How to remove every other line with sed?

Question

How can I remove every odd line, using sed?

remove
keep
remove
keep
remove
...

Answer 1

GNU sed has a suitable addressing mode:

sed -n '1~2!p' file

which means, starting from line 1, and with step 2, print all other lines.

Equivalently, you can drop the -n, and delete matching lines:

sed '1~2d'

It can also be done using awk:

awk 'NR%2==0' file

(Whenever line number is a multiple of 2, print the line)