How can I look up a username by id in linux?

Try

getent passwd "$uid" | cut -d: -f1

You might enjoy this little ditty.

$ id -nu [number]

3.17.3-1-ARCH #1 SMP PREEMPT Fri Nov 14 22:56:01 CET 2014 i686 GNU/Linux

I can confirm that it returns a corresponding user name, if one exists, on Arch Linux. I can also confirm that it does not work on Ubuntu when run as a normal user, although I have not tested this as the superuser. It also does not work on Alpine Linux. Maybe a security feature prevents this from working on some systems.

ls already performs that lookup. You can perform a user information lookup from the command line with getent passwd.

If ls shows a user ID instead of a user name, it's because there's no user by that name. Filesystems store user IDs, not user names. If you mount a filesystem from another system, or if a file belongs to a now-deleted user, or if you passed a numerical user ID to chown, you can have a file that belongs to a user ID that doesn't have a name.

On a shared host, you may have access to some files that are shared between several virtual machines, each with their user database. This is a bit weird (why share files but not the users that own them?), but it's technically possible.