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[Chapter 4] Users, Groups, and the Superuser
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printers.
nobody, which is a user that owns no files and is sometimes used as a default user for unprivileged
operations.
Here is an example of an /etc/passwd file containing these system users:
root:zPDeHbougaPpA:0:1:Operator:/:/bin/ksh
nobody:*:60001:60001::/tmp:
agent:*:1:1::/tmp:
daemon:*:1:1::/tmp:
ftp:*:3:3:FTP User:/usr/spool/ftp:
uucp:*:4:4::/usr/spool/uucppublic:/usr/lib/uucp/uucico
news:*:6:6::/usr/spool/news:/bin/csh
Notice that most of these accounts do not have "people names," and that all except root have a password field
of *. This prevents people from logging into these accounts from the UNIX login: prompt, as we'll discuss
later.[2]
[2] This does not prevent people from logging in if there are trusted hosts/users on that account;
we'll describe these later in the book.
NOTE: There is nothing magical about these particular account names. All UNIX privileges are
determined by the UID (and sometimes the group ID, or GID), and not directly by the account
name. Thus, an account with name root and UID 1005 would have no special privileges, but an
account named mortimer with UID 0 would be a superuser. In general, you should avoid creating
users with a UID of 0 other than root, and you should avoid using the name root for a regular user
account. In this book, we will use the terms "root" and "superuser" interchangeably.
4.1.1 User Identifiers (UIDs)
UIDs are historically unsigned 16-bit integers, which means they can range from 0 to 65535. UIDs between 0
and 9 are typically used for system functions; UIDs for humans usually begin at 20 or 100. Some versions of
UNIX are beginning to support 32-bit UIDs. In a few older versions of UNIX, UIDs are signed 16-bit integers,
usually ranging from -32768 to 32767.
UNIX keeps the mapping between usernames and UIDs in the file /etc/passwd. Each user's UID is stored in the
field after the one containing the user's encrypted password. For example, consider the sample /etc/passwd
entry presented in Chapter 3:
rachel:eH5/.mj7NB3dx:181:100:Rachel Cohen:/u/rachel:/bin/ksh
In this example, Rachel's username is rachel and her UID is 181.
The UID is the actual information that the operating system uses to identify the user; usernames are provided
merely as a convenience for humans. If two users are assigned the same UID, UNIX views them as the same
user, even if they have different usernames and passwords. Two users with the same UID can freely read and
delete each other's files and can kill each other's programs. Giving two users the same UID is almost always a
bad idea; we'll discuss a few exceptions in the next section.
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