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When EusLisp is invoked from Unix, execution is initiated by the toplevel function bound to *toplevel*. This function is eustop in eus and xtop in eusx. You can change this toplevel function by specifying your own function to the third argument to save.
The toplevel function should be programmed to accept arbitrary number of arguments. Each argument on the command line is coerced to a string and transfered to the toplevel function. The program below repeatedly reads expressions from the file given by the first argument and pretty-prints them to the second argument file.
(defun pprint-copy (infile outfile)
(with-open-file (in infile)
(with-open-file (out outfile :direction :output)
(let ((eof (cons nil nil)) (exp))
(while (not (eq (setq exp (read in nil eof)) eof))
(pprint exp out))))))
(defun pprint-copy-top (&rest argv)
(when (= (length argv) 2)
(pprint-copy (first argv) (second argv))))
Once you defined these functions in EusLisp, (save "ppcopy" "" 'pprint-copy-top) creates a unix executable command named ppcopy.
In Solaris based EusLisp, the toplevel evaluator cannot change in this manner, since save is not available. Instead, edit lib/eusrt.l to define the custom toplevel evaluator and set it to *toplevel*. lib/eusrt.l defines initialization procedures evaluated at every invocation of the EusLisp.