Use a opção -l
com tcpdump.
-l Make stdout line buffered. Useful if you want to see the data while capturing it. E.g.,
tcpdump -l | tee dat'' or
tcpdump -l > dat & tail -f dat''.
Claramente declarado na página do manual.
tcpdump -l ... | grep -i "..." | cut -d'&' -f1,2