How can I broadcast to multiple ssh using file descriptor?

Question

I want to be able to broadcast the same commands to multiple servers interactively, meaning I want to send a command, view the result and send another command. The commands are arbitrary and cannot be scripted.

The question How can I automate running commands remotely over SSH? solves this problem very well if you're prone to using the tools suggested like: pssh, sdh, gsh, cluster-ssh.
But I am trying to find out if there a more low level solution, only by using the standard SSH.

My question is can you think of a way in BASH to open multiple SSHs, redirect into them a file, so everytime I write into the file, the new text is redirected into the SSH and executed.
If there is a reasoning behind why the above can't be done, I'd be happy to learn why is it not possible.

Answer 1

One way is to use coprocesses.

$ coproc foo { ssh -T foo.domain; }

Now $foo is an array of two file descriptors; ${foo[0]} is ssh's stdout and writing to fd ${foo[1]} will send to ssh's stdin.

$ echo "whoami" >&${foo[1]}
$ head -n 1 <&${foo[0]}
grawity
$

(Accessing /proc/$$/fd/${foo[1]} would work as well.)

---

Another way - named pipes.

$ mkfifo {foo,bar}.{in.out}
$ ssh foo <foo.in >foo.out &
$ ssh bar <bar.in >bar.out &
$ echo "whoami" | tee foo.in bar.in

Similar - process substitution.

$ echo "whoami" | tee >(ssh foo) >(ssh bar)

For more than one command:

$ mkfifo in
$ tee >(ssh foo) >(ssh bar) <in &
$ echo "whoami" >in

---

But since pipes break easily, you might have to resort to regular files.

(This example will put the file in /dev/shm to avoid hitting the disk every time.)

$ f=/dev/shm/in; touch $f
$ tail -f $f | ssh foo &
$ tail -f $f | ssh bar &
$ echo foo > $f 
$ echo bar >> $f; # note: >> for appending

And:

$ f=/dev/shm/in; touch $f
$ tail -f $f | tee >(ssh foo) >(ssh bar) &
$ echo foo >> $f