2.1. Acknowledgments

2.2. Typographical Conventions

Throughout this book, I have tried to use uniform typographical conventions. Hopefully they aid readability. If you can suggest any improvements please contact me.

Filenames are expressed as: /usr/share/doc/foo.

Command names are expressed as: fsck

Email addresses are expressed as: <stephen@clothcat.demon.co.uk>

URLs are expressed as: http://www.tldp.org

I will add to this section as things come up whilst editing. If you notice anything that should be added then please let me know.

3.1. Various parts of an operating system

A UNIX operating system consists of a kernel and some system programs. There are also some application programs for doing work. The kernel is the heart of the operating system. [2] It keeps track of files on the disk, starts programs and runs them concurrently, assigns memory and other resources to various processes, receives packets from and sends packets to the network, and so on. The kernel does very little by itself, but it provides tools with which all services can be built. It also prevents anyone from accessing the hardware directly, forcing everyone to use the tools it provides. [3] This way the kernel provides some protection for users from each other. The tools provided by the kernel are used via system calls. See manual page section 2 for more information on these.

The system programs use the tools provided by the kernel to implement the various services required from an operating system. System programs, and all other programs, run `on top of the kernel', in what is called the user mode. The difference between system and application programs is one of intent: applications are intended for getting useful things done (or for playing, if it happens to be a game), whereas system programs are needed to get the system working. A word processor is an application; mount is a system program. The difference is often somewhat blurry, however, and is important only to compulsive categorizers.

An operating system can also contain compilers and their corresponding libraries (GCC and the C library in particular under Linux), although not all programming languages need be part of the operating system. Documentation, and sometimes even games, can also be part of it. Traditionally, the operating system has been defined by the contents of the installation tape or disks; with Linux it is not as clear since it is spread all over the FTP sites of the world.

3.2. Important parts of the kernel

The Linux kernel consists of several important parts: process management, memory management, hardware device drivers, filesystem drivers, network management, and various other bits and pieces. Figure 3-1 shows some of them.

Probably the most important parts of the kernel (nothing else works without them) are memory management and process management. Memory management takes care of assigning memory areas and swap space areas to processes, parts of the kernel, and for the buffer cache. Process management creates processes, and implements multitasking by switching the active process on the processor.

At the lowest level, the kernel contains a hardware device driver for each kind of hardware it supports. Since the world is full of different kinds of hardware, the number of hardware device drivers is large. There are often many otherwise similar pieces of hardware that differ in how they are controlled by software. The similarities make it possible to have general classes of drivers that support similar operations; each member of the class has the same interface to the rest of the kernel but differs in what it needs to do to implement them. For example, all disk drivers look alike to the rest of the kernel, i.e., they all have operations like `initialize the drive', `read sector N', and `write sector N'.

Some software services provided by the kernel itself have similar properties, and can therefore be abstracted into classes. For example, the various network protocols have been abstracted into one programming interface, the BSD socket library. Another example is the virtual filesystem (VFS) layer that abstracts the filesystem operations away from their implementation. Each filesystem type provides an implementation of each filesystem operation. When some entity tries to use a filesystem, the request goes via the VFS, which routes the request to the proper filesystem driver.

3.3. Major services in a UNIX system

This section describes some of the more important UNIX services, but without much detail. They are described more thoroughly in later chapters.

4.1. Background

This chapter is loosely based on the Filesystems Hierarchy Standard (FHS) [8] version 2.1, which attempts to set a standard for how the directory tree in a Linux [9] system is organized. Such a standard has the advantage that it will be easier to write or port software for Linux, and to administer Linux machines, since everything should be in standardized places. There is no authority behind the standard that forces anyone to comply with it, but it has gained the support of many Linux distributions. It is not a good idea to break with the FHS without very compelling reasons. The FHS attempts to follow Unix tradition and current trends, making Linux systems familiar to those with experience with other Unix systems, and vice versa.

This chapter is not as detailed as the FHS. A system administrator should also read the full FHS for a complete understanding.

