Replacing Multiple blank lines with a single blank line in vim / sed

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?

Answers:

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Method 1

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!nnn+!^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.

Method 2

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/(nn)n+/1/

Method 3

If in Vim, just do this:

:%!cat -s

The -s flag for cat squeezes multiple blank lines into one.

Method 4

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.

Method 5

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

Method 6

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

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0