Filesystem Hierarchy Standard

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Linux organizes its files and directories into one overall interconnected tree, beginning from the root directory and extending down to system and user directories. The organization and layout for the system directories is determined by the Filesystem Hierarchy Standard (FHS). The FHS provides a standardized layout that all Linux distributions should follow in setting up their system directories. For example, there must be an /etc directory to hold configuration files and a /dev directory for device files. You can find out more about FHS, including the official documentation, at www.pathname.com/fhs. Linux distributions, developers, and administrators all follow the FHS to provide a consistent organization to the Linux file system.

Linux uses a number of specifically named directories for specialized administration tasks. All these directories are at the very top level of your main Linux file system, the file system root directory represented by a single slash, /. For example, the /dev directory holds device files, and the /home directory holds the user home directories and all their user files. You have access to these directories and files only as the system administrator (though users normally have read-only access). You need to log in as the root user, placing yourself in a special root user administrative directory called /root. From here, you can access any directory on the Linux file system, both administrative and user.

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.

Table 30-1: Linux File System Directories

Directory	Function
/	Begins the file system structure—called the root.
/boot	Holds the kernel image files and associated boot information and files.
/home	Contains users' home directories.
/sbin	Holds administration-level commands and any commands used by the root user.
/dev	Holds file interfaces for devices such as the terminal and the printer.
/etc	Holds system configuration files and any other system files.
/etc/opt	Holds system configuration files for applications in /opt.
/etc/X11	Holds system configuration files for the X Window System and its applications.
/bin	Holds the essential user commands and utility programs.
/lib	Holds essential shared libraries and kernel modules.
/lib/modules	Holds the kernel modules.
/mnt	Used to hold directories for mounting file systems like CD-ROMs or floppy disks that are mounted only temporarily.
/opt	Holds added software applications (for example, KDE on some distributions).
/proc	Process directory, a memory-resident directory containing files used to provide information about the system.
/tmp	Holds temporary files.
/usr	Holds those files and commands used by the system; this directory breaks down into several subdirectories.
/var	Holds files that vary, such as mailbox, Web, and FTP files.

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.

Program Directories

Directories with bin in the name are used to hold programs. The /bin directory holds basic user programs, such as login, shells (BASH, TCSH, and zsh), and file commands (cp, mv, rm, ln, and so on). The /sbin directory holds specialized system programs for such tasks as file system management (fsck, fdisk, mkfs) and system operations like shutdown and startup (init). The /usr/bin directory holds program files designed for user tasks. The /usr/sbin directory holds user-related system operation, such as useradd for adding new users. The /lib directory holds all the libraries your system makes use of, including the main Linux library, libc, and subdirectories such as modules, which holds all the current kernel modules.

Configuration Directories and Files

When you configure different elements of your system, such as user accounts, applications, servers, or network connections, you make use of configuration files kept in certain system directories. On Red Hat, configuration files are placed in the /etc directory, with more specific device and service configurations located in the /etc/sysconfig directory (see Chapter 27 for more details).

Table 30-2: System Directories

Directories	Description
/bin	System-related programs
/sbin	System programs for specialized tasks
/lib	System libraries
/etc	Configuration files for system and network services and applications
/home	The location of user home directories and server data directories, such as Web and FTP site files
/mnt	The location where CD-ROM and floppy disk files systems are mounted
/var	The location of system directories whose files continually change, such as logs, printer spool files, and lock files
/usr	User-related programs and files. Includes several key subdirectories, such as /usr/bin, /usr/X11, and /usr/share/doc
/usr/bin	Programs for users
/dev	Device files
/usr/X11	X Window System configuration files
/usr/share	Shared files
/usr/share/doc	Documentation for applications
/tmp	Directory for system temporary files

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.

Table 30-3: /usr Directories

Directory	Description
/usr/bin	Holds most user commands and utility programs.
/usr/sbin	Holds administrative applications.
/usr/lib	Holds libraries for applications, programming languages, desktops, and so on.
/usr/games	Holds games and educational programs.
/usr/include	Holds C programming language header files (.h).
/usr/doc	Holds Linux documentation.
/usr/local	Holds locally installed software.
/usr/share	Holds architecture-independent data such as documentation.
/usr/src	Holds source code, including the kernel source code.
/usr/X11R6	Holds X Window System–based applications and libraries.

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.

Table 30-4: /var Subdirectories

Directory	Description
/var/account	Processes accounting logs.
/var/cache	Holds application cache data for Man pages, Web proxy data, fonts, or application-specific data.
/var/crash	Holds system crash dumps.
/var/games	Holds varying games data.
/var/lib	Holds state information for particular applications.
/var/local	Used for data that changes for programs installed in /usr/local.
/var/lock	Holds lock files that indicate when a particular program or file is in use.
/var/log	Holds log files such as /var/log/messages that contain all kernel and system program messages.
/var/mail	Holds user mailbox files.
/var/opt	Holds variable data for applications installed in /opt.
/var/run	Holds information about system's running processes.
/var/spool	Holds application's spool data such as that for mail, news, and printer queues, as well as cron and at jobs.
/var/tmp	Holds temporary files that should be preserved between system reboots.
/var/yp	Holds Network Information Service (NIS) data files.
/var/www	Holds Web server Web site files.
/var/ftp	Holds FTP server FTP files.
/var/named	Holds DNS server domain configuration files.

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.

