What is the good way to add file extension ".jpg" to extension-less files with bash?

# Strip .jpg from all filenames
for f in *.jpg; do mv "$f" "${f%.jpg}"; done
# Add .jpg to all filenames (even those with .jpg already)
for f in *; do mv "$f" "$f.jpg"; done
# Add .jpg to all filenames...unless they are already .jpg
for f in *; do case "$f" in *.jpg) echo skipped $f;; *) mv "$f" "$f".jpg; esac; done
# Add .jpg to all filenames...unless they already have a . extension
for f in *; do case "$f" in *.*) echo skipped $f;; *) mv "$f" "$f".jpg; esac; done

