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

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