Understanding /etc/group File

Q. Can You tell me more about Linux /etc/group file?

A. /etc/group is a text file which defines the groups to which users belong under Linux and UNIX operating system. Under Unix / Linux multiple users can be categorized into groups. Unix file system permissions are organized into three classes, user, group, and others. The use of groups allows additional abilities to be delegated in an organized fashion, such as access to disks, printers, and other peripherals. This method, amongst others, also enables the Superuser to delegate some administrative tasks to normal users.

/etc/group file

It stores group information or defines the user groups i.e. it defines the groups to which users belong. There is one entry per line, and each line has the format (all fields are separated by a colon (:)

Consider following /etc/group line:

cdrom:x:24:vivek,student13,raj
_____ _  _      _____
|    |  |        |
|    |  |        |
1    2  3        4

1. group_name: It is the name of group. If you run ls -l command, you will see this name printed in the group field.
2. Password: Generally password is not used, hence it is empty/blank. It can store encrypted password. This is useful to implement privileged groups.
3. Group ID (GID): Each user must be assigned a group ID. You can see this number in your /etc/passwd file.
4. Group List: It is a list of user names of users who are members of the group. The user names, must be separated by commas.

More about a user groups
Users on Linux systems are assigned to one or more groups for following reasons:

• To share files or other resource with a small number of users
• Ease of user management
• Ease of user monitoring
• Group membership is perfect solution for large Linux (UNIX) installation.
• Group membership gives you or your user special access to files and directories or devices which are permitted to that group

Consider following figure (Click to enlarge)
User tom is part of both 'Web developers' and 'Sales' group. So tom can access files belongs to both groups.

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