This chapter does not explain all files in detail. The intention is not to describe every file, but to give an overview of the system from a filesystem point of view. Further information on each file is available elsewhere in this manual or in the Linux manual pages.

The full directory tree is intended to be breakable into smaller parts, each capable of being on its own disk or partition, to accommodate to disk size limits and to ease backup and other system administration tasks. The major parts are the root (/), /usr, /var, and /home filesystems (see Figure 4-1). Each part has a different purpose. The directory tree has been designed so that it works well in a network of Linux machines which may share some parts of the filesystems over a read-only device (e.g., a CD-ROM), or over the network with NFS.

The roles of the different parts of the directory tree are described below.

• The root filesystem is specific for each machine (it is generally stored on a local disk, although it could be a ramdisk or network drive as well) and contains the files that are necessary for booting the system up, and to bring it up to such a state that the other filesystems may be mounted. The contents of the root filesystem will therefore be sufficient for the single user state. It will also contain tools for fixing a broken system, and for recovering lost files from backups.

• The /usr filesystem contains all commands, libraries, manual pages, and other unchanging files needed during normal operation. No files in /usr should be specific for any given machine, nor should they be modified during normal use. This allows the files to be shared over the network, which can be cost-effective since it saves disk space (there can easily be hundreds of megabytes, increasingly multiple gigabytes in /usr). It can make administration easier (only the master /usr needs to be changed when updating an application, not each machine separately) to have /usr network mounted. Even if the filesystem is on a local disk, it could be mounted read-only, to lessen the chance of filesystem corruption during a crash.

• The /var filesystem contains files that change, such as spool directories (for mail, news, printers, etc), log files, formatted manual pages, and temporary files. Traditionally everything in /var has been somewhere below /usr, but that made it impossible to mount /usr read-only.

• The /home filesystem contains the users' home directories, i.e., all the real data on the system. Separating home directories to their own directory tree or filesystem makes backups easier; the other parts often do not have to be backed up, or at least not as often as they seldom change. A big /home might have to be broken across several filesystems, which requires adding an extra naming level below /home, for example /home/students and /home/staff.

Although the different parts have been called filesystems above, there is no requirement that they actually be on separate filesystems. They could easily be kept in a single one if the system is a small single-user system and the user wants to keep things simple. The directory tree might also be divided into filesystems differently, depending on how large the disks are, and how space is allocated for various purposes. The important part, though, is that all the standard names work; even if, say, /var and /usr are actually on the same partition, the names /usr/lib/libc.a and /var/log/messages must work, for example by moving files below /var into /usr/var, and making /var a symlink to /usr/var.

The Unix filesystem structure groups files according to purpose, i.e., all commands are in one place, all data files in another, documentation in a third, and so on. An alternative would be to group files files according to the program they belong to, i.e., all Emacs files would be in one directory, all TeX in another, and so on. The problem with the latter approach is that it makes it difficult to share files (the program directory often contains both static and sharable and changing and non-sharable files), and sometimes to even find the files (e.g., manual pages in a huge number of places, and making the manual page programs find all of them is a maintenance nightmare).

4.2. The root filesystem

The root filesystem should generally be small, since it contains very critical files and a small, infrequently modified filesystem has a better chance of not getting corrupted. A corrupted root filesystem will generally mean that the system becomes unbootable except with special measures (e.g., from a floppy), so you don't want to risk it.

The root directory generally doesn't contain any files, except perhaps the standard boot image for the system, usually called /vmlinuz. All other files are in subdirectories in the root filesystems:

4.3. The /etc directory

The /etc directory contains a lot of files. Some of them are described below. For others, you should determine which program they belong to and read the manual page for that program. Many networking configuration files are in /etc as well, and are described in the Networking Administrators' Guide.

4.4. The /dev directory

The /dev directory contains the special device files for all the devices. The device files are named using special conventions; these are described in Chapter 5. The device files are created during installation, and later with the /dev/MAKEDEV script. The /dev/MAKEDEV.local is a script written by the system administrator that creates local-only device files or links (i.e. those that are not part of the standard MAKEDEV, such as device files for some non-standard device driver).

