Added note to UNIX Tips page explaining how to connect a process' STDIO to a TCP...

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# Redirect File Descriptor of Running Process #

This note explains how to redirect `stdin` (or any other file descriptor) of a
pre-existing process using the GNU debugger (`gdb`) and a FIFO. It was tested
on FreeBSD 11.

An example of use would be saving the contents of remote `vi` sessions after
they are detached due to a dropped connection.

First, make a FIFO:

    $ mkfifo /tmp/vififo

Assuming there is a pre-existing `vi` session with PID `91266`, connect
with `gdb`, close file descriptor `0` and reopen it as a connection to the
FIFO with the `call close` and `call open` commands.

    $ gdb -p 91266
    <snip>
    Attaching to process 91266
    <snip>
    (gdb) call close (0)
    $1 = 0
    (gdb) call open ("/tmp/vififo", 0600)

At this point `gdb` will appear to hang. Leave it and open a new terminal.  Use
`echo` to send characters to the process through the FIFO.

Special characters may be escaped by pressing `Ctrl-V` followed by the
character. For example, to send an `Escape`, press `Ctrl-V` followed by
`Escape` which results in an `Escape` code, or `^[`.

Continuing the example, tell `vi` to save the current buffer to a file.

    $ echo "^[:w /tmp/vi_recover.txt" > /tmp/vififo

After this command the `gdb` session should start responding again, returning
to a `(gdb)` prompt. Exit `gdb`.

    $2 = 0
    (gdb) quit
    A debugging session is active.
    
        Inferior 1 [process 91266] will be detached.

    Quit anyway? (y or n) Y
    Detaching from program: /hh/bin/vi, process 91266
    [Inferior 1 (process 91266) detached]

The characters have now been received by `vi` and a file should be waiting at
`/tmp/vi_recover.txt`.


# Expose Process STDIO on TCP Port #

This note explains how to launch a process and connect its
`stdin`/`stdout`/`stderr` directly to a TCP port. It was tested on FreeBSD 12.

As an example, we will launch a `bash` process running on a Debian 10 machine
and connect it to TCP port `4242`. Then, from a FreeBSD machine we will connect
and utilize the `bash` session remotely.

First, on the Debian machine, we use
[`socat`](http://www.dest-unreach.org/socat/), a program which establishes two
bidirectional byte streams and transfers data between them. In our case, we
tell `socat` to build a PTY at `/tmp/pty_test` and connect it to TCP port
`4242`.

    socat PTY,link=/tmp/pty_test,raw TCP-LISTEN:4242,reuseaddr,fork

Note that the `PTY` argument to `socat` must precede the `TCP-LISTEN` argument.
Despite the bidirectional nature, the endpoint definition order is not
interchangeable.

Now we use `setsid` to start a new `bash` process and attach it to the PTY at
`/tmp/pty_test`. Note that we collapse `stdout` and `stderr` into a single
combined stream.

    setsid sh -c 'exec bash --login <> /tmp/pty_test >&0 2>&1'

Replace `bash --login` with whatever other command you desire to execute. 

Now, from the FreeBSD machine, connect the remote TCP port to your local STDIO
with `socat`. Assuming the Debian machine running `bash` is at `192.168.1.107`,
execute the following command.

    socat STDIO,raw TCP:192.168.1.107:4242

At this point you have a fully interative `bash` session running on the Debian
machine, controlled from the FreeBSD machine.