(use posix srfi-13)

(define file "test-file2")
(define index "index")

(define (get-byte-offset line)
  (car (string-split line ":" #t)))

(define (make-index file)
  (call-with-values (lambda () (process (string-append "grep -b '' " file)))
    (lambda (in-port _ _)
      (let ((index-fileno (file-open index (+ open/wronly open/creat))))
	(let loop ((line (read-line in-port)))
	 (unless (eof-object? line)
	   (file-write index-fileno (string-append (string-pad (get-byte-offset line) 16) "\n"))
	   (loop (read-line in-port))))))))

(make-index file))