4.5. The /usr filesystem

The /usr filesystem is often large, since all programs are installed there. All files in /usr usually come from a Linux distribution; locally installed programs and other stuff goes below /usr/local. This makes it possible to update the system from a new version of the distribution, or even a completely new distribution, without having to install all programs again. Some of the subdirectories of /usr are listed below (some of the less important directories have been dropped; see the FSSTND for more information).

4.6. The /var filesystem

The /var contains data that is changed when the system is running normally. It is specific for each system, i.e., not shared over the network with other computers.

4.7. The /proc filesystem

The /proc filesystem contains a illusionary filesystem. It does not exist on a disk. Instead, the kernel creates it in memory. It is used to provide information about the system (originally about processes, hence the name). Some of the more important files and directories are explained below. The /proc filesystem is described in more detail in the proc manual page.

Note that while the above files tend to be easily readable text files, they can sometimes be formatted in a way that is not easily digestible. There are many commands that do little more than read the above files and format them for easier understanding. For example, the free program reads /proc/meminfo and converts the amounts given in bytes to kilobytes (and adds a little more information, as well).

5.1. The MAKEDEV Script

Most device files will already be created and will be there ready to use after you install your Linux system. If by some chance you need to create one which is not provided then you should first try to use the MAKEDEV script. This script is usually located in /dev/MAKEDEV but might also have a copy (or a symbolic link) in /sbin/MAKEDEV. If it turns out not to be in your path then you will need to specify the path to it explicitly.

In general the command is used as:

# /dev/MAKEDEV -v ttyS0 create ttyS0 c 4 64 root:dialout 0660

This will create the device file /dev/ttyS0 with major node 4 and minor node 64 as a character device with access permissions 0660 with owner root and group dialout.

ttyS0 is a serial port. The major and minor node numbers are numbers understood by the kernel. The kernel refers to hardware devices as numbers, this would be very difficult for us to remember, so we use filenames. Access permissions of 0660 means read and write permission for the owner (root in this case) and read and write permission for members of the group (dialout in this case) with no access for anyone else.

5.2. The mknod command

MAKEDEV is the preferred way of creating device files which are not present. However sometimes the MAKEDEV script will not know about the device file you wish to create. This is where the mknod command comes in. In order to use mknod you need to know the major and minor node numbers for the device you wish to create. The devices.txt file in the kernel source documentation is the canonical source of this information.

To take an example, let us suppose that our version of the MAKEDEV script does not know how to create the /dev/ttyS0 device file. We need to use mknod to create it. We know from looking at the devices.txt file that it should be a character device with major number 4 and minor number 64. So we now know all we need to create the file.

# mknod /dev/ttyS0 c 4 64 # chown root.dialout /dev/ttyS0 # chmod 0644 /dev/ttyS0 # ls -l /dev/ttyS0 crw-rw---- 1 root dialout 4, 64 Oct 23 18:23 /dev/ttyS0

As you can see, many more steps are required to create the file. In this example you can see the process required however. It is unlikely in the extreme that the ttyS0 file would not be provided by the MAKEDEV script, but it suffices to illustrate the point.

5.3. Device List

This list which follows is by no means exhaustive or as detailed as it could be. Many of these device files will need support compiled into your kernel for the hardware. Read the kernel documentation to find details of any particular device.

If you think there are other devices which should be included here but aren't then let me know. I will try to include them in the next revision.

