[UPDATE] (shell-expr) is ready and kicking :)

Now you can write

  (import (schemesh all))

  (shell-expr (lisp-expression-writing-to-sh-fd-stdout)) | some-shell-command-and-args | (shell-expr (lisp-expression-reading-from-sh-fd-stdin))

An example of such expressions is: For writing to sh-fd-stdout,

  (fd-write (sh-fd-stdout) (string->utf8b "hello\n"))

For reading from sh-fd-stdin,

  (display (utf8b-bytespan->string (fd-read-until-eof (sh-fd-stdin))))