1. Rename file - basic: 👼
$ cd lesson && for file in *; do mv "$file" "$file.html"; done

2. Replace <a href> to linkify renamed links - advanced: 😁
$ sed -ri 's/href="([-a-z]+)">/href="\1.html">/g' *
$ sed -ri 's/href="lesson\/([-a-z]+)">/href="lesson\/\1.html">/g' index.html

NOTE:
1. REGEX should not be used to parse [X]HTML. 😠
2. XPATH and XML Parser could be useful. 🤔
3. Refer top answer at SO : https://stackoverflow.com/questions/1732348/ 😧

…ibuted to the community

labex-labs#207
labex-labs#154