How to switch/rotate every two lines with sed/awk?

Question

I have been doing this by hand and I just can\'t do it anymore-- I have thousands of lines and I think this is a job for sed or awk.

Essentially, we have a file lik

Answer 1

sed 'N; 
s/\(.*\)\n\(.*\)/\2\
\1/' infile

N - append the next line of input into the pattern space
\(.*\)\n\(.*\) - save the matching parts of the pattern space the one before and the one after the newline.
\2\\ \1 - exchange the two lines (\1 is the first saved part, \2 the second). Use escaped literal newline for portability

With some sed implementations you could use the escape sequence \n: \2\n\1 instead.