Copyright (c) 2003 LinuxIT.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2
or any later version published by the Free Software Foundation;
with the Invariant Sections being History, Acknowledgements, with the Front-Cover Texts being “released under the GFDL by LinuxIT”.
GNU Free Documentation License
Version 1.2, November 2002
Copyright (C) 2000,2001,2002 Free Software Foundation, Inc.
59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
0. PREAMBLE
The purpose of this
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DOCUMENTS
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You may extract a single
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7. AGGREGATION
WITH INDEPENDENT WORKS
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8. TRANSLATION
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9. TERMINATION
You may not copy,
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10. FUTURE
REVISIONS OF THIS LICENSE
The Free Software
Foundation may publish new, revised versions of the GNU Free
Documentation License from time to time. Such new versions will be
similar in spirit to the present version, but may differ in detail to
address new problems or concerns. See http://www.gnu.org/copyleft/.
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published (not as a draft) by the Free Software Foundation.
Introduction:
Acknowledgments
The original material was made available by LinuxIT's technical training
centre www.linuxit.com.
Many thanks to Andrew Meredith for suggesting the idea in the first
place. A special thanks to all the students who have helped dilute
the technical aspects of Linux administration through their many
questions, this has led to the inclusion of more illustrations
attempting to introduce concepts in a userfriendly way. Finally, many
thanks to Paul McEnery for the technical advice and for starting off
some of the most difficult chapters such as the ones covering the X
server (101), modems (102) and the Linux kernel (102).
The manual is available online at http://savannah.nongnu.org/projects/lpi-manuals/.
Thank you to the Savannah Volunteers for assessing the project and providing us with the Web space.
History
First release (version 0.0) October 2003. Reviewed by Adrian Thomasset.
Audience
This course is designed as a 3 to 4 days practical course preparing for the LPI 101 exam. It is recommended that candidates have at least one year experience doing Linux administration professionally. However for those who are ready for a challenge the training is designed to provide as much insight and examples as possible to help non specialists understand the basic concepts and command sets which form the core of Linux computing.
The LPI Certification Program
There are
currently two LPI certification levels. The first level LPIC-1 is
granted after passing both exams LPI 101 and LPI 102. Similarly
passing the LPI 201 and LPI 202 exams will grant the second level
certification LPIC-2.
There are no pre-requisites for LPI 101 and 102. However the exams for LPIC-2 can only be attempted once LPIC-1 has been obtained.
Instructor Notice
There are no instructor notes with this manual. The following issues must be considered.
The installation exercises suggest a network installation (prepare floppies + installation server).
The exercises in the device and filesystem sections both assume that a new partition can be created. Make sure during the installation that a large extended partition with at least 100MB free space is available after all the partitions have been created.
The following RPM packages are needed for the exercises:
rpm-build
sharutils
No Guarantee
The manual comes with no guarantee at all.
Resources
www.lpi.org
www.linux-praxis.de
www.lpiforums.com
www.tldp.org
www.fsf.org
www.linuxit.com
Notations
Commands and filenames will appear in the text in bold.
The <> symbols are used to indicate a non optional argument.
