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RH030 Linux Computing Essentials

RH030 Linux Computing Essentials. Workbook 5 - Part 1 The Linux Filesystem. Objectives chapters 1 - 4. How the linux filesystem structure works Superblock, dentries, inodes, data blocks Linking items in the FSH Link-count Understanding the types of items found in the filesystem

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RH030 Linux Computing Essentials

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  1. RH030 Linux Computing Essentials Workbook 5 - Part 1 The Linux Filesystem

  2. Objectives chapters 1 - 4 • How the linux filesystem structure works • Superblock, dentries, inodes, data blocks • Linking items in the FSH • Link-count • Understanding the types of items found in the filesystem • We already understand • regular files, directories, and symbolic links. • Now we look at /dev: • block and character device nodes. • Attaching items to the FSH • Permanent storage devices • Temporary storage devices Linux+ Guide to Linux Certification, 2e

  3. Command Covered • ls – l - i -s -lt • ln – s • stat • mount • umount • mkfs • df Linux+ Guide to Linux Certification, 2e

  4. First how do you do use ‘c’ to clear the screen ? • alias command: • Has 2 uses: • Used to create shortcut of commands • alias c=clear • List all existing alias’s • alias • unalias command: • Used to remove an existing alias • Needs to be put into the ~/.bashrc to be permanent Linux+ Guide to Linux Certification, 2e

  5. Structure’s of linux filesystem • FSH - main structural sections: • Superblock: Contains general information about the filesystem • Namely the file's user owner, group owner, and permissions • Also the number of inodes and data blocks • Dentry: • Filenames are kept in directory entry data structures called “dentries” • These are associated with an inode. • Inode: • Inode table: Consists of several inodes • Contains the metadata about the item which is the actual attributes or properties of the item. Such as file size, data block locations. • Various times are stored the last time a file was accessed (read), changed, or modified, respectively - atime ctime mtime • Data blocks: Data making up the actual contents of the file • The data is the content of the file Linux+ Guide to Linux Certification, 2e

  6. Working with inodes & files • Each inode has a unique number. • ls –i • Each inode contains a link count. • ls –l • The inode records a file's link count, or the number of dentries (filenames) that refer to the file. • When listing files with ls -l, the second column gives the link count. • Usually, regular files only have one name, and the link count as one. • As we will find out, however, this is not always the case. • Other ls options • To see the size of a filels –s • To sort it into time of last modified (mtime) ls –lt Linux+ Guide to Linux Certification, 2e

  7. stat command • Used to examine the inode information of a file or directory in detail stat [options] itemname • To see the filesystem information instead of the file use: stat -f itemname Linux+ Guide to Linux Certification, 2e

  8. ln command works with inodes • ln <sourcefile> <linkname> • Used for linking items in the FSH • Requires Two arguments: • Source file and a Target file to create as a link to existing file • Files can be created as pointers to another file • Similar to a Microsoft shortcut • Or as a linked duplicate of another file. • There are 2 types of links : symbolic or hard links • Symbolic link: pointer to source file ( shortcut) • Hard link: duplicate of source file Linux+ Guide to Linux Certification, 2e

  9. Hard linked file - duplicate • ln ~/project-file linked-projectfile • ls -i • Two files share the same physical inode & data • Hard linked files share the same inode • Must be in the same filesystem • Directories may not be hard linked The structure of hard linked files Linux+ Guide to Linux Certification, 2e

  10. Symbolic Link - pointer • ln -s /etc/passwd linked-passwd • Use -soption to create symbolic link • Is just is a shortcut to another item • Sometimes called a soft link. • This type can to be used across filesystems • Each entry has it’s own inode entry. • Can be linked to directories The structure of symbolically linked files Linux+ Guide to Linux Certification, 2e

  11. Hard Links verse Soft Links • Hard Links • Create’s a duplicate item which shares the same inode number • Directories may not be hard linked. • Hard links must refer to files in the same filesystem. • Hard links have no concept of "original" and "copy". • Once a hard link has been created, all instances are treated equally. • Symbolic or Soft Links • Create’s a pointer item which has a different inode number • Soft links may refer to directories. • Soft links may span filesystems (partitions). • Soft links have a concept of "referrer" and "referred". • Removing the "referred" file results in a dangling referrer. Linux+ Guide to Linux Certification, 2e

