Q: emacs inferior es echoing lines -- cause?

[alk@et.msc.edu (Anthony L. Kimball)]

I must admit to having a deep mental block when it comes to ttys and ptys
and anything that comes remotely close to a BSD process group.  It's not
surprising that I am truly puzzled, therefore, by the behaviour of es
in in emacs shell mode, to wit:  After entering a line it is echoed back
from the shell, and emacs prints it in the buffer, terminal return and all.
I diffed the trace of an emacs inferior es and an xterm inferior es
(the latter not echoing so), and observed that in both cases es is writing
back the characters to fd 1.

I note that no other interpreter exhibits this behaviour in shell mode,
including rc, and that it's not happening explicitly in the interactive
loop, but rather in the builtin parse (I think).  So what obvious thing am
I missing?  Thanks!

//alk