The [] symbols are used to indicate an optional argument
Commands that can be typed directly in the shell are highlighted as below
or
|
 command
|
Introduction: 6
Acknowledgments 6
History 6
Installation 10
1. The Installation CD 10
2. Local Installations 11
3. Network Installation 11
4. Rescue disk 11
5. Partitioning Schemes 13
6. Easy Dual Booting 13
8. Exercises 15
Hardware Configuration 16
1. Memory Support 16
2. Resource Allocation 16
3. USB Support 17
4. SCSI Devices 18
5. Network cards 18
6. Setting up modems 19
7. Printer Configuration 20
8. Exercises 22
Managing Devices 23
1. Disks and Partitions 23
2. Partitioning Tools: 25
3. Bootloaders 27
4. Managed devices 28
5. Quotas 29
6. Exercises 31
The Linux Filesystem 32
1. The Filesystem Structure 32
2. Formatting and File System Consistency 33
3. Monitoring Disk Usage 35
4. File Permissions 36
5. Exercises 39
The Command Line 41
1. The interactive shell 41
2. Variables 42
3. Input, Output, Redirection 43
4. Metacharacters and Quotes 45
5. The Command History 47
6. Other Commands 47
7. Exercise 49
File Management 52
1. Moving around the filesystem 52
2. Finding Files and Directories 52
3. Handling directories 54
4. Using cp and mv 54
5. Hard Links and Symbolic Links 55
7. Touching and dd-ing 56
8. Exercises 58
Process Management 60
1. Viewing running processes 60
2. Modifying Processes 61
3. Processes and the shell 63
4. Exercises 65
Text Processing 66
1. cat the Swiss Army Knife 66
2. Simple tools 67
3. Manipulating text 68
4. Exercises 70
Software Installation 71
>1. Introduction 71
2. Static and Shared Libraries 72
3. Source Distribution Installation 74
4. The RedHat Package Manager RPM 75
5. The Alien Tool 78
6. Exercises 79
Advanced Text Manipulation 80
1. Regular Expressions 80
2. The grep family 80
3. Working with grep 81
4. egrep and fgrep 81
5. The Stream Editor - sed 81
6. Exercises 84
Using vi 85
1. vi Modes 85
2. Text Items 85
3. Inserting Text 86
4. Deleting Text 86
5. Copy Pasting 86
6. Searching 87
7. Undoing 87
8. Saving 87
9. Exercises 88
The X Environment 89
1. Introduction 89
3. Configuring X11R6 90
4. Controlling X clients 92
5. Starting X 92
6. The Display Manager 93
7. Troubleshooting X Clients 96
8. Choosing a Window Manager 96
9. Exercises 97
Installation
Rather
than discuss a step by step installation we will introduce in this
module the installation CD, the different installation methods and
the “rescue mode”.
1. The Installation CD
The
various Linux distributions have different names for the directories
on the installation CD. The generic structure of the CDROM is as
follows:
packages:
This directory contains the precompiled packages. Here are the
associated names for the main distrubutions:
debian:
dist
mandrake:
Mandrake
redhat:
RedHat
suse:
suse
images:
This directory contains various “images”. These are special flat
files often containing directory structures. An initial ramdisk
(initrd) is an example of an image file. There are different types
of images necessary to:
- boot the installation process
- provide additional kernel modules
- rescue the system
Some of these files can be copied to a floppy disk when the installation
is started using floppies rather than the CDROM. The Linux tool used
to do this is dd. There is a tool called rawrite which does the same under DOS.
The image is a special file which may contain subdirectories (much like an archive file).
|
Image file structure
|
|
  Image file
|
An image file can be mounted on a loop device. If the image file name is called Image then the
following command will allow one to view the content of this file in the /mnt/floppy directory:
mount -o loop /path/to/Image /mnt/floppy
dosutils:
this directory contains DOS tools which may be used to prepare a
Linux installation such as the
rawrite.exe tool mentioned above. Another tool is the fips utility which non destructively partions a C:\ drive in two provided the underlying filesystem type is FAT and not NTFS.
2. Local Installations
The easiest and most
common type of installation is a local installation. Most
distributions are a CD iso image with an automatic installation
script. On machines with no CD-ROM hardware it is still possible to
start an installation from a floppy.
CD-ROM installation
Change the settings in
the BIOS for the computer to boot from CD. The installation is menu
driven and allows for advanced and basic configuration.
Floppy Installation
If for some reason you
don't boot using the CD-ROM you will need to create a floppy
installation image. This can happen if the CD is not bootable or you
have downloaded a non-iso image of the distribution.
|
Making a bootable installation disk
|
|
dd if=/path/to/<image_name> of=/dev/fd0
|
on a linux box
|
|
rawrite.exe
|
under Windows (not NT)
|
For RedHat distributions the installation images are in the images directory. The basic image is boot.img. Other images are more specialised like bootnet.img or pcmcia.img.
In a Suse distribution the floppy image is in the disks directory and the image is called bootdisk.
3. Network Installation
For a RedHat installation this is only a specialised floppy installation. Make a bootable
floppy using the bootnet.img image:
|
dd /mnt/cdrom/images/bootnet.img of=/dev/fdo
|
The installation is text based and will allow you to setup the network parameters needed. The rest of the installation can be done via FTP, NFS or HTTP.
4. Rescue disk
If a Linux system is corrupt it is possible to boot the computer using a rescue disk. This is a small version of Linux that will mount a minimal virtual filesystem into memory.
