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

For BSD-derived systems (including GNU):

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

cat -s

From man page: -s --squeeze-blank: suppress repeated empty output lines.

grep -A1 . <yourfile> | grep -v "^--$"

This grep solution works assuming you want the following:

Input

line1

line2
line3


line4



line5

Output

line1

line2
line3

line4

line5

I just solved this problem by sed. Even if this is a 7 year old question, someone may find this helpful, so I am writing my solution by sed here:

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