Concepts Glossary 148 Getting Started Guide Groups Linux uses groups to organize users for security purposes. Each user can belong to a primary group, as defined by the group ID number (GID) in that user account. You can review a user’s primary group in a graphical user administra- tion tool or by reviewing the /etc/passwd file. Each user can also belong to additional, secondary groups by being added to the membership lists of those groups. All groups on your Linux system are defined in the file /etc/groups. You can use the command groupadd or a graphical utility to create and manage groups. Groups are important in Linux because many file permissions (see Access Rights) are granted based on the groups that a user is a member of. For example, a user can create a file and grant read access to all users who are part of that user’s primary group. gzip and gunzip Utilities Linux includes several different compressions tools.Two of the most widely used are gzip and gunzip. To use these utilities, simply enter the command with a file to compress: gzip filename The file extension .gz is added to the compressed file. Use gunzip with the file- name to uncompress it. Other Linux compression utilities include compress and uncompress, and zip and unzip (these are also compatible with the PKZIP format commonly used on DOS/Windows systems). Halt See shutdown Home directories Each regular user account in Linux has a home directory created for it. These are normally stored in the /home directory. For example, if you create a new user account named jbutler, a new subdirectory is created named /home/jbutler. When this user logs in to the system, he or she has complete rights within the home directory for that account. Although users can look around the rest of the