The Linux operating system runs entirely in RAM. The aim is to access the root filesystem on the PC hard drive. Most rescue disks can determine this automatically. Assuming the root filesystem was found on the first logical partion of the computer's first IDE disk (/dev/hda5), the rescue disk script can then mount this resource on a subdirectoty of the filesystem in RAM, say /mnt/system. To use the root filesystem on the hard drive as our top directory we need to change our perspective (change root). The chroot tool does just that:
chroot/mnt/system
Getting started
Old Method:
- Make a bootable floppy using the boot.img image file: dd if=boot.img of=/dev/fd0
- Copy the rescue.img image file to a second floppy: dd if=rescue.img of=/dev/fd0
- Boot the system using with the boot.img diskette
- At the LILO prompt type "linux rescue". You should see something like
Insert root file system disk:
- Insert the rescue.img diskette and press enter
- The boot process will continue until you get a shell prompt
- You may still need to determine where the root filesystem is on the hard drive
New Method:
- Insert the Linux installation disk (Suse, RedHat, Mandrake ...)
- At the prompt type “linux rescue”
- Follow the instructions.
- The instuction should say where the root filesystem is mounted
- If the root filesystem is mounted on /mnt/sysimage then enter the following command
|
chroot
/mnt/sysimage
|
5. Partitioning Schemes
The figure below shows a possible partitioning scheme. The File System layout is a tree of directories and subdirectories. The physical resources with the data are mounted at specific locations on the file system called mount points.
The root of the tree structure is called root and is represented by a forward slash “/”. At boot time, the boot loader is told which device to mount at root. The leaves in this tree structure are subdirectories.
During installation you will partition the hard drive and assign a size and a mount point for each partition.
Fig 2: Mount points on the file system
6. Easy Dual Booting
(This section is not for exam purposes).
If Windows9x/2k is already installed on the system the installation setup will automatically configure LILO for dual booting.
Pre-installation:
Before altering the system you should run a defragmentation program over the whole disk. This will make sure that all the blocks used by Windows are rearranged at the beginning of the disk.
Next, using PartitionMagic or fips, partition the C:\ drive in two. The Windows programs are located at the beginning and the second half must be large enough to hold a Linux installation.
Notice: The average amount of space needed for a recent Linux distribution is 1GB.
Starting the installation from DOS:
For non-NT systems restart your computer in DOS command mode. If you are installing RedHat then you can run E:\DOSUTILS\AUTOBOOT.BAT. This will start the installation program. Similarly if you are installing Suse you can run E:\setup.exe under DOS.
The hard drive from a Windows' perspective:
When running Windows the OS will only see the C:\ drive. The rest of the disk where Linux
is installed will be inaccessible.
The hard drive from a Linux point of view:
When running Linux the Windows partition should be called /dev/hda1 (since it's the first partition on the first physical disk). By default this partition is not mounted. You can make a directory /dos or /mnt/dos and mount this
partition. The disk partition corresponding to C:\ is then accessible.
8. Exercises
1. Do a network installation using the ready prepared bootnet.img floppy disk.
(i) Choose “Custom System” installation
(ii) Partition the disk with Disk Druid:
This is a suggestion for a partitioning scheme using about 3GB of hard disk space. If you have more space available then make /usr larger and consider installing more packages than those suggested in
step (iv)
IMPORTANT:Leave a free partition of at least 100MB. We will need this later!!
/boot 20M
/ 250M
/usr 2300M
/home 50M
/tmp 100M
/var 150M
SWAP 128M (Notice that SWAP is a filesystem type and that no mount point is defined)
(iii) Install LILO on /dev/hda2 or /dev/hda3. In all cases do not use the suggested /dev/hda, which is the MBR.
We deliberately don't want the installation to boot properly. The bootloader will be fixed in step 2(i) in rescue mode.
(iv) Packages to install: (the names may vary from one distribution to another)
“Network Support”
“Classic X Window System”
“X Window System”
“Software Development” [This is important, we will need this to compile packages later]
(v) Don’t create a bootable floppy
2. Rescue the system:
(i) Reboot with the bootnet.img floppy disk (or the installation CDROM of you have it). This time type linux rescue at the prompt.
(ii) Read all the instructions until you get to a prompt. Use the chroot command as suggested.
(iii) Edit /etc/lilo.conf (use vi). You should have
boot=/dev/fd0
prompt
linear
timeout=50
image=/boot/vmlinuz-<kernel-version>
label=linux
read-only
root=/dev/<root-partition>
(v) Run /sbin/lilo. If an error occurs you may have to replace linear by lba32 depending on your disk.