6.1. Two kinds of devices

UNIX, and therefore Linux, recognizes two different kinds of device: random-access block devices (such as disks), and character devices (such as tapes and serial lines), some of which may be serial, and some random-access. Each supported device is represented in the filesystem as a device file. When you read or write a device file, the data comes from or goes to the device it represents. This way no special programs (and no special application programming methodology, such as catching interrupts or polling a serial port) are necessary to access devices; for example, to send a file to the printer, one could just say

$ cat filename > /dev/lp1 $

and the contents of the file are printed (the file must, of course, be in a form that the printer understands). However, since it is not a good idea to have several people cat their files to the printer at the same time, one usually uses a special program to send the files to be printed (usually lpr). This program makes sure that only one file is being printed at a time, and will automatically send files to the printer as soon as it finishes with the previous file. Something similar is needed for most devices. In fact, one seldom needs to worry about device files at all.

Since devices show up as files in the filesystem (in the /dev directory), it is easy to see just what device files exist, using ls or another suitable command. In the output of ls -l, the first column contains the type of the file and its permissions. For example, inspecting a serial device might give

$ ls -l /dev/ttyS0 crw-rw-r-- 1 root dialout 4, 64 Aug 19 18:56 /dev/ttyS0 $

The first character in the first column, i.e., `c' in crw-rw-rw- above, tells an informed user the type of the file, in this case a character device. For ordinary files, the first character is `-', for directories it is `d', and for block devices `b'; see the ls man page for further information.

Note that usually all device files exist even though the device itself might be not be installed. So just because you have a file /dev/sda, it doesn't mean that you really do have an SCSI hard disk. Having all the device files makes the installation programs simpler, and makes it easier to add new hardware (there is no need to find out the correct parameters for and create the device files for the new device).

6.2. Hard disks

This subsection introduces terminology related to hard disks. If you already know the terms and concepts, you can skip this subsection.

See Figure 6-1 for a schematic picture of the important parts in a hard disk. A hard disk consists of one or more circular platters, [12] of which either or both surfaces are coated with a magnetic substance used for recording the data. For each surface, there is a read-write head that examines or alters the recorded data. The platters rotate on a common axis; typical rotation speed is 5400 or 7200 rotations per minute, although high-performance hard disks have higher speeds and older disks may have lower speeds. The heads move along the radius of the platters; this movement combined with the rotation of the platters allows the head to access all parts of the surfaces.

The processor (CPU) and the actual disk communicate through a disk controller. This relieves the rest of the computer from knowing how to use the drive, since the controllers for different types of disks can be made to use the same interface towards the rest of the computer. Therefore, the computer can say just "hey disk, give me what I want", instead of a long and complex series of electric signals to move the head to the proper location and waiting for the correct position to come under the head and doing all the other unpleasant stuff necessary. (In reality, the interface to the controller is still complex, but much less so than it would otherwise be.) The controller may also do other things, such as caching, or automatic bad sector replacement.

The above is usually all one needs to understand about the hardware. There are also other things, such as the motor that rotates the platters and moves the heads, and the electronics that control the operation of the mechanical parts, but they are mostly not relevant for understanding the working principles of a hard disk.

The surfaces are usually divided into concentric rings, called tracks, and these in turn are divided into sectors. This division is used to specify locations on the hard disk and to allocate disk space to files. To find a given place on the hard disk, one might say "surface 3, track 5, sector 7". Usually the number of sectors is the same for all tracks, but some hard disks put more sectors in outer tracks (all sectors are of the same physical size, so more of them fit in the longer outer tracks). Typically, a sector will hold 512 bytes of data. The disk itself can't handle smaller amounts of data than one sector.

Each surface is divided into tracks (and sectors) in the same way. This means that when the head for one surface is on a track, the heads for the other surfaces are also on the corresponding tracks. All the corresponding tracks taken together are called a cylinder. It takes time to move the heads from one track (cylinder) to another, so by placing the data that is often accessed together (say, a file) so that it is within one cylinder, it is not necessary to move the heads to read all of it. This improves performance. It is not always possible to place files like this; files that are stored in several places on the disk are called fragmented.

The number of surfaces (or heads, which is the same thing), cylinders, and sectors vary a lot; the specification of the number of each is called the geometry of a hard disk. The geometry is usually stored in a special, battery-powered memory location called the CMOS RAM, from where the operating system can fetch it during bootup or driver initialization.

