tcpdump -w command doesn't write to file
 

Anyone know why when I run < tcpdump -w file > I get know output in the file specified. I get network traffic on standard output but nothing when I try to redirect to a file.

I don't know why, but, from man tcpdump:

"In all cases, only packets that match expression will be processed by tcpdump."

With expression being the last thing on the command line, or so it seems.

If you check the man pages, this writes the *raw* package to file, hence if you view the file, you'll get a load of binary. The quickest / easiest way to write to a file is:
# tcpdump > filename