How to remove every other line?

Question

How can I remove every odd line, using sed?

remove
keep
remove
keep
remove
...

Answer 1

sed -n '1~2!p' file

awk 'NR%2==0' file

Answer 2

Here is the shortest I can think of:

sed -n 'g;n;p' file

It should work with non-GNU versions of sed (as well as GNU sed).