Hardware Configuration
1. Memory Support
The system’s RAM is first detected by the BIOS. All types of RAM (EDO, DRAM and SDRAM) are recognised by the Linux kernel. There can be problems with old hardware when the BIOS cannot detect 64MB of RAM or more. In this case one needs to passe parameters to the kernel at boot time.
When using LILO insert the following into /etc/lilo.conf:
append=”mem=<amount of ram>M”
Remember to run /sbin/lilo.
If you are using GRUB add the following to /etc/grub.conf on the
line beginning with kernel:
kernel vmlinuz mem=<amount of ram>M
2. Resource Allocation
To allow peripherals and devices on the PC to communicate directly with system resources, in particular the CPU, the system allocates resources such as lines and channels for each device. These resources are Interrupt Request Lines (IRQ), Input/Output addresses and Direct Memory Access channels (DMA).
IRQs: The Interrupt Request Lines allow devices to request CPU time. The CPU will stop its current activity and process the instructions sent by the device. IRQs range from 0 to 15.
I/O address: These represent specific addresses in the system’s memory map. The CPU will then communicate with the device by reading and writing to memory at the specified address.
DMA: Certain devices can access the system’s memory through a DMA channel, allowing them to write and process data without accessing the CPU. This can enhance performance.
Listing Allocated Resources
The kernel keeps information related to allocated resources in the /proc
directory. The relevant files are:
/proc/dma
/proc/interrupts
/proc/ioports
/proc/pci
Allocated resources can also be listed using tools such as lspci and dmesg:
lspci: lists chipset information of all attached PCI components. Lists I/O and IRQ settings with the -v flag. Also notice the -b (BUS centric) option which shows allocations assigned by the BIOS rather than the kernel.
dmesg. This displays the kernel message logged at boot time. The kernel scans all the hardware on the system and can automatically allocate modules (drivers) for given chipsets. These messages are also available in /var/log/dmesg.
Typical Resources
Device |
I/O port |
IRQ |
/dev/ttyS0 |
0x03f8 |
4 |
/dev/ttyS1 |
0x02f8 |
3 |
/dev/lp0 |
0x378 |
7 |
/dev/lp1 |
0x278 |
5 |
soundcard |
0x220 |
|
Manual Resourse Allocation
|
NOTICE:
This is a very common example, however since kernel modules are only discussed in LPI 102 some may find it difficult. You may skip this example and go to § 3
|
Example: configuring two ethernet cards
1. For statically compiled modules, parameters can be passed to the kernel at boot time. A typical example is when two ethernet cards are present and only the first one is detected. The following line tells
the kernel that:
- there is an ethernet card using IRQ 10 and I/O 0x300
- there is another ethernet card using IRQ 9 and I/O 0x340
ether=10,0x300,eth0 ether=9,0x340,eth1
You type this line at the LILO/GRUB ‘boot:’ prompt, or else, as with the RAM settings before, edit
/etc/lilo.conf (use an append= statement) or /etc/grub.conf.
Notice that the ether= statement is a generic kernel command similar to root=, mem= or init=.
Also notice that you need not specify any information about the ethernet card (Intel, Netgear ...)
2. For dynamically compiled modules, IRQ and I/O address settings can be defined using /etc/modules.conf (or /etc/conf.modules). Assuming that in the above example both cards where using the e100.o kernel module, then /etc/modules.conf would contain the following:
alias eth0 e100
alias eth1 e100
options eth0 io=0x300 irq=10
options eth1 io=0x340 irq=9
3. USB Support
The
Universal Serial Bus (USB) is a communication architecture designed to connect devices to a PC. These devices are divided into five classes:
- Display Devices
- Communication Devices
- Audio Devices
- Mass Storage Devices
- Human Interface Devices (HID)
The
devices are pluuged into a USB port which is driven by a USB
controller. Support for USB controllers is present in the Linux
kernel since version 2.2.7 ( The Linux USB sub-system HOWTO)
There
are 3 types of USB host controlers:
|
Host Controler Kernel Module
|
|
OHCI (Compaq)
|
usb-ohci.o
|
|
UHCI (Intel)
|
usb-uhci.o
|
|
EHCI (USB v 2.0)
|
ehci-hdc.o
|
4. SCSI Devices
Types
of SCSI devices
There are two types of
SCSI interfaces:
- an 8-bit interface with
a bus that supports 8 devices, this includes the controler, so there
is only space for 7 block devices (tapes, disks, etc)
- a 16-bit interface
(WIDE) with a bus that supports 16 devices including the controler,
so there can only be 15 block devices.
