What:	Cause non-arbitrary commands on an hpterm by writing to the tty
When:	Discovered 7/15/96
Who:	David Ljung
Description:

  If someone is using an hpterm with a writable tty, then you can
send Escape codes to that terminal to make it execute specific commands.

If you send an Escape-a to an hpterm tty, then it will sometimes
try to execute a command of the format xxxcyyyR (more on this later).

Sometimes, however, it will hang the hpterm instead.  I'm not sure
what the dependency is for this.  It doesn't seem to be OS revision,
shell type or tty/pty type.  This needs to be controllable or determinable
at some point, obviously.

All machines will hang on an Escape-c, and the machines that hang on
Escape-a will also hang on Escape-d.

The command executed is xxxcyyyR where xxx is the column and yyy is the
row.  The column is from the left side of the window, but the row is
from the top of the 'virtual window' that the hpterm is in, not the
actual window (so it includes scrollback regions).

You can control this with the Escape-h command, which is home.  The command:

  echo (escape)h(escape)a >> their_tty

will execute the command at the home position, hence it will try to
execute 000c000R.  If we have access to their path we can put a command
in the path called 000c000R that will be run as their user.  Then when
you do the Escape-h Escape-a combination, it will force their hpterm to
send it to their shell as a command.

Important note:  If they aren't at a shell prompt then it will go to
whatever application they are currently running.

This also means that if they have a line partially typed into
their shell then it will run the command appended to that line -
probably not what you want.

A solution to the second problem is to do an Escape-a sequence first,
that will execute some garbage command and clear the buffer, then
you can run the h,a combination to get what you want, as long as
they are at a shell prompt.

--------------------------------------------------

TO DO:

Figure out the dependencies that cause some windows to take Escape-a
as a command and some take it as a hang.

More needs to be learned about the escape codes.  It would be
*extremely* nice if the command was arbitrary, so you didn't need
to have access to executables in their path.