get all the odd lines in a file

sed can be used to get all the odd or even lines in a file. Consider, for example,

$cat linenums.txt
 line 1
 line 2
 line 3
 line 4
 line 5
 line 6
 line 7
 line 8
 line 9
 line 10

To print all the odd lines

$sed -n '1~2p' linenums.txt 
 line 1
 line 3
 line 5
 line 7
 line 9

$sed -n '2~2p' linenums.txt
 line 2
 line 4
 line 6
 line 8
 line 10