TCP connection, bash only

Question

I found this line in a script. While I globally understand what it does--opening a bidirectional TCP connection--, I need some explanations on the syntax. Here\'s the line:<

Answer 1

I can only answer for the exec part:

exec without a command given may be used to change I/O redirections. <> in this case means open for read+write. 5 is the channel number (or file descriptor). This makes sense if other commands send their output / read their input from channel 5.

For "/dev/tcp/${SERVER}/${PORT}" I don't know if it's a feature of a specific Linux version or if it's a bash feature (I assume the latter).

-- EDIT: from the bash manual page: --

 Bash handles several filenames specially when they are  used
 in redirections, as described in the following table:

      /dev/fd/fd
           If fd is a valid integer, file  descriptor  fd  is
           duplicated.
      /dev/stdin
           File descriptor 0 is duplicated.
      /dev/stdout
           File descriptor 1 is duplicated.
      /dev/stderr
           File descriptor 2 is duplicated.
      /dev/tcp/host/port
           If host is a valid hostname or  Internet  address,
           and  port  is  an  integer  port number or service
           name, bash attempts to open a  TCP  connection  to
           the corresponding socket.
      /dev/udp/host/port
           If host is a valid hostname or  Internet  address,
           and  port  is  an  integer  port number or service
           name, bash attempts to open a  UDP  connection  to
           the corresponding socket.