readme-update-toc.sh

#!/bin/bash

set -ueo pipefail

# Usage: bash readme-update-toc.sh README.md
# Purpose:  update the TOC in markdown files

gentoc() {
    echo ""
    # This sed command finds the headings "##" style.
    # The awk command then processes those headings into a table of contents.
    sed -n  -e "/^## / {
        s/^\(##\) */\1|/
        p
        }
        h
" "$1" | \
    awk -F \| '{
    sub("#", "", $1)
    gsub("#", "", $1)
    link=tolower($2)
    gsub(" +", "-", link)
    gsub("[^a-zA-Z0-9_-]","",link)

    print $1 "* [" $2 "](#" link ")"

    }'
    echo ""
}

modify() {
    sed -i.bak -ne "\
   /^## Contents/{
      p
      r $2
      bloop
   }
   /^[=-][=-]*/ {
    x
    /Contents/ {
    x;p;
    r $2
    bloop
    }
    x
   }
   h
   p
   d
   :loop
     n

     /^#/{
        p
        d
     }
     /^[=-][=-]*/ {
       x;p;x;p
       d
     }
     h
     bloop
" \
"$1"

}

for file in "$@"; do
    gentoc "$file" > tmp_$$
    modify "$file" tmp_$$
done

rm -f ./tmp_*