将文件了里连续的空行删除成一个空行

1.cat -s filename
2.[xxxx@localhost test]$ sed -e '/^$/{
> $!N
> /^\n$/D
> }'  filename
$!N:对最后一行不进行（N）操作，也就是出来最后一行的意思

man sed:
n N    Read/append the next line of input into the pattern space.
D      Delete  up  to  the first embedded newline in the pattern space.
       Start next cycle, but skip reading from the input  if  there  is
       still data in the pattern space.