  12. How Linux Identifies Device’s • All physical devices on a linux system are represented by a file on the hardrive called a Device Node: • The system uses this Device Node to refer to the physical device. • One file per device – typically found in /dev directory. • Use’s a /dev pathname to refer to each device nodes • These pathname is used to refer to specific devices in the filesystem • pathname = /dev/<device node/file> • Naming device files • Can be just ASCII letters • But commonly use a combination of letters and numbers • Letters = type of physical device • Numbers = unique identification for this specific type of device. Examples = /dev/tty2 /dev/cdrom /dev/hda1 Linux+ Guide to Linux Certification, 2e

  13. Each partition is associated with a different device node. • IDE HDDs are configured in one of the following: • Primary master (/dev/hda) • Primary slave (/dev/hdb) • Secondary master (/dev/hdc) • Secondary slave (/dev/hdd) • SCSI hard disks are configured in one of the following: • First SCSI HDD (/dev/sda) • Second SCSI HDD (/dev/sdb) • Third SCSI HDD (/dev/sdc) • And so on Linux+ Guide to Linux Certification, 2e

  14. OK - /dev holds Device Nodes • But what do they do? • A device node specifies how data is transferred to/from the device. • The job of a device node is to act as a conduit to a particular device driver within the kernel. • When a user writes to a device node, the device node transfers the information to the appropriate device driver in the kernel. • When a user would like to collect information from a particular device, they read from that device's associated device node, just as reading from a file. Linux+ Guide to Linux Certification, 2e

  15. Types of Device Nodes • There are 2 types of Device Files: Block or Character • c = Character devices: • Transfer’s data to and from system character by character • b = Block devices: • Transfer chunks or blocks of data • CD-ROM, HDD, floppy disks • This is identified by the first character infront of the permissions. ls –l /dev/fd0 $ brw-r--r-- 1 root root 2, 0 Feb 23 16:02 /dev/fd0 Linux+ Guide to Linux Certification, 2e

  16. Character Device Files in /dev Directory Table 6-1 (continued): Common device files Linux+ Guide to Linux Certification, 2e

  17. Block Device Files in the /dev Directory • Disk devices are represented by device files that reside in /dev Table 6-1: Common device files Linux+ Guide to Linux Certification, 2e

  18. Quick Device Node exercise • ps • who • ps –u sheila_a • Lets assume Sheila is on puseudo terminal pts/874 • And Sheila_a is on puseudo terminal pts/875 cal > /dev/pts/875 • As chapter 3 does not have an online exercise. • Do this exercise for yourself. Linux+ Guide to Linux Certification, 2e

  19. Storage devices • For access storage devices attached to the FSH. • This requires an empty directory on the FSH. • Whilst Linux does allow low level access to disk drives through device nodes in the /dev directory they are commonly mounted to the FSH. • The mount command is used to both show what devices are available and to attach new devices to the FSH. • The umount command is used to unattach a device. Linux+ Guide to Linux Certification, 2e

  20. umounted / not attached floppy Empty mount point Figure 6-1: The directory structure prior to mounting Linux+ Guide to Linux Certification, 2e

  21. mounted / attached floppy Figure 6-2: The directory structure after mounting a floppy device Linux+ Guide to Linux Certification, 2e

  22. mount command • Only the root user can mount new filesystems. • But everyone can see what is mounted and available. • Mount has 2 uses: • To display the already available or mounted filesystems • mount • With no options or arguments, it lists the currently mounted filesystems - Displays the /etc/mtab • To mount a new file system and allow access to it. Linux+ Guide to Linux Certification, 2e

