Question

I’ve got a file descriptor that points to a socket (example code below).

exec 3<>/dev/tcp/localhost/9999
echo -e "Some Command\n" >&3

Sometimes that socket closes and needs to be re-opened (a restart of the server).

How can I test if the socket (fd #3 in this case) is writable?

The echo will always succeed, regardless of whether the socket has been closed already or not.

Answer 1

Solution is the feedback from the server.

When you send a request to the server, it needs to answer to it.

exec 3<>/dev/tcp/localhost/9999
echo -e "Some Command\n" >&3
sleep 5 # example max time given to server to respond
cat <&3 #receive an answer
check is correct, restart server otherwise

EDIT:
using netcat to determine is the port is open

netcat -w 3 -z www.google.com 80
if[ $? -eq 0 ]
then
    echo port open
else
    echo port closed
fi