> 3. from what I understand, shell syntax is available only at REPL top level. Once you switch to Lisp syntax with `(`, you can return to shell syntax only with `)`. Thus means you cannot embed shell syntax inside Lisp syntax, i.e. you cannot do `(define j {find -type f | less})`

It's possible I misunderstand what you mean because I'm not sure what piping to less is supposed to accomplish here, but this is not true. The following program works just fine:

    #lang rash

    (require racket/port
             racket/string)

    (define (echo!!! message)
      (define output {echo $message |> port->string |> string-trim})
      (string-append output "!!!"))

    (echo!!! "Hello")