Can't get wireshark -R working in unix shell

Question

I would like to get the following wireshark command working in unix shell (bash).

tshark -i any host $IP -R 'udp.port == $PORT'

For some reason the tshark command doesn't see the value in $PORT that in between single quotes, just the variable id $PORT, which then my script fails.

Any info much appreciated.

Answer 1

Single quotes stop environment variables from being evaluated. Change to double quotes to allow the shell to expand the variable. See the following for shell expansion differences

MYPORT=1234
echo '$MYPORT'
echo "$MYPORT"