Why is sed giving me this error: ... unknown command: `^'

Question

I would like sed to remove empty lines from a file. I used this command:

cat file | sed '; ^$ ; d'

and was given the following error:

 sed: -e expression #1, char 3: unknown command: `^'

Why is this happening? Isn't sed supposed to interpret ^ as "the beginning of the line"? If so, why am I getting this error message?

Answer 1

The (more) correct way is

cat file | sed '/^$/d'

But I am sure people will have lots of answers to the way of deleting empty lines.