Reads a markdown file from STDIN and outputs the TOC to STDOUT. Install Racket to run it.

markdown-toc.rkt

#!/usr/bin/env racket

#lang racket

(define (contains-heading? line)
  (regexp-match #rx"^#+" line))

(define (heading-level line)
  (length (regexp-match* #rx"#" line)))

(define (heading-title line)
  (string-trim (last (string-split line "#"))))

(define (make-heading line)
  (cons (heading-title line) (heading-level line)))
  
(define (make-heading-padding heading)
  (make-string (* 2 (sub1 (cdr heading))) #\space))
  
(define (heading-link heading)
  (let ((padding (make-heading-padding heading))
        (href (string-downcase (string-replace (car heading) " " "-"))))
    (format "~a* [~a](#~a)" padding (car heading) href)))

(let* ((lines (sequence->list (in-lines (current-input-port))))
      (headings (map make-heading (filter contains-heading? lines))))
  (for ([toc-line (map heading-link headings)])
    (printf "~a\n" toc-line)))