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Every account on the system has an entry in the file /etc/passwd. This file contains entries, one line per user, that specify several attributes for each account, such as the username, real name, and so forth.

Each entry in this file is of the following format:

username:password:uid:gid:gecos:homedir:shell

The following list explains each field:

username

A unique character string, identifying the account. For personal accounts, this is the name the user logs in with. On most systems it is limited to eight alphanumeric characters—for example, larry or kirsten.

password

An encrypted representation of the user's password. This field is set using the passwd program to set the account's password; it uses a one-way encryption scheme that is difficult (but not impossible) to break. You don't set this by hand; the passwd program does it for you. Note, however, that if the first character of the password field is * (an asterisk), the account is "disabled"; the system will not allow logins as this user. See "Creating Accounts," later in this chapter.

uid

The user ID, a unique integer the system uses to identify the account. The system uses the uid field internally when dealing with process and file permissions; it's easier and more compact to deal with integers than byte strings. Therefore, both the user ID and the username identify a particular account: the user ID is more important to the system, whereas the username is more convenient for humans.

gid

The group ID, an integer referring to the user's default group, found in the file /etc/group. See "The Group File," later in this chapter.

gecos

Miscellaneous information about the user, such as the user's real name, and optional "location information" such as the user's office address or phone number. Such programs as mail and finger use this information to identify users on the system; we'll talk more about it later. By the way, gecos is a historical name dating back to the 1970s; it stands for General Electric Comprehensive Operating System. GECOS has nothing to do with Unix, except that this field was originally added to /etc/passwd to provide compatibility with some of its services.

homedir

The user's home directory , for the user's personal use; more on this later. When the user first logs in, the shell finds its current working directory in the named home directory.

shell

The name of the program to run when the user logs in; in most cases, this is the full pathname of a shell, such as /bin/bash or /bin/tcsh.

Many of these fields are optional; the only required fields are username, uid, gid, and homedir. Most user accounts have all fields filled in, but "imaginary" or administrative accounts may use only a few.

Here are two sample entries you might find in /etc/passwd:

root:ZxPsI9ZjiVd9Y:0:0:The root of all evil:/root:/bin/bash

aclark:BjDf5hBysDsii:104:50:Anna Clark:/home/aclark:/bin/bash

The first entry is for the root account. First of all, notice that the user ID of root is 0. This is what makes root root: the system knows that uid 0 is "special" and that it does not have the usual security restrictions. The gid of root is also 0, which is mostly a convention. Many of the files on the system are owned by root and the root group, which have a uid and gid of 0, respectively. More on groups in a minute.

On many systems, root uses the home directory /root, or just /. This is not usually relevant because you most often use su to access root from your own account. Also, it is traditional to use a Bourne-shell variant (in this case /bin/bash) for the root account, although you can use the C shell if you like. (Shells are discussed in Chapter 4.) Be careful, though: Bourne shells and C shells have differing syntax, and switching between them when using root can be confusing and lead to mistakes.

The second entry is for an actual human being, username aclark. In this case, the uid is 104. The uid field can technically be any unique integer; on many systems, it's customary to have user accounts numbered 100 and above and administrative