  23. mount command • Only the root user can mount new filesystems. • To mount a new file system and allow access to it. • mount –t ext3 /dev/fd0 /mnt/floppy • This Associates a device node with the mountpoint thru which it will be accessed. • Can also used to specify the type of filesystem you wish to use. mount /mnt/floppy • If no device node settings are given it reads these settings from the /etc/fstab Linux+ Guide to Linux Certification, 2e

  24. umount command • Only the root user can umount new filesystems. • umount command: • Used to unmount devices from their mount point directories • Remember to umount the disk before removing it from the drive. • A temporary device is NOT written until it is unmounted. Linux+ Guide to Linux Certification, 2e

  25. fuser command • To umount a device from the FSH tree the empty mount point / directory needs to be free for use. • If you get a “device busy” error message when you try to mount a filesystem it means the empty mount point is not free. • fuser -u /mnt/floppy • Checks whether the /mnt/floppy is being used by any user. • Identifies the user Linux+ Guide to Linux Certification, 2e

  26. Mount Points & Mounting • Mount point: • Refers to en empty directory by which a device or storage area is attached to the FSH. • Any existing directory can be a mount point • But this should really be an empty directory = empty mount point. • Mounting: • Mounting refers to attaching a filesystem to the FSH. • Before they can be accessed. • All devices/filesystems are required to be mounted onto the FSH. • The mount command is used to map the device node of a partition or storage device to an already existing empty directory. • That directory is then referred to as a mount point. • So the term ‘mounting’ - refers to the act of making a device accessible to users via the FSH or logical directory tree structure. Linux+ Guide to Linux Certification, 2e

  27. external partitioning are permanent storage devices Figure 6-7: A sample dual-boot Linux partitioning strategy Linux+ Guide to Linux Certification, 2e

  28. Remember this ? Empty mount point Empty mount point Figure 6-1: The directory structure prior to mounting Linux+ Guide to Linux Certification, 2e

  29. /etc/fstab is run at boot time. • All permanent external filesystems are mounted to the / directory. • Through the /etc/fstab system configuration file. • cat /etc/fstab • System Administrators edit the /etc/fstab file • to add new partitions • to mount new filesystems automatically at boot time. • It is also consulted when users do not specify enough argumentswhen they use the mount command to mount a temporary filesystem. • Six fields: Device to mount, mount point, type, mount options, dump#, fsck# Linux+ Guide to Linux Certification, 2e

  30. Summary of mount commands Table 6-4: Useful commands when mounting and unmounting filesystems Linux+ Guide to Linux Certification, 2e

  31. Mounting temporary storage devices • A temporary device must have a mount point already available. • CDROM • Typically they use iso9660 filesystem type and are not writable. • And are mount with –r (read-only) option • Cannot be ejected until properly unmounted • mount –t iso9660 /dev/cdrom /mnt/cdrom • USB • Typically they use vfat filesystem • Identified as scsi devices • mount –t vfat /dev/sda1 /mnt/usb • Floppy • Typically they use either ext3 or vfat filesystem • mount –t ext3 /dev/fd0 /mnt/floppy Linux+ Guide to Linux Certification, 2e

  32. All storage devices need a filesystems on them. • For access storage devices need to be formatted with a filesystem. • Filesystem: Is an organization imposed on physical storage media • There are many different types of filesystems. • The default filesystem of Red Hat Enterprise Linux is the ext3 filesystem. Linux+ Guide to Linux Certification, 2e

  33. mkfs command • Can be used to format a device or partition. • mkfs /dev/fd0 • With no options assumes the default filesystem ext2. • mkfs –t ext3 /dev/fd0 • Also used to specify the type of filesystem you wish to use. • There are other commands also available which directly identify the filesystem to be created on the device. • Basically these two formats of commands do exactly the same thing. Table 6-3: Commands used to create filesystems Linux+ Guide to Linux Certification, 2e

  34. df commnd • df (disk free space) command: • Monitor free space used by mounted filesystems • –h option: More user friendly Linux+ Guide to Linux Certification, 2e

  35. Workbook5 - Command Summary • ls – n –i –s • stats • ln –s • mount umount • mkfs df Linux+ Guide to Linux Certification, 2e

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