Each device is assigned a
unique SCSI ID that can be set using jumpers on the disk. The
IDs range from 0 to 7 for 8-bit controllers and from 0 to 15 for
16-bit controllers.
Logical
units
A
group of disks for example, using RAID is called a logical
unit and is seen as a single
device with a unique SCSI ID. To make the distinction between logical
units a SCSI logical unit number
or LUN is used.
Booting
SCSI disks
The
system will boot from the device with SCSI ID 0 by default. This can
be changed in the SCSI BIOS at boot time.
5. Network cards
The Network Interface
The
network interface card (NIC) must be supported by the kernel. You can
get information about your current card using either of the
following:
dmesg,
lspci, scanpci, /proc/interrupts, /sbin/lsmod.or
/etc/modules.conf:
|
dmesg
|
|
Linux
Tulip driver cersion 0.9.14 (February 20, 2001)
|
|
PCI:
Enabled device 00:0f.0 (0004 ->0007)
|
|
PCI:
Found IRQ 10 for device 00:0f.0
|
|
eth0:
Lite-On 82cl68 PNIC rev 32 at 0xf800, 00:0A:CC:D3:6E:0F,
|
|
IRQ
10
|
|
eth0:
MII transceiver #1 config 3000 status 7829 advertising
|
|
cat /proc/interrupts
0: 8729602 XT-PIC timer
1: 4 XT-PIC keyboard
2: 0 XT-PIC cascade
7: 0 XT-PIC parport0
8: 1 XT-PIC rtc
10: 622417 XT-PIC eth0
11: 0 XT-PIC usb-uhci
14: 143040 XT-PIC ide0
15: 180 XT-PIC ide1
|
/sbin/lsmod
Module Size Used by
tulip 37360
1 (autoclean)
From
the examples above we see that the Ethernet card’s chipset is
Tulip, the i/o address is 0xf800 and the IRQ is 10. This information
can be used either if the wrong module is being used or if the
resources (i/o or IRQ) are conflicting.
This
information can either be used to insert a module with a different
i/o address (using the modprobe or insmod utilities) or
can be saved in /etc/modules.conf (this will save the settings
for the next bootup).
6. Setting up modems
The Modem device
We
will only consider serial modems. The following table shows the
equivalence between DOS COM ports and Linux serial devices.
Table
1: Serial port equivalence DOS-Linux
|
DOS
|
Linux
|
|
COM1
|
/dev/ttyS0
|
|
COM2
|
/dev/ttyS1
|
|
COM3
|
/dev/ttyS2
|
Most
Linux distributions have hardware browser tools (GUIs) which can
detect modems. But one can also use setserial
to scan the serial devices. With the -g option this
utility will tell you which serial devices are in use:
setserial -g /dev/ttyS*
/dev/ttyS0, UART: 16550A, Port: 0x03f8, IRQ: 4
/dev/ttyS1,
UART: 16550A, Port: 0x02f8, IRQ: 3
A
symbolic link called /dev/modem pointing
to used serial portcan be
used to reference the modem.
Manually
linking the modem device
ln -s /dev/ttyS1 /dev/modem
The
setserial tool is also used
to set the speed of the serial port.
Dialup Configuration (The LPI101 objectives only cover hardware
detection and not configuration)
The
wvdial commandline tool has
a setup script called wvdialconf
which will scan the system for modems (all serial and USB ports are
scanned). Once the script has run a skeleton configuration file is
generated as below:
Sample
/etc/wvdial.conf file:
[Dialer Defaults]
>Modem = /dev/ttyS1
Baud = 115200
Init1 = ATZ
Init2 = ATQ0 V1 E1
S0=0 &C1 &D2 S11=55 +FCLASS=0
; Phone = <Target
Phone Number>
; Username = <Your
Login Name>
; Password = <Your
Password>
A
quick way to get started is to replace Defaults with the name
of your provider say WorldISP, fill in the Usernam/Password entries
and type the following:
|
wvdial WorldISP
|
7. Printer Configuration
Printing
is covered in depth in LPI 102. From a hardware perspective, the
printers are detected at boot time automatically and can be seen in
the dmesg output.
