2021-05-24-bash-rename-file-extension

# Rename all *.html to *.markdown
for f in *.html; do 
    mv -- "$f" "${f%.html}.markdown"
done

-- marks the end of the option list.

* This avoids issues with filenames starting with hyphens.

${f%.html} is a parameter expansion

* and is replaced by the value of f with .html removed from the end.