42

Question more or less says it all. I'm aware that /^$/d will remove all blank lines, but I can't see how to say 'replace two or more blank lines with a single blank line'

Any ideas?

48

If you aren't firing vim or sed for some other use, cat actually has an easy builtin way to collapse multiple blank lines, just use cat -s.

If you were already in vim and wanted to stay there, you could do this with the internal search and replace by issuing: :%s!\n\n\n\+!^M^M!g (The ^M is the visual representation of a newline, you can enter it by hitting Ctrl+vEnter), or save yourself the typing by just shelling out to cat: :%!cat -s.

  • wow, did not know the -s option of cat - just a historic note, it is not in POSIX, but seems to be available in BSD and GNU cat. – maxschlepzig May 7 '11 at 20:40
  • 2
    Marking as 'winning' for the :%!cat -s. Learn something GNnew everyday! – Andrew Bolster May 7 '11 at 20:43
  • 3
    I think you can also use \r instead of ^M like this: %s!\n\n\n\+!\r\r!g – Niko Bellic May 28 '17 at 22:32
  • I needed this today. Thank you, Caleb! :) – Dɑvïd Jun 11 at 14:16
18

Use \n to indicate a newline in the search pattern. Use Ctrl+M in the replacement text, or a backreference. See :help pattern and :help sub-replace-special (linked from :help :s).

%s/\(\n\n\)\n\+/\1/
  • Can you please explain how does the search pattern? – Sagar Jain Sep 22 '15 at 7:14
  • @sjmp Requirement: replace two or more blank lines with a single blank line. Implementation: replace three or more consecutive newlines with just two newlines. See the manual for an explanation of the constructs used in the command. – Gilles Sep 22 '15 at 8:21
  • But why do you need to use a group when its contents are fixed? – Sabuncu Nov 23 '17 at 18:01
  • @Sabuncu You don't need to use a group. But why not use a group? Here, I used a group because you can't copy-paste a command with a newline in the replacement text: you need to insert a literal Ctrl+M character, \n doesn't work in the replacement text. – Gilles Nov 23 '17 at 19:24
  • Now I understand, thank you for taking the time to answer. – Sabuncu Nov 23 '17 at 19:52
6

If in Vim, just do this:

:%!cat -s
  • 2
    You might want to add a little detail around how this works, even if it is just a quote from the man page. – jasonwryan Nov 21 '12 at 5:22
  • Didn't work on windows 7 – user674669 Dec 2 '17 at 2:31
0

Using Perl:

perl -00 -pe ''

-00 command line option turns paragraph slurp mode on, meaning Perl reads text paragraph by paragraph rather than line by line.

0

With sed (GNU sed) 4.2.2:

sed -r '
  /^\s*$/ { 
    # blank line
:NEXT
    N # append next line to pattern space - if none, autoprint PS and exit
    s/^\s*$\n^\s*$//g;t NEXT # if 2 blank lines, clear PS and loop to NEXT
  }
  # else, autoprint PS and next/exit
' < $MYFILE
-2

I know this is silly code, but I wanted to solve this issue in less than 10 min, and it worked

for file in /directory/*
do
  originalname=$file
  us='_'
  tempname=$file$us
  echo $originalname
  mv $originalname $tempname
  uniq $tempname $originalname
  rm $tempname
done
  • 1
    uniq removes adjacent equal lines. Which is not what OP wants. – pacholik Mar 30 '17 at 9:01
  • @pacholik he clearly said "Replacing Multiple blank lines with a single blank line" NOT remove adjacent equal lines!!! – Mahmoud Zalt Mar 30 '17 at 17:43
  • 3
    Yes, exactly... – pacholik Mar 31 '17 at 7:17

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