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A unique identifier for the user.</FONT>

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password

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The user's password (encrypted).</FONT>

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user ID (UID)

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A unique number that identifies the user to the operating system.</FONT>

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group ID (GID)

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A unique number that identifies the user's group (for file permissions).</FONT>

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comment

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Usually the user's real name, but sometimes phone numbers, departments, and so on.</FONT>

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home directory

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The directory in which users are placed when they log in.</FONT>

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login command

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The command executed when the user logs in, normally a shell.</FONT>

</TABLE><P>We can look at each field in a little more detail. You should know what each field does and how it is used by other programs on your Linux system. Note that this type of user file is used with almost every UNIX system in the world, so once you 
know it for Linux, you know it for most UNIX versions.

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<FONT SIZE=4 COLOR="#FF0000"><B>User Names</B></FONT></CENTER></H4>

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<P>The user name is a single string, usually eight characters or less, that uniquely identifies each user. Since the user name is the basis of most communications between users and other machines, the user name you use (or assign to others) should be 
simple and obvious. Usually, this means a permutation of the user's real name. A typical user name may be a combination of the user's first and last names, such as tparker or timp. The former example, composed of the first initial and last name, is fairly 
common in large networks.

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<P>Note that the characters in these examples are all lowercase. Case is important in Linux (as with all UNIX versions), so tparker and Tparker are two different logins. Since most Linux commands are lowercase, convention is to also keep user names 
lowercase. Underscores, periods, numbers, and some special characters are allowed, but should be avoided.

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<P>Small systems, such as on a single machine, may use more familiar names, such as the user's first name only. A small system may have users with the names tim, bill, yvonne, and so on. If two users have the same name, then there must be some method found 
to differentiate between the two (such as bill and billy).

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<P>A few users like to create cryptic user names that reflect their hobbies, nicknames, pets, lifestyle, or personality. You may find user names such as vader, grumpy, wizard, and hoops. This type of naming is fine on small systems that are used by one or 
two users, but quickly becomes awkward on larger systems where other users may not know their coworkers' user names. On the whole, if your system is used by more than a couple of friends, discourage this type of user name.

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<FONT SIZE=4 COLOR="#FF0000"><B>Passwords</B></FONT></CENTER></H4>

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<P>The system stores the user's encrypted password in this field. (Actually, the password is encoded, not encrypted, although the convention has always been to use the term encrypted.) This field is very sensitive to changes, and any modification 
whatsoever can render the login useless until the system administrator performs a password change. A user's password can only be changed by the system administrator by using the passwd command when logged in as root (or by the users themselves).

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<NOTE>Some versions of UNIX do not keep the passwords in the /etc/passwd file because of potential security problems. If the password fields on your system are all set to x, then another file (called a shadow password file) is in use. However, all versions 
of Linux currently available do use this field normally.

<BR>Systems running either Yellow Pages or NIS (Network Information Service), both of which rely on a central file of user names and passwords, do not use this field. However, few Linux systems will use either YP or NIS, so this distinction can be ignored 
for the moment.</NOTE>

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<P>When a user logs in, the login program logically compares the password the user typed to a block of zeros, and then compares that result to the entry in the password field. If they match, the user is granted access. Any deviation causes login to refuse 
access.

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<P>This field can be used to restrict access to the system. If you want a login to never be used for access, such as a system login like lp or sync, place an asterisk between the two colons for this field. This restricts all access. In the example 
/etc/passwd file shown earlier, you can see that many system logins have an asterisk as their password, effectively blocking access.

<BR>

<P>This field can also be used to allow unrestricted access by leaving it blank. If there is no password, anyone using the user name is granted access immediately, with no password requested. This is a very bad habit to get into! Do not leave passwords 
open unless you are using your Linux system for your own pleasure and have nothing of value on the file system.

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<P>Don't attempt to put a password in the password field&#151;you cannot recreate the encryption method, and you'll end up locking the user out. Then, only the system administrator is able to change the password and allow access.

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<FONT SIZE=4 COLOR="#FF0000"><B>User ID</B></FONT></CENTER></H4>

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<P>Every user name has an associated, unique user ID. The user ID, also called the UID, is used by Linux to identify everything associated with the user. The user ID is preferable to the user name because numbers are easier to work with than the characters 
in a name, and they take up much less space. Linux tracks all processes started by a user, for example, by the user ID and not the user name. A translation can take place in some utilities to display the user name, but the utility generally examines the 
/etc/passwd file to match the UID to the name.