Linux
printing happens in two stages. First the raw data is filtered into a
postscript format, then the printing itself is handled by the
ghostscript, or gs utility.
Using
printtool (not
examined)
This
utility creates an entry in /etc/printcap. The main features
which need to be specified are the location of the input_filter=if,
the spool_directory=sd and the printer_device=lp.
If
the printtool fails to detect which parallel port corresponds
to the printer device you can use the dmesg utility to recall
the kernel's initial parallel port scan.
Here
is an example of a system with a local printer plugged into the first
parallel port /dev/lp0
|
Parallel
port scan at the end of dmesg
|
|
parport0:
PC-style at 0x378 (0x778) [SPP,ECP,ECPEPP,ECPPS2]
parport0:
detected irq 7; use procfs to enable interrupt-driven operation.
parport_probe:
succeeded
parport0:
Printer, HEWLETT-PACKARD DESKJET 610C
lp0:
using parport0 (polling)
|
|
Sample
/etc/printcap file
|
|
#
This file can be edited with the printtool in the control-panel.
##PRINTTOOL3##
LOCAL cdj550 300x300 a4 {} DeskJet550 3 {}
lp:\
:sd=/var/spool/lpd/lp:\
:mx#0:\
:sh:\
:lp=/dev/lp0:\
:if=/var/spool/lpd/lp/filter:
|
Figure
7: The gtk-based printtool GUI
Using cups
Cups
is a newer administration and configuration tool for printers. It's
main configuration files are stored in
/etc/cups. The printing process is the same except that cups
uses its own filters situated in /usr/lib/cups.
The
configuration tool for CUPS is a Web based GUI runing on port 631.
When using cups lpd is replaced by the cupsd daemon.
|
NOTICE
|
|
A
local printer is physically detected at boot time for both USB and
parallel connections. Information on the boot process is displayed
at any time with dmesg
|
8. Exercises
1. Use the dmesg
command to view the /var/log/dmesg file. Search for keywords
such as USB, tty or ETH0.
- What are the names of the USB controllers used?
- What are the IRQs for the first two serial ports?
2.
Investigate the contents of the following files:
/proc/ioports
/proc/interrupts
/proc/pci
/proc/dma
3. The PCI bus:
- Investigate the output of lspci -v and scanpci –v. What type of ethernet card in present?
- Verify that there are as many ‘bus ’ entries in /proc/pci. Does this file give as much information as the
commands above?
4. USB
tools:
- Use lsmod and
lsusb to determine which type of host controller is used on your system, UHCI, OHCI or
EHCI (for USB v 2.0).
- Use usbmodules to list the kernel module which can handle the
plugged in interface.
On the exam you may be asked questions on IRQ settings for devices such
as the ethernet card, the parallel and the serial ports.
Managing Devices
1. Disks and Partitions
Physical disks:
On a running Linux system, disks are represented by entries in the /dev
directory. The kernel communicates with devices using a unique
major/minor pair combination. All major numbers are listed in
/proc/devices. For example the first IDE controller‘s major
number is 3:
Block devices:
Hard disk descriptors in /dev begin with hd (IDE) or sd (SCSI), a SCSI
tape would be st, and so on. Since a system can have more than
one block device, an additional letter is added to the descriptor to
indicate which device is considered.
|
Table 1
|
Physical block devices
|
|
hda
|
Primary Master
|
|
hdb
|
Primary Slave
|
|
hdc
|
Secondary Master
|
|
hdd
|
Secondary Slave
|
|
sda
|
First SCSI disk
|
|
sdb
|
Second SCSI disk
|
Disk
Partitions:
Disks
can further be partitioned. To keep track of the partitions a number
is added at the end of each physical device.
|
Table 2
|
Partitions
|
|
hda1
|
First partition on first hard disk
|
|
hda2
|
Second partition on first hard disk
|
|
sdc3
|
Third partition on third SCSI disk
|
IDE type disks allow 4 primary partitions, one of which can be
extended. The extended partition can further be divided into
logical partitions. There can be a maximum of 62 partitions
(primary and logical, excluding the extended).
