The system keeps track of a number of pieces of information about each user. They are summarized below.
The file /etc/passwd contains this information about users. Each line in the file contains information about a single user; the format of each line is
username:encrypted password:UID:GID:full name:home directory:login shell
An example might be:
As we can see, the first field, ``kiwi'', is the username.
The next field, ``Xv8Q981g71oKK'', is the encrypted password. Passwords are not stored on the system in any human-readable format. The password is encrypted using itself as the secret key. In other words, you need to know the password to decrypt it. This form of encryption is fairly secure.
Some systems use ``shadow password'' in which password information is relegated to the file /etc/shadow. Because /etc/passwd is world-readable, /etc/shadow provides some degree of extra security because it is not. Shadow password provides some other features such as password expiration and so on; we will not go into these features here.
The third field, ``102'', is the UID. This must be unique for each user. The fourth field, ``100'', is the GID. This user belongs to the group numbered 100. Group information, like user information, is stored in the file /etc/group. See Section 4.4.5 for more information.
The fifth field is the user's full name, ``Laura Poole''. The last two fields are the user's home directory (/home/kiwi) and login shell (/bin/bash), respectively. It is not required that the user's home directory be given the same name as the username. It does help identify the directory, however.