RH030 Linux Computing Essentials

Workbook 5 - Part 1The Linux Filesystem

RH030 Linux Computing Essentials

Linux+ Guide to Linux Certification, 2e

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


3

Command Covered

ls – l - i -s -lt ln – s stat mount umount mkfs df


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


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


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 file ls –s To sort it into time of last modified (mtime) ls –lt


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


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


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


10

Symbolic Link - pointer ln -s /etc/passwd linked-passwd Use -s option 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


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.


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


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


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.


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


16

Character Device Files in /dev Directory

Table 6-1 (continued): Common device files


17

Block Device Files in the /dev Directory

Table 6-1: Common device files

Disk devices are represented by device files that reside in /dev


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.


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.


20

umounted / not attached floppy

Figure 6-1: The directory structure prior to mounting

Empty mount point


21

mounted / attached floppy

Figure 6-2: The directory structure after mounting a floppy device


22

mount command Only the root user can mount new filesystems. But everyone can see what is mounted and available.

Mount has 2 uses:1. To display the already available or mounted filesystems

mount With no options or arguments, it lists the currently mounted

filesystems - Displays the /etc/mtab

2. To mount a new file system and allow access to it.


23

mount command Only the root user can mount new filesystems.

1. 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


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.


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


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.


27

external partitioning are permanent storage devices

Figure 6-7: A sample dual-boot Linux partitioning strategy


28

Remember this ?

Figure 6-1: The directory structure prior to mounting

Empty mount point

Empty mount point


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 arguments when they use the mount command to mount a temporary filesystem.

Six fields: Device to mount, mount point, type, mount options, dump#, fsck#


30

Summary of mount commands

Table 6-4: Useful commands when mounting and unmounting filesystems


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


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.


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


34

df commnd df (disk free space) command:

Monitor free space used by mounted filesystems –h option: More user friendly


35

Workbook5 - Command Summary ls – n –i –s

stats ln –s mount umount mkfs df

RH030 Linux Computing Essentials

Documents

Transcript of RH030 Linux Computing Essentials