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<P>The user ID numbers are usually assigned in specific ranges. Most UNIX systems, for example, allocate the numbers from zero to 99 for machine-specific logins, and the user ID numbers from 100 and up for users. This is a good working model and makes your 
system consistent with others. In the example /etc/passwd file shown earlier, you can see that root has a UID of 0, while the other system-created logins have numbers ranging upward. The login nobody is a special login used for NFS (Network File System) 
and has a UID of &#151;1, an invalid number. When you assign user ID numbers, it is a good idea to assign them sequentially, so the first user is 100, the second 101, and so on.

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<FONT SIZE=4 COLOR="#FF0000"><B>Group ID</B></FONT></CENTER></H4>

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<P>The group ID (GID) is used to track the users' startup group (in other words, the ID of the group the users belongs to when they log in). A group, as you will see later, is used for organization purposes to set file permissions, although many 
organizations don't bother with them. Group ID numbers range from zero and upwards. Linux systems assign a group called users with the group number 100 for this purpose.

<BR>

<P>The GID is used by the system when tracking file permissions, access, and file creation and modification specifications. If your system has only a single user group, then you need not worry about the GID. If you work with several groups (as might be 
implemented on a large system), then you need to examine the /etc/group file.

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<FONT SIZE=4 COLOR="#FF0000"><B>Comments</B></FONT></CENTER></H4>

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<P>This field is used for the system administrator to add any information necessary to make the entry more self-explanatory. Typically, this area is used to enter the user's full name, although some system administrators like to add department or extension 
numbers for convenience. (This field is sometimes called the GECOS field, after the operating system that first used it.)

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<P>The comment field is used by some utilities to display information about users, so make sure you don't place any sensitive information there. Electronic mail systems, for example, can access this field to show who is sending mail. While you don't have 
to use the field, on larger systems it can make things much easier for administrators and other users when they can discover the real name of the person the user name belongs to.

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<FONT SIZE=4 COLOR="#FF0000"><B>Home Directory</B></FONT></CENTER></H4>

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<P>The home directory field indicates to the login process where to place users when they log in. This is usually their home directory. Each user on the system should have her own dedicated home directory, and then the startup files will initialize the 
environment variable HOME to this value. The directory indicated in this field is the user's initial working directory only, and places no restrictions on the user (unless file permissions have been set to restrict movement).

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<P>For the most part, user home directories are located in a common area. Linux tends to use the /home directory, so you will find home directories such as /home/tparker, /home/ychow, and so on. Other versions use /usr, /user, or /u as user home 
directories. In some cases where the system administrator has experience with another type of UNIX that uses an alternate directory structure, you may find the home directories changed to make life easier (and more familiar) for that administrator. As far 
as Linux is concerned, it doesn't care what the name of the home directory is, as long as it can be entered.

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<FONT SIZE=4 COLOR="#FF0000"><B>Login Command</B></FONT></CENTER></H4>

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<P>The login command is the command to be executed when login terminates. In most cases this is a shell command that is started, such as the C Shell or Bourne Shell, to provide the user with a shell environment. In some cases, it may be a single 
application or front-end system that restricts what the user can do. For example, the uucp login (used for e-mail and other simple networking tasks) executes the uucp command only. If the login command field is left empty, the operating system usually 
defaults to the Bourne shell (although this may change depending on the manner in which the operating system is set up).

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<P>Many versions of Linux enable users to change their login shell with the command chsh or passwd -s. When either command is used, the file /etc/shells is searched for a match. Only those commands in the /etc/shells file are allowed as valid entries when 
the user tries to change his startup shell. (You can add or remove lines in the /etc/shells file using any editor.) This helps you keep tighter security on the system. The superuser account has no restrictions on the entry in this field (or any other 
user's field). If your system uses the /etc/shells file, make sure it has the same file permissions and ownership as the /etc/passwd file, or a user can sneak through the system security by modifying the startup command for her login.

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<FONT SIZE=5 COLOR="#FF0000"><B>Default System User Names</B></FONT></CENTER></H3>

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<P>The extract from the /etc/passwd file shown in the preceding section lists over a dozen system-dependent user names. These all serve special purposes on the Linux system. A few of these logins are worth noting because they have specific uses for the 
operating system and for system administrators:

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root

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The superuser account (UID 0) with unrestricted access and owns many system files.

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daemon

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