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rutabaga$

starts a new shell with the group ID of bozo. If the password field is blank, or the first character is an asterisk, you receive a permission denied error if you attempt to newgrp to that group.

However, the password field of the group file is seldom used and is really not necessary. (In fact, most systems don't provide tools to set the password for a group; you could use passwd to set the password for a dummy user with the same name as the group in /etc/passwd and copy the encrypted password field to /etc/group.) Instead, you can make a user a member of multiple groups simply by including the username in the members field for each additional group. In the previous example, the users linus and mdw are members of the bozo group, as well as whatever group they are assigned to in the /etc/passwd file. If we wanted to add linus to the megabozo group as well, we'd change the last line of the previous example to:

megabozo:*:52:kibo,linus

The command groups tells you which groups you belong to:

rutabaga$ groups

users bozo

Giving a list of usernames to groups lists the groups to which each user in the list belongs.

When you log in, you are automatically assigned to the group ID given in /etc/passwd, as well as any additional groups for which you're listed in /etc/group. This means you have "group access" to any files on the system with a group ID contained in your list of groups. In this case, the group permission bits (set with chmod g+...) for those files apply to you (unless you're the owner, in which case the owner permission bits apply instead).

Now that you know the ins and outs of groups, how should you assign groups on your system? This is really a matter of style and depends on how your system will be used. For systems with just one or a handful of users, it's easiest to have a single group (called, say, users) to which all personal user accounts belong. Note that all the system groups—those groups contained within /etc/group when the system is first installed—should probably be left alone. Various daemons and programs may depend upon them.

If you have a number of users on your machine, there are several ways to organize groups. For example, an educational institution might have separate groups for students, faculty, and staff. A software company might have different groups for each design team. On other systems, each user is placed into a separate group, named identically to the username. This keeps each pigeon in its own hole, so to speak. Files can also be assigned to special groups; many users create new groups and place files into them for sharing the files between users. However, this requires adding users to the additional groups, a task that usually requires the system administrator to intervene (by editing /etc/group or using utilities, such as gpasswd on Debian systems). It's really up to you.

Another situation in which groups are often used is special hardware groups. Let's say that you have a scanner that is accessed via /dev/scanner. If you do not want to give everybody access to the scanner, you could create a special group called scanner, assign /dev/scanner to this group, make this special file readable for the group and nonreadable for everybody else, and add everybody who is allowed to use the scanner to the scanner group in the /etc/groups file.

Creating Accounts

Creating a user account requires several steps: adding an entry to /etc/passwd, creating the user's home directory, and setting up the user's default configuration files (such as .bashrc) in her home directory. Luckily, you don't have to perform these steps manually; nearly all Linux systems include a program called adduser to do this for you. Some Linux systems, such as Red Hat or SUSE, use a different set of tools for account creation and deletion. If the sequence of inputs in this section does not work for you, check the documentation for your distribution. (Red Hat allows accounts to be managed through the control-panel tool, and SUSE does it via yast2; Debian includes an adduser script