bash {} expansion

❯ echo filepath.ext{,.bak}
filepath.ext filepath.ext.bak

❯ echo filepath{.ext,.bak.ext}
filepath.ext filepath.bak.ext

❯ FILENAME=myfile.ext
❯ FILE=${FILENAME%.*}
❯ EXTN=${FILENAME##*.}
❯ echo $FILE $EXTN
myfile ext
❯ echo $FILE.{$EXTN,bak.$EXTN}
myfile.ext myfile.bak.ext