Comment by izabera

  if [ exec 1>/dev/null 2>/dev/null 3>/dev/tcp/localhost/4000 ] ; then

This code runs `[ exec ]` (which tests if "exec" is a non-empty string, thus it returns true) and redirects its output and error to /dev/null, and fd 3 to /dev/tcp/whatever. If the socket opening fails, `[ exec ]` is not even executed, and the overall command returns nonzero. Completely equivalent to `: 3>/dev/tcp/localhost/4000`, which also returns nonzero if the socket can't be opened.

It happens to work for the intended purpose, but not for the reasons the author intended.