In last month’s Guru Guidance column, we looked briefly at Linux user accounts in the context of the user authentication process. In this month’s column, we will take a look at the fundamental Linux multiuser entity: groups.
In last month’s Guru Guidance column, we looked briefly at Linux user accounts in the context of the user authentication process. In this month’s column, we will take a look at the fundamental Linux multiuser entity: groups.
Groups are the mechanism that Unix systems provide to enable arbitrary collections of users to share files and other system resources. As such, they provide one of the cornerstones of system security.
Groups may be defined in two ways:
* Implicitly, by Group ID (GID): whenever a new GID appears in the fourth field of the password file, a new group is defined.
* Explicitly, by name and GID, via an entry in the file /etc/group.
The best administrative practice is to define all groups in the /etc/group file.
Each individual entry in /etc/group consists of a single line having the following form:
group-name:x:GID:additional-users
The meanings of these fields are:
group-name: A name identifying the group. For example, a development group that is working on new simulation software might have the group-name simulation.
“!”: The second field is the traditional group password field. It holds an exclamation point when a group password file is in use (see below), as is usually the case under Linux.
GID: This is the group’s identification number. User groups generally start numbering at 100. Note that usernames and group names are independent of each other, even when the same name is both a username and a group name….
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