Unfortunately, the BIOS [13] has a design limitation, which makes it impossible to specify a track number that is larger than 1024 in the CMOS RAM, which is too little for a large hard disk. To overcome this, the hard disk controller lies about the geometry, and translates the addresses given by the computer into something that fits reality. For example, a hard disk might have 8 heads, 2048 tracks, and 35 sectors per track. [14] Its controller could lie to the computer and claim that it has 16 heads, 1024 tracks, and 35 sectors per track, thus not exceeding the limit on tracks, and translates the address that the computer gives it by halving the head number, and doubling the track number. The mathematics can be more complicated in reality, because the numbers are not as nice as here (but again, the details are not relevant for understanding the principle). This translation distorts the operating system's view of how the disk is organized, thus making it impractical to use the all-data-on-one-cylinder trick to boost performance.

The translation is only a problem for IDE disks. SCSI disks use a sequential sector number (i.e., the controller translates a sequential sector number to a head, cylinder, and sector triplet), and a completely different method for the CPU to talk with the controller, so they are insulated from the problem. Note, however, that the computer might not know the real geometry of an SCSI disk either.

Since Linux often will not know the real geometry of a disk, its filesystems don't even try to keep files within a single cylinder. Instead, it tries to assign sequentially numbered sectors to files, which almost always gives similar performance. The issue is further complicated by on-controller caches, and automatic prefetches done by the controller.

Each hard disk is represented by a separate device file. There can (usually) be only two or four IDE hard disks. These are known as /dev/hda, /dev/hdb, /dev/hdc, and /dev/hdd, respectively. SCSI hard disks are known as /dev/sda, /dev/sdb, and so on. Similar naming conventions exist for other hard disk types; see Chapter 5 for more information. Note that the device files for the hard disks give access to the entire disk, with no regard to partitions (which will be discussed below), and it's easy to mess up the partitions or the data in them if you aren't careful. The disks' device files are usually used only to get access to the master boot record (which will also be discussed below).

6.3. Floppies

A floppy disk consists of a flexible membrane covered on one or both sides with similar magnetic substance as a hard disk. The floppy disk itself doesn't have a read-write head, that is included in the drive. A floppy corresponds to one platter in a hard disk, but is removable and one drive can be used to access different floppies, and the same floppy can be read by many drives, whereas the hard disk is one indivisible unit.

Like a hard disk, a floppy is divided into tracks and sectors (and the two corresponding tracks on either side of a floppy form a cylinder), but there are many fewer of them than on a hard disk.

A floppy drive can usually use several different types of disks; for example, a 3.5 inch drive can use both 720 kB and 1.44 MB disks. Since the drive has to operate a bit differently and the operating system must know how big the disk is, there are many device files for floppy drives, one per combination of drive and disk type. Therefore, /dev/fd0H1440 is the first floppy drive (fd0), which must be a 3.5 inch drive, using a 3.5 inch, high density disk (H) of size 1440 kB (1440), i.e., a normal 3.5 inch HD floppy.

The names for floppy drives are complex, however, and Linux therefore has a special floppy device type that automatically detects the type of the disk in the drive. It works by trying to read the first sector of a newly inserted floppy using different floppy types until it finds the correct one. This naturally requires that the floppy is formatted first. The automatic devices are called /dev/fd0, /dev/fd1, and so on.

The parameters the automatic device uses to access a disk can also be set using the program setfdprm. This can be useful if you need to use disks that do not follow any usual floppy sizes, e.g., if they have an unusual number of sectors, or if the autodetecting for some reason fails and the proper device file is missing.

Linux can handle many nonstandard floppy disk formats in addition to all the standard ones. Some of these require using special formatting programs. We'll skip these disk types for now, but in the mean time you can examine the /etc/fdprm file. It specifies the settings that setfdprm recognizes.

