For file compression, the obvious choice is gzip or, for a even better compression ratio,bzip2. Just enter gzip testarchive.tar (or bzip2 testarchive.tar,but gzip is used in this example). With ls, now see that the file testarchive.taris no longer there and that the file testarchive.tar.gz has been created instead.This file is much smaller and therefore much better suited for transfer via e-mail orstorage on a USB stick.Now, unpack this file in the test2 directory created earlier. To do so, enter cptestarchive.tar.gz test2 to copy the file to that directory. Change to thedirectory with cd test2. A compressed archive with the .tar.gz extension canbe unzipped with the gunzip command. Enter gunzip testarchive.tar.gz,which results in the file testarchive.tar, which then needs to be extracted oruntarred with tar -xvf testarchive.tar. You can also unzip and extract acompressed archive in one step with tar -xvf testarchive.tar.gz (addingthe -z option is no longer required). With ls, you can see that a new test directoryhas been created with the same contents as your test directory in your home directory.17.1.6 Cleaning UpAfter this crash course, you should be familiar with the basics of the Linux shell orcommand line. You may want to clean up your home directory by deleting the varioustest files and directories using the rm and rmdir commands. In Section 17.3, “ImportantLinux Commands” (page 347), find a list of the most important commands and a briefdescription of their functions.17.2 Users and Access PermissionsSince its inception in the early 1990s, Linux has been developed as a multiuser system.Any number of users can work on it simultaneously. Users need to log in to the systembefore starting a session at their workstations. Each user has a username with a corre-sponding password. This differentiation of users guarantees that unauthorized userscannot see files for which they do not have permission. Larger changes to the system,such as installing new programs, are also usually impossible or restricted for normalusers. Only the root user, or super user, has the unrestricted capacity to make changesto the system and unlimited access to all files. Those who use this concept wisely, onlylogging in with full root access when necessary, can cut back the risk of unintentionalloss of data. Because under normal circumstances only root can delete system files orWorking with the Shell 343
