(query-replace-regexp "^

\\{2,\\}" "

" nil min max)

)

)

)

Any suggestions on improving these?

— Sameer Morar · reply

Sameer,

Not bad for a first attempt! I think the first one can be simplified to this:
(defun flush-blank-lines (start end)
  (interactive "r")
  (flush-lines "^\\s-*$" start end nil))
This way we use Emacs's built-in support for working with regions by using the interactive "r" code letter which will pass in the start/end of point and mark automatically -- this is important for people who don't use transient-mark-mode. This approach also lets use pass in explicit ranges from lisp.

The second function works in much the same way as the first, although I have opted to use the interactionversion instead of the "proper" elisp regexp function:
(defun collapse-blank-lines (start end)
  (interactive "r")
  (replace-regexp "^\n\\{2,\\}" "\n" nil start end))

— mickey · reply

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Removing blank lines in a buffer

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