The operating system must know when a disk has been changed in a floppy drive, for example, in order to avoid using cached data from the previous disk. Unfortunately, the signal line that is used for this is sometimes broken, and worse, this won't always be noticeable when using the drive from within MS-DOS. If you are experiencing weird problems using floppies, this might be the reason. The only way to correct it is to repair the floppy drive.

6.4. CD-ROMs

A CD-ROM drive uses an optically read, plastic coated disk. The information is recorded on the surface of the disk [15] in small `holes' aligned along a spiral from the center to the edge. The drive directs a laser beam along the spiral to read the disk. When the laser hits a hole, the laser is reflected in one way; when it hits smooth surface, it is reflected in another way. This makes it easy to code bits, and therefore information. The rest is easy, mere mechanics.

CD-ROM drives are slow compared to hard disks. Whereas a typical hard disk will have an average seek time less than 15 milliseconds, a fast CD-ROM drive can use tenths of a second for seeks. The actual data transfer rate is fairly high at hundreds of kilobytes per second. The slowness means that CD-ROM drives are not as pleasant to use as hard disks (some Linux distributions provide `live' filesystems on CD-ROMs, making it unnecessary to copy the files to the hard disk, making installation easier and saving a lot of hard disk space), although it is still possible. For installing new software, CD-ROMs are very good, since maximum speed is not essential during installation.

There are several ways to arrange data on a CD-ROM. The most popular one is specified by the international standard ISO 9660. This standard specifies a very minimal filesystem, which is even more crude than the one MS-DOS uses. On the other hand, it is so minimal that every operating system should be able to map it to its native system.

For normal UNIX use, the ISO 9660 filesystem is not usable, so an extension to the standard has been developed, called the Rock Ridge extension. Rock Ridge allows longer filenames, symbolic links, and a lot of other goodies, making a CD-ROM look more or less like any contemporary UNIX filesystem. Even better, a Rock Ridge filesystem is still a valid ISO 9660 filesystem, making it usable by non-UNIX systems as well. Linux supports both ISO 9660 and the Rock Ridge extensions; the extensions are recognized and used automatically.

The filesystem is only half the battle, however. Most CD-ROMs contain data that requires a special program to access, and most of these programs do not run under Linux (except, possibly, under dosemu, the Linux MS-DOS emulator, or wine, the Windows emulator. [16] There is also VMWare, a commercial product which emulates an entire x86 machine in software [17]) .

A CD-ROM drive is accessed via the corresponding device file. There are several ways to connect a CD-ROM drive to the computer: via SCSI, via a sound card, or via EIDE. The hardware hacking needed to do this is outside the scope of this book, but the type of connection decides the device file.

6.5. Tapes

A tape drive uses a tape, similar [18] to cassettes used for music. A tape is serial in nature, which means that in order to get to any given part of it, you first have to go through all the parts in between. A disk can be accessed randomly, i.e., you can jump directly to any place on the disk. The serial access of tapes makes them slow.

On the other hand, tapes are relatively cheap to make, since they do not need to be fast. They can also easily be made quite long, and can therefore contain a large amount of data. This makes tapes very suitable for things like archiving and backups, which do not require large speeds, but benefit from low costs and large storage capacities.

6.6. Formatting

Formatting is the process of writing marks on the magnetic media that are used to mark tracks and sectors. Before a disk is formatted, its magnetic surface is a complete mess of magnetic signals. When it is formatted, some order is brought into the chaos by essentially drawing lines where the tracks go, and where they are divided into sectors. The actual details are not quite exactly like this, but that is irrelevant. What is important is that a disk cannot be used unless it has been formatted.

The terminology is a bit confusing here: in MS-DOS and MS Windows, the word formatting is used to cover also the process of creating a filesystem (which will be discussed below). There, the two processes are often combined, especially for floppies. When the distinction needs to be made, the real formatting is called low-level formatting, while making the filesystem is called high-level formatting. In UNIX circles, the two are called formatting and making a filesystem, so that's what is used in this book as well.

