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What is a User?

Every process (running program) on the system runs as a particular user. Every file is owned by a particular user. Access to files and directories are restricted by user. The user associated with a running process determines the files and directories accessible to that process.
The id command is used to show information about the current logged-in user. Basic information about another user can also be requested by passing in the username of that user as the first argument to the id command. 
[student@desktopX ~]$ id
uid=1000(student) gid=1000(student) groups=1000(student),10(wheel) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
To view the user associated with a file or directory, use the ls -l command. The third column shows the username:
[student@serverX ~]$ ls -l /tmp
drwx------. 2 gdm     gdm      4096 Jan 24 13:05 orbit-gdm
drwx------. 2 student student  4096 Jan 25 20:40 orbit-student
-rw-r--r--. 1 root    root    23574 Jan 24 13:05 postconf
To view process information, use the ps command. The default is to show only processes in the current shell. Add the a option to view all processes with a terminal. To view the user associated with a process, include the u option. The first column shows the username:
[student@serverX ~]$ ps au
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root       428  0.0  0.7 152768 14400 tty1     Ss+  Feb03   0:04 /usr/bin/Xorg 
root       511  0.0  0.0 110012   812 ttyS0    Ss+  Feb03   0:00 /sbin/agetty 
root      1805  0.0  0.1 116040  2580 pts/0    Ss   Feb03   0:00 -bash
root      2109  0.0  0.1 178468  2200 pts/0    S    Feb03   0:00 su - student
student   2110  0.0  0.1 116168  2864 pts/0    S    Feb03   0:00 -bash
student   3690  0.0  0.0 123368  1300 pts/0    R+   11:42   0:00 ps au
The output of the previous commands displays users by name, but internally the operating system tracks users by a UID number. The mapping of names to numbers is defined in databases of account information. By default, systems use a simple "flat file," the /etc/passwd file, to store information about local users.
The format of /etc/passwd (seven colon-separated fields): 
1username:2password:3UID:4GID:5GECOS:6/home/dir:7shell
Item Description
1 username is a mapping of a UID to a name for the benefit of human users.
2 password is where, historically, passwords were kept in an encrypted format. Today, they are stored in a separate file called /etc/shadow.
3 UID is a user ID, a number that identifies the user at the most fundamental level.
4 GID is the user's primary group ID number. Groups will be discussed in a moment.
5 GECOS field is arbitrary text, which usually includes the user's real name.
6 /home/dir is the location of the user's personal data and configuration files.
7 shell is a program that runs as the user logs in. For a regular user, this is normally the program that provides the user's command line prompt.

What is a Group?

Like users, groups have a name and a number (GID). Local groups are defined in /etc/group.
Primary Groups
  • Every user has exactly one primary group.
  • For local users, the primary group is defined by the GID number of the group listed in the fourth field of /etc/passwd.
  • Normally, the primary group owns new files created by the user.
  • Normally, the primary group of a newly created user is a newly created group with the same name as the user. The user is the only member of this User Private Group (UPG).
Supplementary Groups
  • Users may be a member of zero or more supplementary groups.
  • The users that are supplementary members of local groups are listed in the last field of the group's entry in /etc/group. For local groups, user membership is determined by a comma-separated list of users found in the last field of the group's entry in /etc/group: 
    groupname:password:GID:list,of,users,in,this,group
  • Supplementary group membership is used to help ensure that users have access permissions to files and other resources on the system.

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