{"id":142,"date":"2018-07-13T06:08:21","date_gmt":"2018-07-13T06:08:21","guid":{"rendered":"http:\/\/itp7.epgpbooks.inflibnet.ac.in\/?post_type=chapter&p=142"},"modified":"2019-05-16T07:18:02","modified_gmt":"2019-05-16T07:18:02","slug":"user-environment-local-computer-security-in-gnu-linux","status":"publish","type":"chapter","link":"https:\/\/ebooks.inflibnet.ac.in\/itp7\/chapter\/user-environment-local-computer-security-in-gnu-linux\/","title":{"rendered":"User Environment & Local Computer security in GNU\/Linux"},"content":{"raw":"
\"epgp<\/a>\r\n<\/span><\/div>\r\n \r\n\r\nIntroduction<\/strong>\r\n\r\n \r\n

As we are aware that Linux supports multiuser, at the same time more than one user can work concurrently into the system. Linux provides the facility to create multiple users and classify them into groups. Linux organizes the users into groups, groups are collections of accounts where certain permissions can be shared among users. The list of groups and their members are stored in a file \/etc\/group file. By default, every user has its own primary or default group. When a user logs into the system, the user gets all permissions assigned to their primary group and all other groups that a user belongs to. A user can belong to one or more groups.<\/p>\r\n \r\n

All Linux users are assigned a user ID(uid) which is unique. The groups also have unique IDs (gid). These IDs are integer numbers and start from zero (0). Most of the users ID start either from 500 (Fedora family) or from 1000(other distributions). The uid and gid can be seen in the file \/etc\/passwd or \/etc\/group files.<\/p>\r\n \r\n\r\nwho command<\/strong>\r\n\r\n \r\n

The command \u201cwho\u201d can be used to list online users, those users who are currently logged on to the system. Few options of the \u201cwho\u201d command are listed in following table<\/p>\r\n

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As shown in figure 1, the who command lists users who are online. Each line indicates the terminals from where the user has logged in. So, if a user logs in from multiple terminals, the output of who command will display multiple records. In addition to the username, who command lists the login time and the IP address from which the user has logged in.<\/p>\r\n \r\n

Now let us study the format of \/etc\/passwd file in which the details of users created within the system are stored. This file contains details that are separated by \u2018:\u2019 sign. Each line of the file represents a user. The file stores details like username, uid, gid, alias of username, default login directory and default shell. When a user logs onto the system, he logs onto a default directory (say \/home\/user) and he uses a default shell. The \/etc\/passwd file contains list of all users who are created within the system. Each user has a single entry in this file.<\/p>\r\n \r\n

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Figure 2: Snapshot of \/etc\/passwd file<\/strong><\/p>\r\n\r\n<\/div>\r\n

Let us understand the line containing \u201cDemo\u201d in figure 2. The user name is Demo whose UID=502 and GID=502. When the user \u201cDemo\u201d logs onto the system, he is placed in \/home\/Demo directory and the user gets bash shell to interact with the system. Let us take another case of user \u201chardik\u201d, where the UID and GID are different. Since the user \u201chardik\u201d belongs to more than one group, we can see that UID and GID need not be equal. If we want to know the group to which user \u201chardik\u201d belongs to, we can view the content of \/etc\/groups file.<\/p>\r\n

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Figure 3 illustrates the content of \/etc\/group file. The \/etc\/group file lists the groups created within the system. The output of \/etc\/group file signifies that the Demo group has GID 502 and no other user belongs to this group except the Demo user. While, the group \u201cfaculty\u201d has another user \u201chardik\u201d as a member of \u201cfaculty\u201d group supply username as an argument to the id command, it shows the details of the username supplied to the command. The syntax of id command can be:<\/p>\r\n

\"\"<\/p>\r\n \r\n\r\nUser Management<\/strong>\r\n\r\n \r\n

In this section, we will learn how to create users, how to modify\/delete the users existing in the system and how to set password to the user. In Linux, the users can be classified into following categories:<\/p>\r\n\r\n