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installation. It is unlikely that all of the 15 possible partitions on a disk would be necessary just to support Linux. On the Exam Be sure that you understand how partition numbering works. In particular, pay attention to the differences in numbering between primary, extended, and logical partitions. The root filesystem and mount points As a Linux system boots, the first filesystem that becomes available is the top level, or root filesystem, denoted with a single forward slash.* In a simple installation, the root filesystem could contain nearly everything on the system. However, such an arrangement could lead to system failure if the root filesystem fills to capacity. Instead, multiple partitions are typically defined, each containing one of the directories under /. As the Linux kernel boots, the partitions are mounted to the root filesystem, and together create a single unified filesystem. (Mounting is the subject of Objective 3.) Everything on the system that is not stored in a mounted partition is stored locally in /. The mounted filesystems are placed on separate partitions and possibly multiple disk drives. The choice of which directories are placed into separate partitions is both a personal and technical decision. Here are some guidelines for individual partitions: / (the root directory) Since the only filesystem mounted at the start of the boot process is /, certain directories must be part of it in order to be available for the boot process. These include: /bin and /sbin Contain required system binary programs /dev Contains device files /etc Contains boot configuration information /lib Contains program libraries These directories are always part of the single / partition. See the description of the FHS in Objective 8 for more on the requirements for the root filesystem. * The root filesystem /, often called the root directory, shouldn t be confused with the root superuser account or the superuser s home directory, /root. The distinct directories / and /root are unrelated and are not required to share the same disk partition. 74 Devices, Linux Filesystems, and the Filesystem Hierarchy Standard (Topic 2.4)
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Disk partitions On each disk in a PC, there may be between 1 and 16 partitions. A partition can be thought of as a container on the disk, into which a filesystem (or in one circumstance, more partitions) can be placed. Unlike MS-DOS, which assigns letter names to partitions, each partition under Linux is assigned an integer number on the disk, which is appended to the disk s device name. For example, the first partition on IDE disk /dev/hda is /dev/hda1. There are three types of partitions found on PCs: Primary partitions This type of partition contains a filesystem. At least one primary partition must exist, and up to four can exist on a single physical disk. If all four primary partitions exist, they are numbered as follows: /dev/hda1 /dev/hda2 /dev/hda3 /dev/hda4 One of these primary partitions may be marked active, in which case the PC BIOS will be able to select it for boot. Extended partitions An extended partition is a variant of the primary partition but cannot contain a filesystem. Instead, it contains logical partitions. Only one extended partition may exist on a single physical disk. If an extended partition exists, it takes one of the four possible spots for primary partitions, leaving room for only three primary partitions. The partitions on a disk with one primary partition and the sole extended partition are numbered as follows: /dev/hda1 (primary) /dev/hda2 (extended) Logical partitions Logical partitions exist within the extended partition. 1 to 12 logical partitions may be created. Logical partitions are numbered from 5 to 16. The partitions on a disk with one primary partition, the sole extended partition, and four logical partitions are numbered as follows: /dev/hda1 (primary) /dev/hda2 (extended) /dev/hda5 (logical) /dev/hda6 (logical) /dev/hda7 (logical) /dev/hda8 (logical) Under this PC partitioning scheme, a maximum of 15 partitions with filesystems may exist on a single physical disk (3 primary plus 12 logical), more than enough for any Linux installation. In practice, the last example is typical for a Linux Study Guide101 Create Partitions and Filesystems 73
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attaching two disk drives to a system. One device is named master and the other is the slave (an unfortunate naming convention). Most PCs have a primary and secondary IDE interface. Together, these interfaces allow up to four devices (primary master, primary slave, secondary master, and secondary slave). At a minimum, these devices will include a hard disk and a CD-ROM, leaving two available positions for CD-R, Zip, tape, or other IDE devices. Adding additional IDE controllers or specialized IDE subsystems can further expand a PC s capabilities. Compared to IDE, SCSI offers excellent performance, lower CPU utilization, and a much more flexible connection scheme capable of handling up to 15 devices on a single bus. These conveniences allow SCSI systems to grow as space requirements increase without major hardware reconfiguration. Unfortunately, SCSI usually implies higher cost, which reduces demand for SCSI in the cost-sensitive PC market. Typically, IDE is considered appropriate for desktop use. SCSI is usually specified for servers, for high-performance workstations, and in situations in which expansion capability is a concern. Hard disk devices By default, Linux defines IDE device files as follows: /dev/hda Primary master IDE (often the hard disk) /dev/hdb Primary slave IDE /dev/hdc Secondary master IDE (often a CD-ROM) /dev/hdd Secondary slave IDE SCSI device files are similar, except that there is no four-device limitation: /dev/sda First SCSI drive /dev/sdb Second SCSI drive /dev/sdc Third SCSI drive (and so on) Under Linux, a typical PC with a single hard disk on the primary IDE interface and a single CD-ROM on the secondary IDE interface would have disk drive /dev/hda and CD-ROM /dev/hdc. On the Exam You should be prepared to identify IDE and SCSI devices based on their device filenames. 72 Devices, Linux Filesystems, and the Filesystem Hierarchy Standard (Topic 2.4)
