Finding out the username by user id (uid) in Unix is not as common a task as determining the uid by a username, but if you need to do it – I'll show you how.
How to find the username using user id (uid)
Simply use the getent command. Most common use for it is to query the passwd source for a username, like this:
ubuntu$ getent passwd greys greys:x:1000:113:Gleb Reys,,,:/home/greys:/bin/bash
however, if you query for a user id instead (1000 in this case), getent will work just as good:
ubuntu$ getent passwd 1000 greys:x:1000:113:Gleb Reys,,,:/home/greys:/bin/bash
That's all there is to it! Enjoy!