Use bash to test if a port is open

This one is pretty simple. You can use bash to open a TCP port as a simple test to see if a port is open/closed:

</dev/tcp/127.0.0.1/80 3>&1 && echo "Connected!" || echo "Not Connected!"

Important things here are:

<, which will opening the socket and input to nothing, no data will be sent over the connection. This is better than using Telnet to test sockets as no data is sent down the connection - Telnet will try and negotiate a terminal and on some connections you don't know how that input will behave.

&& executes the statement if the previous statement didn't return an error, in this case the socket opened.

|| executes the statement if the previous statement failed, in this case if the socket didn't connect.

Combining all of this we have a simple tcp connection test. Here it is in function form with a timeout value also:

function tc {
    HOST=$1
    PORT=$2
    TIMEOUT=$([ -z "$3" ] && echo 1 || echo $3)   
    timeout ${TIMEOUT} bash -c "echo </dev/tcp/${HOST}/${PORT}" && echo "Connected!" || echo "Not Connected!"
}

This takes the host and port as the first two parameters. The third parameter is a timeout value, if it's not specified it will default to 1 second.