Chapter 4. How Linux Works

4.2.2 Filesystems

Whether you're using Windows or Linux, you must format a partition before you can store data on it. The installation procedure automatically formats the partitions you create during system installation. When the installation procedure formats a partition, it also writes special data, called a filesystem , on the partition. The filesystem organizes the available space and provides a directory that lets you assign a name to each file , which is a set of stored data. A filesystem also enables you to group files into directories, which function much like the folders you create using the Windows Explorer: directories store information about the files they contain.

Just as every partition must have a filesystem, every CD-ROM and floppy diskette must have a filesystem. The filesystem of a CD-ROM is written when the disk is created; the filesystem of a floppy diskette is rewritten each time you format it.

Windows 98 lets you choose to format a partition as FAT or FAT32. Windows NT, 2000, and XP also support the NTFS filesystem type. Linux supports a wider variety of filesystem types; Table 4-2 summarizes the most common ones. The most important filesystem types are ext3 , which is used for Linux native partitions; msdos , which is used for FAT partitions (and floppy diskettes) of the sort created by MS-DOS and Microsoft Windows; and iso9660 , which is used for CD-ROMs. Linux also provides the vfat filesystem, which is used for FAT32 partitions of the sort created by Windows 9 x . Linux can also read Windows NT/2000/XP NTFS filesystems; however, the support for doing so is not enabled in the standard Red Hat Linux kernel.

Table 4-2. Common filesystem types

4.2.3 Directories and Paths

If you've used MS-DOS, you're familiar with the concepts of file and directory and with various MS-DOS commands that work with files and directories. Under Linux, files and directories work much as they do under MS-DOS.

4.2.3.1 Home and working directories

When you log in to Linux, you're placed in a special directory known as your home directory . Generally, each user has a distinct home directory, where the user creates personal files. This makes it simple for the user to find files previously created, because they're kept separate from the files of other users and system files.

The current directory— or current working directory , as it's sometimes called—is the directory you're currently working in. When you log in to Linux, you're automatically placed in your home directory.

4.2.3.2 The directory tree

The directories of a Linux system are organized as a hierarchy. Unlike MS-DOS, which provides a separate hierarchy for each partition, Linux provides a single hierarchy that includes every partition. The topmost directory of the directory tree is the root directory , which is written using a forward slash ( / ), not the backward slash ( \ ) used by MS-DOS to designate a root directory.

Figure 4-1 shows a hypothetical Linux directory tree; a real Linux system contains many more directories. The root directory contains six subdirectories: /bin , /dev , /etc , /home , /tmp , and /usr . The /home directory has two subdirectories; each is the home directory of a user and has the same name as the user who owns it. The user named bill has created two subdirectories in his home directory: books and school . The user named patrick has created the single school subdirectory in his home directory.

Figure 4-1. A hypothetical Linux directory tree

Each directory (other than the root directory) is contained in a directory known as its parent directory. For example, the parent directory of the bill directory is home .

The root user has a special home directory, /root . This directory is commonly called "slash root" to distinguish it from the root directory, / .

4.2.3.3 Absolute and relative pathnames

If you look closely at Figure 4-1, you'll see that two directories named school exist; one is a subdirectory of bill and the other is a subdirectory of patrick . To avoid confusion that could result when several directories have the same name, directories are specified using pathnames .

There are two kinds of pathnames: absolute and relative . The absolute pathname of a directory traces the location of the directory beginning at the root directory; you form the pathname as a list of directories, separated by forward slashes ( / ). For example, the absolute pathname of the unique directory named bill is /home/bill . The absolute pathname of the school subdirectory of the bill directory is /home/bill/school . The absolute pathname of the identically named school subdirectory of the patrick directory is /home/patrick/school .

When a subdirectory is many levels below the root directory, its absolute pathname may be long and cumbersome. In such a case, it's often more convenient to use a relative pathname, which gives the current directory, rather than the root directory, as its starting point. For example, suppose that the bill directory is the current directory; you can refer to its books subdirectory by the relative pathname books . A relative pathname must never begin with a forward slash, whereas an absolute pathname must begin with a forward slash. As a second example, suppose that the /home directory is the current directory. The relative pathname of the school subdirectory of the bill directory would be bill/school ; the relative pathname of the identically named subdirectory of the patrick directory would be patrick/school .

Linux provides two special directory names . Using a single dot (.) as a directory name is equivalent to specifying the current directory. Using two dots ( . .) within a pathname refers to the directory up one level in the current path ; that is, to the parent directory. For example, if the current directory is /home/bill , then . . refers to the / home directory. Similarly, if the current directory is /home/bill and the directory tree is that shown in Figure 4-1, the path ../patrick/school refers to the /home/patrick/school directory .

4.2.4 File Permissions

Unlike Windows 9 x , but like other varieties of Unix and Windows NT/2000, Linux is a multiuser operating system. Therefore, it includes mechanisms to protect data from unauthorized access. The primary protection mechanism restricts access to directories and files based on the identity of the user who requests access and on access modes assigned to each directory and file.

Each directory and file has an associated user, called the owner . The user who initially creates a file is the owner of the file. Each user belongs to one or more sets of users known as groups . Each directory and file has an associated group, which is assigned when the directory or file is created. The user and the group can be changed later.

Access permissions, also known as modes , determine what operations a user can perform on a directory or file. Table 4-3 lists the most common permissions and explains the meaning of each. Notice that permissions work differently for directories than for files. For example, permission r denotes the ability to list the contents of a directory or read the contents of a file. A directory or file can have multiple permissions. Only the listed permissions are granted; any other operations are prohibited . For example, a user who had file permission rw could read or write the file but could not execute it, as indicated by the absence of the execute permission, x .

Table 4-3. Common access permissions

r

w

x

The access modes of a directory or file consist of three sets of permissions:

User/Owner

Applies to the owner of the file

Group

Applies to users who are members of the group assigned to the file

Other

Applies to other users, sometimes referred to as world

The ls command, which you'll meet in Chapter 7, lists the file access modes in the second column of its long output format, as shown in Figure 4-2. The GNOME and KDE file managers use this same format. The column contains nine characters : the first three specify the access allowed the owner of the directory or file, the second three specify the access allowed users in the same group as the directory or file, and the final three specify the access allowed to other users (see Figure 4-3).

Figure 4-2. Access modes as shown by the ls command

Figure 4-3. Access modes specify three permissions

4.2.5 Mounting and Unmounting Filesystems

You cannot access a hard drive partition, CD-ROM, or floppy disk until the related device or partition is mounted . Mounting a device checks the status of the device and readies it for access when it boots or when you launch a desktop environment. By default, the GNOME and KDE desktop environments automatically mount devices that use removable media when you insert new media.

Before you can remove media from a device, you must unmount it. You can unmount a device by using a desktop environment or issuing a command. For your convenience, the system automatically unmounts devices when it shuts down. A device can be unmounted only if it's not in use. For example, if a user's current directory is a directory of the device, the device cannot be unmounted.

A common error is attempting to mount a CD-ROM or floppy without first inserting the media. If you're unable to mount a device that uses removable media, check that the media is in place. Also, be sure to unmount media before ejecting it, especially when using USB or Firewire drives . If you fail to do so, data on the media may be corrupted.