Table 30-5: /proc Subdirectories and Files

Files	Description
/proc/num	There is a directory for each process labeled by its number. /proc/1 is the directory for process 1.
/proc/cpuinfo	Contains information about the CPU, such as its type, make, model, and performance.
/proc/devices	Lists the device drivers configured for the currently running kernel.
/proc/dma	Displays the DMA channels currently used.
/proc/filesystems	Lists file systems configured into the kernel.
/proc/interrupts	Displays the interrupts in use.
/proc/ioports	Shows the I/O ports in use.
/proc/kcore	Holds an image of the physical memory of the system.
/proc/kmsg	Contains messages generated by the kernel.
/proc/ksyms	Holds the symbol table for the kernel.
/proc/loadavg	Lists the system load average.
/proc/meminfo	Displays memory usage.
/proc/modules	Lists the kernel modules currently loaded.
/proc/net	Lists status information about network protocols.
/proc/stat	Contains system operating statistics, such as page fault occurrences.
/proc/uptime	Displays the time the system has been up.
/proc/version	Displays the kernel version.

Tip 

You can use redhat-config-proc, the Kernel Tuning tool (More System Tools menu), to set proc file values you are allowed to change, like the maximum number of files, or turning on IP forwarding.

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.

Table 30-6: Device Name Prefixes

Device Name	Description
hd	IDE hard drives; 1–4 are primary partitions; 5 and up are logical partitions
sd	SCSI hard drives
scd	SCSI CD-ROM drives
fd	Floppy disks
st	SCSI tape drives
nst	SCSI tape drives, no rewind
ht	IDE tape drives
tty	Terminals
lp	Printer ports
pty	Pseudoterminals (used for remote logins)
js	Analog joysticks
midi	Midi ports
ttyS	Serial ports
md	RAID devices
rd/cndn	The directory that holds RAID devices is rd; cn is the RAID controller and dn is the RAID disk for that controller
cdrom	Link to your CD-ROM device file
cdwriter	Link to your CD-R or CD-RW device file
modem	Link to your modem device file
floppy	Link to your floppy device file
tape	Link to your tape device file
scanner	Link to your scanner device file

Floppy Devices

The device name for your floppy drive is fd0; it is located in the directory /dev. /dev/fd0 references your floppy drive. Notice the numeral 0 after fd. If you have more than one floppy drive, additional drives are represented by fd1, fd2, and so on.

Hard Disk Devices

IDE hard drives use the prefix hd, whereas SCSI hard drives use the prefix sd. RAID devices, on the other hand, use the prefix md. The prefix for a hard disk is followed by a letter that labels the hard drive and a number for the partition. For example, hda2 references the second partition on the first IDE hard drive, where the first hard drive is referenced with the letter a, as in hda. The device sdb3 refers to the third partition on the second SCSI hard drive (sdb). RAID devices, however, are numbered from 0, like floppy drives. Device md0 references the first RAID device, and md1 references the second. On an IDE hard disk device, Linux supports up to four primary IDE hard disk partitions, numbered 1 through 4. You are allowed any number of logical partitions. To find the device name, you can use df to display your hard partitions or examine the /etc/fstab file.

CD-ROM Devices

The device name for your CD-ROM drive varies depending on the type of CD-ROM you have. The device name for an IDE CD-ROM has the same prefix as an IDE hard disk partition, hd, and is identified by a following letter that distinguishes it from other IDE devices. For example, an IDE CD-ROM connected to your secondary IDE port may have the name hdc. An IDE CD-ROM connected as a slave to the secondary port may have the name hdd. The actual name is determined when the CD-ROM is installed, as happened when you installed your Linux system. SCSI CD-ROM drives use a different nomenclature for their device names. They begin with scd for SCSI drive and are followed by a distinguishing number. For example, the name of a SCSI CD-ROM could be scd0 or scd1. The name of your CD-ROM was determined when you installed your system. You can find out what it is by examining the /etc/fstab file.

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.

Tip 

On GNOME, you can use the Disk Management tool on the System Settings window and menu to mount and unmount file systems, including floppy disks and CD-ROMs. On KDE, you can use the KDiskFree utility (More System Tools menu), which also lists your mountable file systems as well as their disk usage.

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

The df command reports file system disk space usage. It lists all your file systems by their device names, how much disk space they take up, and the percentage of the disk space used, as well as where they are mounted. With the -h option, it displays information in a more readable format; such as measuring disk space in megabytes instead of memory blocks. The df command is also a safe way to obtain a listing of all your partitions, instead of using fdisk (with fdisk you could erase partitions). df shows only mounted partitions, however, whereas fdisk shows all partitions.

$ 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

You can also use df to tell you to what file system a given directory belongs. Enter df with the directory name or df . for the current directory.

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

e2fsck and fsck

To check the consistency of the file system and repair it if it is damaged, you can use file system checking tools. fsck checks and repairs a Linux file system. e2fsck is designed to support ext2 and ext3 file systems, whereas the more generic fsck also works on any other file systems. The ext2 and ext3 file systems are the file systems normally used for Linux hard disk partitions and floppy disks. Linux file systems beginning with Red Hat 7.3 are normally ext3, which you would use e2fsck to check. fsck and e2fsck take as their argument the device name of the hard disk partition that the file system uses.

fsck   device-name 

Before you check a file system, be sure that the file system is unmounted. e2fsck should not be used on a mounted file system. To use e2fsck, enter e2fsck and the device name that references the file system. The -p option automatically repairs a file system without first requesting approval from the user for each repair task. The following examples check the disk in the floppy drive and the primary hard drive:

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

With fsck, the -t option lets you specify the type of file system to check, and the -a option automatically repairs systems, whereas the -r option first asks for confirmation. The -A option checks all systems in the /etc/fstab file.

Tip 

In earlier distribution versions, fsck and e2fsck were also used to recover file systems after disk crashes or reset-button reboots. With recent releases, journaling capabilities were introduced with file systems like ext3 and ReiserFS. Journaling provides for fast and effective recovery in case of disk crashes, so recovering with fsck or e2fsck is no longer necessary.

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