Typical output of
fdisk -l
|
Device
Boot Start End Blocks Id System
/dev/hda1
* 1 748 6297448+ b Win95 FAT32
/dev/hda2
785 788 32130 83 Linux
/dev/hda3
789 2432 13205430 5 Extended
/dev/hda5
789 1235 3590496 83 Linux
/dev/hda6
1236 1618 3076416 83 Linux
/dev/hda7
1619 1720 819283+ 83 Linux
/dev/hda8
1721 1784 514048+ 83 Linux
/dev/hda9
1785 1835 409626 83 Linux
/dev/hda10
1836 1874 313236 83 Linux
/dev/hda11
1875 1883 72261 82 Linux swap
|
On this system the main
feature to notice is that there are 3 primary partitions. The third
partition is extended (/dev/hda3) and holds 8 logical partitions. The
primary partition /dev/hda3 is not used. In fact /dev/hda3 acts as a
'container' and a filesystem exists only on the enclosed logical
partitions.
|
NOTICE
|
|
Make
sure to distinguish between primary, extended and logical
partitions. Also make sure you understand the naming convention
for the IDE disks and controllers.
|
2. Partitioning Tools:
1. Before
installation: (not for exam purpose)
PartitionMagic
fips
Notice that fips only handles fat16 and fat32. On the
other hand, PartitionMagic is much more versatile and can
handle most common UNIX formats as well.
No partitioning is needed if for example C:\ and D:\ exist and the D:\ drive is
empty.
Partitioning before
installation:
2. During
installation: (not for exam purpose)
During the installation
process the Linux partition is partitioned again. Why do Linux
systems require further partitioning? To answer this question we
first define mount points.
Defining a mount
point: (also see figure page5)
One has the choice to
associate a piece of hardware (or resource) to a directory. For
example the root directory “/” which is more or less like
the C:\ drive for DOS could correspond to the /dev/hda2 partition,
and the subdirectory /boot could correspond to the partition
/dev/hda3.
“/dev/hda3 is said
to be mounted on /boot”. The directory on which a
block device is mounted is then called a mount point.
While installing Linux
you will have the choice of creating new partitions and associating
each partition to a mount point.
For advanced users this is done in two steps:
1. Use the fdisk
tool to create new partitions
2. Associate a mount
point to each partition
For intermediate users
most distributions include a userfriendly tool that does both these
steps at once:
diskdrake (Mandrake)
DiskDruid (RedHat)
The very early success of
RedHat over other projects such as Debian was the introduction of
intuitive installation tools such as DiskDruid.
Finally, for beginners
and busy sysadmin’s, the latest Linux distributions will
automatically assign a partition scheme.
3. On a Running
System:
Once the operating system
is installed you can use the fdisk utility to configure new
partitions.
We will next look at the
basic syntax for fdisk
Example:
1) Start partitioning the
first hard drive:
fdisk
/dev/hda
2) Type m for
help. Then create a new partition with n.
3) To write the changes
to disk type w.
4) REBOOT.
These four points outline
the steps you would follow to create new partitions. The last point is often overlooked. This
forces the partition table in the master boot record MBR to be
reread.
|
NOTICE
|
|
You need to create a filesystem on a new partition with mkfs or
mke2fs before using it
|
This ends the survey of available partitioning tools. We next take a look at bootloaders.
3. Bootloaders
The MBR occupies the first sector of the disk (512 bytes) and contains
the partition tables together with a bootloader. At boot time the
bootloader reads the partition tables looking for a partition marked
“active” and loads the first sector of this partion.
There
are roughly 3 parts envolved:
1. LILO - This is the loader itself. LILO is installed on the MBR and loads the
second stage bootloader, generally situated in /boot/boot.b.
2. /etc/lilo.conf - The main options are specified here
boot* where
LILO should be installed (/dev/hda is the MBR)
install which
second stage to install (boot.b is the default)
prompt give
the user a chance to choose an OS to boot
default name
of the image that will be booted by default
timeout used
with prompt, causes LILO to pause (units are 1/10 of a sec)
image* path
to the kernel to boot (one can use ‘other’ to chain load)
label* name
of the image. This is the name a user can type at the boot prompt
root* the
name of the disk device which contains the root filesystem /
read-only* mount
the root filesystem read-only for fsck to work properly
append give
kernel parameters for modules that are statically compiled.
linear/lba32 these
options are mutually exclusive. Both ask LILO to read the disk using Linear Block Addressing.
linear is typically used for very large disks.
3.
/sbin/lilo
This binary reads it’s configuration file /etc/lilo.conf and installs
the LILO bootloader.
