Root Directory: /

The subdirectories held in the root directory, /, are listed in Table 30-1, along with other useful subdirectories. Directories that you may commonly access as an administrator are the /etc directory, which holds configuration files; the /dev directory, which holds device files; and the /var directory, which holds server data files for DNS, Web, mail, and FTP servers, along with system logs and scheduled tasks. For managing different versions of the kernel, you may need to access the /boot and /lib/modules directories as well as /usr/src/linux. The /boot directory holds the kernel image files for any new kernels you install, and the /lib/modules directory holds modules for your different kernels.

System Directories

Your Linux directory tree contains certain directories whose files are used for different system functions. For basic system administration, you should be familiar with the system program directories where applications are kept, the system configuration directory (/etc) where most configuration files are placed, and the system log directory (/var/log) that holds the system logs, recording activity on your system. Both are covered in detail in Chapter 27. Table 30-2 lists the system directories.

The /usr Directory

The /usr directory contains a multitude of important subdirectories used to support users, providing applications, libraries, and documentation. /usr/bin holds numerous user-accessible applications and utilities. /usr/sbin holds user-accessible administrative utilities. The /usr/share directory holds architecture-independent data that includes an extensive number of subdirectories, including those for documentation, such as man, info, and doc files. Table 30-3 lists the subdirectories of the /usr directory.

The /mnt Directory

The /mnt directory is usually used for mountpoints for your CD-ROM, DVD, floppy, or Zip drives, as well as for other mounted file systems such as Windows partitions. These are file systems you may be changing frequently, unlike partitions on fixed disks. Red Hat Linux installs by default floppy and CD-ROM subdirectories for mounting floppies and CD-ROMs, /mnt/floppy and /mnt/cdrom. Additional drives have a number attached to their name, as in /mnt/cdrom1 for a second CD-ROM drive. You can also create directories for any partitions you want to mount, such as /mnt/windows for a Windows partition.

The /home Directory

The /home directory holds user home directories. When a user account is set up, a home directory is set up here for that account, usually with the same name as the user. As the system administrator, you can access any user's home directory, and so you have control over their files.

The /var Directory

The /var directory holds subdirectories for tasks whose files change frequently, such as lock files, log files, Web server files, or printer spool files. For example, the /var directory holds server data directories, such as /var/www for the Apache Web server Web site files or /var/ftp for your FTP site files, as well as /var/named for the DNS server. The /tmp directory is simply a directory to hold any temporary files programs may need to perform a particular task.

The /var directories are designed to hold data that changes with the normal operation of the Linux system. For example, spool files for documents that you are printing are kept here. A spool file is created as a temporary printing file and is removed after printing. Other files, such as system log files, are changed constantly. Table 30-4 lists the subdirectories of the /var directory.

The /proc File System

The /proc file system is a special file system that is generated in system memory. It does not exist on any disk. /proc contains files that provide important information about the state of your system. For example, /proc/cpuinfo holds information about your computer's CPU processor. /proc/devices lists those devices currently configured to run with your kernel. /proc/filesystems lists the file systems. /proc files are really interfaces to the kernel, obtaining information from the kernel about your system. Table 30-5 lists the /proc subdirectories and files.

Device Files: /dev

To mount a file system, you have to specify its device name. The interfaces to devices that may be attached to your system are provided by special files known as device files. The names of these device files are the device names. Device files are located in the /dev directories and usually have abbreviated names ending with the number of the device. For example, fd0 may reference the first floppy drive attached to your system. The prefix sd references SCSI hard drives, so sda2 would reference the second partition on first SCSI hard drive. In most cases, you can use the man command with a prefix to obtain more detailed information about this kind of device. For example, man sd displays the Man pages for SCSI devices. A complete listing of all device names can be found in the devices file located in the linux/doc/device-list directory at the www.kernel.org Web site, and in the devices.txt file in the /etc/usr/linux-2.4/Documentation directory on your Red Hat system. Table 30-6 lists several of the commonly used device names.

Mounting File Systems

Attaching a file system on a storage device to your main directory tree is called mounting the device. The file system is mounted to an empty directory on the main directory tree. You can then change to that directory and access those files. If the directory does not yet exist, you have to create it. The directory in the file structure to which the new file system is attached is referred to as the mountpoint. For example, to access files on a CD-ROM, first you have to mount the CD-ROM.

Mounting file systems can be done only as the root user. This is a system administration task and cannot be performed by a regular user. As the root user, you can, however, make a particular device, like a CD-ROM, user-mountable. In this way, any user could mount a CD-ROM. You could do the same for a floppy drive.

Even the file systems on your hard disk partition must be explicitly mounted. When you install your Linux system and create the Linux partition on your hard drive, however, your system is automatically configured to mount your main file system whenever it starts. When your system shuts down, they are automatically unmounted. You have the option of unmounting any file system, removing it from the directory tree, and possibly replacing it with another, as is the case when you replace a CD-ROM.

Once a file system it actually mounted, an entry for it is made by the operating system in the /etc/mstab file. Here you will find listed all file systems currently mounted.

File System Information

The file systems on each storage device are formatted to take up a specified amount of space. For example, you may have formatted your hard drive partition to take up 3GB. Files installed or created on that file system take up part of the space, while the remainder is available for new files and directories. To find out how much space you have free on a file system, you can use the df command or, on GNOME, you can use either the Procman System Monitor or the KDE DiskFree utility (More System Tools menu). For the Procman System Monitor (System Tools menu), click the System Monitor tab to display a bar graph of the free space on your file systems. KDE DiskFree displays a list of devices showing how much space is free on each partition, and the percentage used.

$ df -h
Filesystem Size Used Avail Use% Mounted on
/dev/hda3  9.7G 2.8G 6.4G  31%   /
/dev/hda2  99M  6.3M 88M   7%    /boot
/dev/hda2  22G  36M  21G   1%    /home
/dev/hdc  525M  525M  0    100%  /mnt/cdrom

$ df .
Filesystem 1024-blocks Used Available Capacity Mounted on
/dev/hda3 297635 169499 112764 60% /

fsck   device-name

# e2fsck /dev/fd0
# e2fsck /dev/hda1