bash - Find out if user name exists

How can I find out, in a shell script, whether a given user name exists on the current system?

---

/etc/passwd and /etc/shadow are incomplete. Consider OS X's Directory Services, or Linux with Likewise Active Directory integration.

Answer

One of the most basic tools to be used for that is probably id.

#!/bin/bash
if id "$1" >/dev/null 2>&1; then
        echo "user exists"
else
        echo "user does not exist"
fi

Which produces

$ ./userexists root
user exists
$ ./userexists alice
user does not exist
$ ./userexists
user does not exist