How to use sed to remove only double empty lines?

Question

I found this question and answer on how to remove triple empty lines. However, I need the same only for double empty lines. Ie. all double blank lines should be deleted completely, but single blank lines should be kept.

I know a bit of sed, but the proposed command for removing triple blank lines is over my head:

sed '1N;N;/^\n\n$/d;P;D'

Answer 1

This would be easier with cat:

cat -s 

Answer 2

I've commented the sed command you don't understand:

sed '     ## In first line: append second line with a newline character between them.     1N;     ## Do the same with third line.     N;     ## When found three consecutive blank lines, delete them.      ## Here there are two newlines but you have to count one more deleted with last "D" command.     /^\n\n$/d;     ## The combo "P+D+N" simulates a FIFO, "P+D" prints and deletes from one side while "N" appends     ## a line from the other side.     P;     D ' 

Remove 1N because we need only two lines in the 'stack' and it's enought with the second N, and change /^\n\n$/d; to /^\n$/d; to delete all two consecutive blank lines.

A test:

Content of infile:

1   2 3  4    5  6   7 

Run the sed command:

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

That yields:

1 2 3  4  5  6 7