How to insert a new line after every second line?

Question

sed/bash/wharever :D

INPUT:

sadf
asdf
yxcv
cxv
eqrt
asdf

OUTPUT:

sadf
asdf

yxcv
cxv

eqrt
asdf

Answer 1

With awk:

awk ' {print;} NR % 2 == 0 { print ""; }' inputfile

With sed(GNU extension):

sed '0~2 a\ ' inputfile

With bash:

#!/bin/bash
lines=0
while IFS= read -r line
do
    printf '%s\n' "${line}"
    # Based on @manatwork's comment
    ((lines++ % 2)) && echo
done < "$1" 

Answer 2

Another awk flavour:

awk '{ l=$0; getline; printf("%s\n%s\n\n", l, $0) }'