How can I have tcpdump write to file and standard output the appropriate data?

Question

Here’s a neat way to do what you want:

tcpdump -w - -U | tee somefile | tcpdump -r -

What it does:

-w - tells tcpdump to write binary data to stdout
-U tells tcpdump to write each packet to stdout as it is received, rather than buffering them and outputting in chunks
tee writes that binary data to a file AND to its own stdout
-r - tells the second tcpdump to get its data from its stdin