How can I replace mutliple empty lines with a single empty line in bash?

Question

I have a file that contains:

something



something else

something else again

I need a bash command, sed/grep w.e that will produce the following output

something

something else

something else again

In other words, I need to remove multiple blank lines with just a single blank line. gred/sed are line based. I've never found a BASH solution that would work on multi-line regex patterns. Any suggestions?

Answer 1

Use python:

s = file("filename.txt").read()
while "\n\n\n" in s: s = s.replace("\n\n\n", "\n\n")
import sys
sys.stdout.write(s)

Answer 2

Super easy to do with vim. Just open the file and type the following:

:%s/\n\n\n*/\r\r/

That will reduce all blocks of more than 2 new lines to 2 new lines. Hope this helps!

Answer 3

Actually, if you replace multiple newlines with a single newline, the output would be:

something
something else
something else again

You can achieve this by:

sed /^$/d FILE

Answer 4

I take it that you'll probably want to remove lines that only have whitespace.

That can be done with:

sed /^[:space:]*$/d FILE

Answer 5

A solution with awk, which replaces several blank lines with a single blank line:

awk 'BEGIN{bl=0}/^$/{bl++;if(bl==1)print;else next}/^..*$/{bl=0;print}' myfile

Answer 6

Usually, if I find that sed can't do something I need, I turn to awk:

awk '
BEGIN {
    blank = 0;
}

/^[[:blank:]]*$/ {
     if (!blank) {
          print;
     }
     blank = 1;
     next;
}

{
     print;
     blank = 0;
}' file

Answer 7

grep -A1 . | grep -v "^--$"

This grep solution works assuming you want the following:

Input

line1

line2
line3


line4



line5

Output

line1

line2
line3

line4

line5

Answer 8

Pipelining it to |uniq may be solution (if other than empty lines don't duplicate)

Answer 9

You just need cat with the -s option which causes it to remove repeated empty lines from its output:

cat -s