There are 15 questions tagged command-line and 33 tagged commandline. I think command-line is clearer, but the two tags should be rolled up either way.
In favour of command-line only because it's less of a jumarble of the words when reading.
Feeling is that tags that are multiple words should just include the hyphen (-) instead of jammingthemallintoonewordfor the sake of it.
There's also cmd to consider, and from the looks of things, deal with the command line anyway.
Yeah, I agree about going with
command-line. (It's the better option for a couple of reasons, one small one being consistency between SU and SO: https://stackoverflow.com/questions/tagged/command-line.)