去除文件中的连续空白行

[root@book sh]# cat a.txt
ddddddddddd



ssssssss                     dddddddddddddd






ssssssssssssss


ddddd                 sssssssssssaaaaaaaaaaa




[root@book sh]# cat a.txt -s
ddddddddddd

ssssssss                     dddddddddddddd

ssssssssssssss

ddddd                 sssssssssssaaaaaaaaaaa

[root@book sh]# cat -s a.txt | sed '/^[[:space:]]*$/d'
ddddddddddd
ssssssss                     dddddddddddddd
ssssssssssssss
ddddd                 sssssssssssaaaaaaaaaaa