http://www.purl.org/tcl/home/man/tcl8.4/TclCmd/tclvars.htm
[Someone want to comment on where this global variable gets set and to what good uses one can put it to?]
RS: Contains the error message and traceback of the last error. Silly example:
% proc foo {} {bar x}
% proc bar {input} {grill$input}
% foo
invalid command name "grillx"
% set errorInfo
invalid command name "grillx"
while executing
"grill$input"
(procedure "bar" line 1)
invoked from within
"bar x"
(procedure "foo" line 1)
invoked from within
"foo"
See also errorCode.