Bash redirections

 echo Hello world! | (exec 3<>/dev/tcp/termbin.com/9999; cat >&3; cat <&3;
 exec 3<&-)

If host is a valid hostname or Internet address, and port is an integer port number or service name, Bash attempts to open the corresponding TCP socket

1. open new descriptor

   The redirection operator [n]<>word causes the file whose name is the expansion of word to be opened for both reading and writing on file descriptor n

2. redirect cat output to socket

3. redirect response to cat input

   If >&- or <&- is preceded by {varname}, the value of varname defines the file descriptor to close.

4. close

Manual