The term 'login' is spelled as one word when referring to an object. When used as an action it is spelled as two words. For example.
After booting, the system displays the login prompt. Follow the procedure below to log in to the system.
Similarly, the nouns backup and setup are each one word, but the verbs back up and set up are two words.
If it’s a verb, then it’s two words but one word if it’s a noun.
- Back up your hard drive (verb)
- Don't forget to create a backup before you proceed (noun)