distinguishing shells

[Dave Mason <dmason@plg.uwaterloo.ca>]

DaviD W Sanderson <dws@ssec.wisc.edu> asked for a way to differentiate between
rc/es and the other shells.

This works:

 y=`echo`false;$y&&echo rc or es||echo sh

(it *does* expect there to be a false accessible to /bin/sh, but that
seems fairly likely, and it doesn't require anything for the rc side.)

In trying a few things, one of my ideas was to:
 $SHELL -c '~ 5 5 2>/dev/null' && echo rc || echo sh

To my surprise, rc gave a syntax error because the ~ builtin doesn't
accept I/O redirection.  (It actually doesn't work anyway because
/bin/sh complains about '~ not found' before the stderr redirect.)

../Dave

(I forwarded this to the rc list in case anyone there needed it too.)