How To Remove Single Blank Line Only - Keep Multiple Blank Lines

Question

I have some data where I want to keep the multiple blank lines and remove the single blank lines only. My data look like this:

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I would like the output to be:

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I have tried working with the following commands, but I can't seem to get it to work:

awk ' /^$/ { print; } /./ { printf("%s ", $0); }'

And

sed 'N;/^\n$/d;P;D'

Also, I tried using cat -s but that doesn't necessarily take out the blank lines. Furthermore, I have played around with sed '/^$/' but cant specify single lines only. Any help is greatly appreciated.

Answer 1

Using gnu-awk it is pretty simple:

awk -v RS='\n+' '{printf "%s", $0 (length(RT) != 2 ? RT : "\n")}' file

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• Using -v RS='\n+' we constitute 1 or more line breaks as record separator
• Using length(RT) we check how many line breaks are after each record
• we print RT (original captured value) if length != 2

Alternative awk command:

awk -v RS='\n{3,}' '{gsub(/\n{2}/, "\n", $0); printf "%s", $0 RT}' file

Answer 2

This might work for you (GNU sed):

sed '/\S/!b;n;N;/^\s*\n\s*\S/D' file

If a line is empty print it. Otherwise, print the line and fetch the next two lines and if the first of these is empty but the second is not, delete the first and repeat.

Answer 3

Using GNU sed (minor modifications needed for BSD/OSX):

$ sed -E ':a; N; $!ba; :b; s/([^\n])\n\n([^\n])/\1\n\2/g; tb' input
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How it works

• :a; N; $!ba

  Read the whole file in at once.

  Here, a is a label. N reads in the next line. $!ba branches back to label unless we are on the last line.

• :b; s/([^\n])\n\n([^\n])/\1\n\2/g; tb

  Replace double-newlines with single newlines. Repeat as often as necessary.

  Here b is a label. The s command removes the double newlines. If any double-newlines were removed, then tb branches back to label b.