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Objective 5: Use File Permissions to Control Access to Files Linux file permissions are a critical part of any system s security policy. This Objective covers permissions on files and directories, including special modes. Weight: 3. Objective 6: Manage File Ownership File ownership is a fundamental part of the access control described in Objective 5. This Objective covers the management of user and group ownership. Weight: 2. Objective 7: Create and Change Hard and Symbolic Links The Linux filesystem allows the creation of filesystem links. Links allow multiple filenames to point to the same file, a handy way of having the same file appear in more than one place or under different names. This Objective covers both hard and soft (symbolic) links. Weight: 2. Objective 8: Find System Files and Place Files in the Correct Location Linux distributions share a common Filesystem Hierarchy Standard (FHS), which describes where files are located in the filesystem and how they are named. This Objective covers that standard, as well as methods of locating files. Weight: 2. Objective 1: Create Partitions and Filesystems In many PCs, disk organization schemes use a single disk containing a single file- system. This filesystem contains all data on the computer, mixing system files and user files together. On MS-DOS and Windows systems, that volume is usually labeled C: and thought of as the C drive. If additional space is made available, it is seen as one or more additional volumes, each with a separate drive letter. If these separate drives are intended for user data, it is the user s responsibility to remember which drive letter to use when storing files. While this simplicity has some value for most users, others prefer the ability to create filesystems across multiple partitions, devices, and even multiple computers. Linux offers this ability. Disk Drives Under Linux Linux supports many types of disk devices and formats. Any SCSI or IDE hard disk will work with Linux, as will floppy disks, CD-ROMs, CD-Rs, Zip and Jaz disks, and other types of removable media. These media can contain the standard Linux ext2 filesystem, FAT, FAT32, NTFS, as well as other filesystem types. This flexibility makes Linux coexist nicely with other operating systems on multiboot systems. The most commonly found hard disks on PCs are IDE (Integrated Device Electronics) drives. These disks feature a relatively simple system interface, and most of the smarts of the disk are onboard the disk itself. The IDE standard allows disk manufacturers to sell their product at a very attractive price point. Also used on PCs are Small Computer System Interface (SCSI, pronounced scuzzy ) drives. SCSI is an older standard for connecting peripherals; however, modern SCSI versions are quite fast and flexible. In general, IDE disks offer reasonable performance at a low price point, which is highly desirable for consumer products. A single IDE interface is capable of Study Guide101 Create Partitions and Filesystems 71
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Devices, Linux Filesystems, and the Filesystem Hierarchy Standard (Topic 2.4) Filesystem management is among the most critical activities that you must perform to maintain a stable Linux system. In simple situations, after a successful installation, you may never have a problem or need to manage filesystem specifics. However, understanding how to configure and maintain Linux filesystems is essential to safely manage your system and to pass Exam 101. This section contains these Objectives: Objective 1: Create Partitions and Filesystems Most Linux distributions will automate initial filesystem creation on your system for you. However, subsequent management of partitions, particularly on large systems and multiboot configurations, requires specific knowledge. This Objective involves the creation of disk partitions using fdisk, and file- system creation using mkfs. Weight: 3. Objective 2: Maintain the Integrity of Filesystems At one time or another, you will probably find yourself stuck with an ailing filesystem. It could be a small problem resulting from a system crash, or it could be a total disk failure. Whatever the cause, you must be prepared to work with fsck to repair problems. This Objective also covers the handy du and df commands, which will help you with monitoring filesystem properties. Weight: 5. Objective 3: Control Filesystem Mounting and Unmounting Under Linux, a filesystem is not available for use unless it is mounted. When the system boots, it mounts its filesystems according to instructions in the important /etc/fstab file. This Objective covers the management of this file, manual mounting of filesystems, and configuration of user-mountable removable filesystems. Weight: 3. Objective 4: Set and View Disk Quotas When running a system with multiple users, you may find some of them competing for disk space. Managing that problem gets much easier when you enforce disk quotas, which allocate finite amounts of space to individual user accounts. Setup and management of quotas is covered by this Objective. Weight: 1. 70
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Example 7 Translate the characters a, b, and c to the @ character from line 11 to 20, wherever they occur on the line: 11,20y/abc/@@@/ On the Exam Make certain you are clear about the difference between file globbing and the use of regular expressions. Study Guide101 Making Use of Regular Expressions 69