For IDE and some SCSI disks the formatting is actually done at the factory and doesn't need to be repeated; hence most people rarely need to worry about it. In fact, formatting a hard disk can cause it to work less well, for example because a disk might need to be formatted in some very special way to allow automatic bad sector replacement to work.

Disks that need to be or can be formatted often require a special program anyway, because the interface to the formatting logic inside the drive is different from drive to drive. The formatting program is often either on the controller BIOS, or is supplied as an MS-DOS program; neither of these can easily be used from within Linux.

During formatting one might encounter bad spots on the disk, called bad blocks or bad sectors. These are sometimes handled by the drive itself, but even then, if more of them develop, something needs to be done to avoid using those parts of the disk. The logic to do this is built into the filesystem; how to add the information into the filesystem is described below. Alternatively, one might create a small partition that covers just the bad part of the disk; this approach might be a good idea if the bad spot is very large, since filesystems can sometimes have trouble with very large bad areas.

Floppies are formatted with fdformat. The floppy device file to use is given as the parameter. For example, the following command would format a high density, 3.5 inch floppy in the first floppy drive:

$ fdformat /dev/fd0H1440 Double-sided, 80 tracks, 18 sec/track. Total capacity 1440 kB. Formatting ... done Verifying ... done $

Note that if you want to use an autodetecting device (e.g., /dev/fd0), you must set the parameters of the device with setfdprm first. To achieve the same effect as above, one would have to do the following:

$ setfdprm /dev/fd0 1440/1440 $ fdformat /dev/fd0 Double-sided, 80 tracks, 18 sec/track. Total capacity 1440 kB. Formatting ... done Verifying ... done $

It is usually more convenient to choose the correct device file that matches the type of the floppy. Note that it is unwise to format floppies to contain more information than what they are designed for.

fdformat will also validate the floppy, i.e., check it for bad blocks. It will try a bad block several times (you can usually hear this, the drive noise changes dramatically). If the floppy is only marginally bad (due to dirt on the read/write head, some errors are false signals), fdformat won't complain, but a real error will abort the validation process. The kernel will print log messages for each I/O error it finds; these will go to the console or, if syslog is being used, to the file /usr/log/messages. fdformat itself won't tell where the error is (one usually doesn't care, floppies are cheap enough that a bad one is automatically thrown away).

$ fdformat /dev/fd0H1440 Double-sided, 80 tracks, 18 sec/track. Total capacity 1440 kB. Formatting ... done Verifying ... read: Unknown error $

The badblocks command can be used to search any disk or partition for bad blocks (including a floppy). It does not format the disk, so it can be used to check even existing filesystems. The example below checks a 3.5 inch floppy with two bad blocks.

$ badblocks /dev/fd0H1440 1440 718 719 $

badblocks outputs the block numbers of the bad blocks it finds. Most filesystems can avoid such bad blocks. They maintain a list of known bad blocks, which is initialised when the filesystem is made, and can be modified later. The initial search for bad blocks can be done by the mkfs command (which initializes the filesystem), but later checks should be done with badblocks and the new blocks should be added with fsck. We'll describe mkfs and fsck later.

Many modern disks automatically notice bad blocks, and attempt to fix them by using a special, reserved good block instead. This is invisible to the operating system. This feature should be documented in the disk's manual, if you're curious if it is happening. Even such disks can fail, if the number of bad blocks grows too large, although chances are that by then the disk will be so rotten as to be unusable.

6.7. Partitions

A hard disk can be divided into several partitions. Each partition functions as if it were a separate hard disk. The idea is that if you have one hard disk, and want to have, say, two operating systems on it, you can divide the disk into two partitions. Each operating system uses its partition as it wishes and doesn't touch the other ones. This way the two operating systems can co-exist peacefully on the same hard disk. Without partitions one would have to buy a hard disk for each operating system.

Floppies are not usually partitioned. There is no technical reason against this, but since they're so small, partitions would be useful only very rarely. CD-ROMs are usually also not partitioned, since it's easier to use them as one big disk, and there is seldom a need to have several operating systems on one.

6.8. Filesystems