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CHAPTER 14 UNDERSTANDING LINUX FILES AND USERS
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The -l option tells the ls command to list nearly all the details about the files. If you do this in GNOME Terminal, you ll see that the listing is color-coded. Table 14-1 shows what each color indicates. The command returns a lot of additional information, including who owns which file and what you and others can do with it. This requires an understanding of users and file permissions, which we ll discuss next.
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Tip The command ls -la will give you even more information perhaps too much for general use.
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In most instances, ls -l should show enough information.
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Table 14-1. Color-Coding Within GNOME Terminal
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Color
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Black text Light-blue text Black outline with yellow text Green text Cyan text Pink text Red text
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Type of File
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Standard file Directory Virtual device1 Program or script2 Symbolic link to another file3 Image file Archive4
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1 This is found only
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in the /dev directory. program or script that has merely been marked as
2 Technically speaking, green text indicates a
being executable.
3 This is similar to a Windows desktop shortcut. 4 Installation files are also marked red because
they re usually contained in archives.
Users and File Permissions
The concept of users and permissions is as important to Ubuntu as the idea of a central and allencompassing file system. In fact, the two are implicitly linked. When initially installing Linux, you should have created at least one user account. By now, this will have formed the day-to-day login that you use to access Linux and run programs. Although you might not realize it, as a user, you also belong to a group. In fact, every user on the system belongs to a group. Under Ubuntu, ordinary users belong to a group based on their username (under other versions of Linux, you might find that you belong to a group called users).
CHAPTER 14 UNDERSTANDING LINUX FILES AND USERS
Note Groups are yet another reminder of Ubuntu s Unix origins. Unix is often used on huge computer systems with hundreds or thousands of users. By putting each user into a group, the system administrator s job is a lot easier. When controlling system resources, the administrator can control groups of users rather than hundreds of individual users. On most home user PCs, the concept of groups is a little redundant, because there s normally a single user, or at most, two or three. However, the concept of groups is central to the way that Linux handles files.
A standard user account under Ubuntu is normally limited in what it can do. As a standard user, you can save files to your own private area of the disk, located in the /home directory, as shown in Figure 14-2, but usually nowhere else. You can move around the file system, but some directories are strictly out of bounds. In a similar way, some files can be opened as read-only, so you cannot save changes to them. All of this is achieved using file permissions.
Figure 14-2. Your personal directory within home is your area on the hard disk. This is enforced via file permissions. Every file and directory is owned by a user. In addition, files and directories have three separate settings that indicate who within the Linux system can read them, who can write to them, and, if the file in question is runnable (usually a program or a script), who can run it ( execute it). In the case of directories, it s also possible to set who can browse them, as well as who can write files to them. If you try to access a file or directory for which you don t have permission, you ll be turned away with an access denied error message.
CHAPTER 14 UNDERSTANDING LINUX FILES AND USERS
ROOT VS. SUDO
Most versions of Linux have two types of user accounts: standard and root. Standard users are those who can run programs on the system but are limited in what they can do. Root users have complete run of the system, and as such, are often referred to as superusers. They can access and/or delete whatever files they want. They can configure hardware, change settings, and so on. Most versions of Linux create a user account called root and let users log in as root to perform system maintenance. However, for practical as well as security reasons, most of the time the user is logged in as a standard user. Ubuntu is different in that it does away with the root account. Instead, it allows certain users, including the one created during installation, to temporarily adopt root-like powers. You will already have encountered this when configuring hardware. As you ve seen, all you need to do is type your password when prompted in order to administer the system. This way of working is referred to as sudo, which is short for superuser do. In fact, the command sudo will let you adopt root powers at the shell prompt simply preface any command with sudo in order to run it with root privileges. In some ways, the sudo system is slightly less secure than using a standard root account. But it s also a lot simpler. It reduces the chance of serious errors, too. Any command or tweak that can cause damage will invariably require administrative powers, and therefore require you to type your password or preface the command with sudo. This serves as a warning and prevents mistakes. If you re an experienced Linux user and want to invoke the root account, simply type the following at the command prompt: sudo passwd root Then type a password. If you subsequently want to deactivate the root account, type this: sudo passwd l root
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