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):

username:password:UID:GID:GECOS:/home/dir:shell 

Item	Description
	username is a mapping of a UID to a name for the benefit of human users.
	password is where, historically, passwords were kept in an encrypted format. Today, they are stored in a separate file called /etc/shadow.
	UID is a user ID, a number that identifies the user at the most fundamental level.
	GID is the user's primary group ID number. Groups will be discussed in a moment.
	GECOS field is arbitrary text, which usually includes the user's real name.
	/home/dir is the location of the user's personal data and configuration files.
	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.