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Example 5 Match a dollar amount, using an escaped dollar sign, zero or more spaces or digits, an escaped period, and two more digits: $[ 0-9]*.[0-9]{2} Example 6 Match the month of June and its abbreviation, Jun. The question mark matches zero or one instance of the e: June? Using regular expressions as addresses in sed These examples are commands you would issue to sed. For example, the commands could take the place of command1 in this usage: $ sed [options] ‘command1′ [files] These commands could also appear in a standalone sed script. Example 1 Delete blank lines: /^$/d Example 2 Delete any line that doesn t contain #keepme:: /#keepme/!d Example 3 Delete lines containing only whitespace (spaces or tabs). In this example, tab means the single tab character and is preceded by a single space: /^[ tab]*$/d Example 4 Delete lines beginning with periods or pound signs: /^[.#]/d Example 5 Substitute a single space for any number of spaces wherever they occur on the line: s/ */ /g Example 6 Substitute def for abc from line 11 to 20, wherever it occurs on the line: 11,20s/abc/@@@/g 68 GNU and Unix Commands (Topic 1.3)
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Example 4 Display lines from file1 that contain file (because ? can match zero occurrences), file1, or file2: $ grep -E ‘file[12]?’ file1 Example 5 Display all lines from file1 containing at least one digit: $ grep -E ‘[0-9]+’ file1 Example 6 Display all lines from file1 that contain 111, 1111, or 11111 on a line by itself: $ grep ‘^1{3,5}$’ file1 Example 7 Display all lines from file1 that contain any three-, four-, or five-digit number: $ grep ‘<[0-9]{3,5}>‘ file1 Example 8 Display all lines from file1 that contain Happy, happy, Sad, sad, Angry, or angry : $ grep -E ‘[Hh]appy|[Ss]ad|[Aa]ngry’ file1 Basic regular expression patterns Example 1 Match any letter: [A-Za-z] Example 2 Match any symbol (not a letter or digit): [^0-9A-Za-z] Example 3 Match an uppercase letter, followed by zero or more lowercase letters: [A-Z][a-z]* Example 4 Match a U.S. Social Security Number (123-45-6789) by specifying groups of three, two, and four digits separated by dashes: [0-9]{3}-[0-9]{2}-[0-9]{4} Study Guide101 Making Use of Regular Expressions 67
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Example 6 Display all nonblank lines from file1 (i.e., that have at least one character): $ grep ‘.’ file1 Example 7 Display all lines from file1 that contain a period (normally a metacharacter) using escape: $ grep ‘.’ file1 Modifiers Modifiers change the meaning of other Table 1-14 lists these modifiers. Table 1-14: Regular Expression Modifiers characters in a regular expression. Regular Expression * ? + {n,m} | Description Match an unknown number (zero or more) of the single character (or single-character regex) that precedes it. Match zero or one instance of the preceding regex. This modifier is an extended feature and available in grep only when the E command-line option is used. Match one or more instances of the preceding regex. This modi fier is an extended feature and available in grep only when the E command-line option is used. Match a range of occurrences of the single character or regex that precedes this construct. {n} matches n occurrences,{n,} matches at least n occurrences, and {n,m} matches any number of occurrences between n and m, inclusively. The back- slashes are required and enable this interpretation of{ and }. Alternation. Match either the regex specified before or after the vertical bar. This modifier is an extended feature and available in grep only when the E command-line option is used. Example 1 Display all lines from file1 that contain ab, abc, abcc, abccc, and so on: $ grep ‘abc*’ file1 Example 2 Display all lines from file1 that contain abc, abcc, abccc, and so on, but not ab : $ grep ‘abcc*’ file1 Example 3 Display all lines from file1 that contain two or more adjacent digits: $ grep ‘[0-9][0-9][0-9]*’ file1 66 GNU and Unix Commands (Topic 1.3)
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Table 1-13: Regular Expression Character Sets Regular Expression [abc] [a z] [^abc] [^a z] . (the single dot) Description Single-character groups and ranges. In the first form, match any single character from among the enclosed characters a, b,or c. In the second form, match any single character from among the range of characters bounded by a and z. The brackets are for grouping only and are not matched themselves. Inverse match. Match any single character not among the enclosed characters a, b, and c or in the range a z. Be careful not to confuse this inversion with the anchor character ^, described earlier. Match words. Words are essentially defined as being character sets surrounded by whitespace and adjacent to the start of line, the end of line, or punctuation marks. The backslashes are required and enable this interpretation of < and >. Match any single character except a newline. As mentioned in the section on quoting earlier, turn off (escape) the special meaning of the character that follows, turning metacharacters in to literals. Study Guide101 Example 1 Display all lines from file1 containing either Linux, linux, TurboLinux, and so on: $ grep ‘[Ll]inux’ file1 Example 2 Display all lines from file1 which contain three adjacent digits: $ grep ‘[0-9][0-9][0-9]’ file1 Example 3 Display all lines from file1 beginning with any single character other than a digit: $ grep ‘^[^0-9]’ file1 Example 4 Display all lines from file1 that contain the whole word Linux or linux, but not LinuxOS or TurboLinux : $ grep ‘<[Ll]inux>‘ file1 Example 5 Display all lines from file1 with five or more characters on a line (excluding the newline character): $ grep ‘…..’ file1 Making Use of Regular Expressions 65
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