/sbin/lilo should be run every time a change is made to /etc/lilo.conf
GRUB is also installed on the MBR. You can either alter this MBR with the
/sbin/grub shell or use a configuration file called
/boot/grub/grub.conf which will be read by /sbin/grub-install
Detailed information about GRUB can be found in the info pages
GRUB keywords (used in /boot/grub/rub.conf):
1. General/Global
default image that will boot by default (the first entry is 0)
timeout prompt timeout in seconds
2. Image
title name of the image
root where the 2nd stage bootloader and kernel are e.g (hd0,0) is /dev/hda
kernel path for the kernel starting from the previous root e.g /vmlinuz
ro read-only
root the filesystem root
|
Example grub.conf
|
|
default=0
|
|
timeout=10
|
|
splashimage=(hd0,0)/grub/splash.xpm.gz
|
|
title Linux (2.4.18-14)
|
|
root (hd0,0)
|
|
kernel /vmlinuz-2.4.18-14 ro root=/dev/hda5
|
|
initrd /initrd-2.4.18-14.img
|
4. Managed devices
At boot time the /etc/fstab file assigns mount points for block devices.
The /etc/fstab format
|
device mount-point fstype options dump-number
fsck-number
|
Sample /etc/fstab
LABEL=/ / ext2 defaults 1
1
LABEL=/boot
/boot ext2 defaults 1 2
LABEL=/home /home
ext3 defaults 1 2
/dev/fd0 /mnt/floppy
auto noauto,owner 0 0
LABEL=/usr /usr ext2 defaults 1
2
LABEL=/var /var ext3 defaults 1
2
none /proc proc defaults 0
0
none /dev/shm tmpfs defaults 0
0
none /dev/pts
devpts gid=5,mode=620 0 0
/dev/hdc9 swap,pri=-1 swap defaults 0
0
/dev/cdrom
/mnt/cdrom iso9660 noauto,owner,kudzu,ro 0 0
On a running system the /etc/fstab file also acts as a shortcut for assigning a resource to a specific directory. For example:
mount
/dev/cdrom
The mount utility reads fstab and deduces where to mount
the resource. Notice that some of the devices are accessed using a
label. Labels are assigned to devices with the tune2fs tool:
tune2fs
-L /usr/local /dev/hdb12
|
Option summary for mount:
|
|
rw,ro
|
read-write and read-only
|
|
users
|
the device can be read and unmounted by all users
|
|
user
|
the device can unmounted only be the user
|
|
owner
|
the device will change it's permission and belong to the user that
mounted it
|
|
usrquota
|
start user quotas on the device
|
|
grpquota
|
start group quotas on the device
|
|
NOTICE
|
|
Remember that mount -a will mount
all filesytems in /etc/fstab that have not been mounted and do not have the option noauto
|
5. Quotas
The
quota tools allow administrators to set up quotas without having to
reboot. Here are the steps.
1.
Edit /etc/fstab and add usrquota
to the options
2.
Remount the partition:
mount
-o remount <device>
3.
Start the quota stats:
quotacheck
-ca
The
preliminary aquota.user file is generated at the top of
the directory.
4.
Edit quotas for each user:
edquota
-u <user>
Here
a soft/hard limit must be set for both the number of blocks and
inodes available for each user.
The system will allow the user to exceed the soft limit during a certain
grace period. After the grace period has expired the soft
limit will be enforced as a hard limit.
5.
START enforcing quotas:
quotaon –a
Users
can query the quota status with quota. The system
administrator can generate reports with repquota or
quotastats.
6. Exercises
1. Create 1 new partition on the /dev/hda device using fdisk.
fdisk /dev/hda
HINT: To create a new partition type n. The partition type defaults to 83(Linux)
To write the new partition table type w.
The partition table needs to be read: REBOOT the computer !
2. Make a new filesystem (format) on one of the partitions:
mkfs <device>
3. (i) Make a directory called data
mkdir /data
(ii) Edit /etc/fstab and allocate the mount point /data to this new resource
<device> /data ext2 defaults 0 2
4.Force mount to read /etc/fstab:
mount –a
If this doesn't work check that each entry is correct in the fstab
and make sure that the directory /data exists (2 (i))
5. Follow the steps in this chapter to enforce quotas on this
device.
After
step (2) run the mount command and look at the output. Which option from
/etc/fstab can be seen showing that quotas can be enforced on the device? _________
After step (3) which file is created in the /data directory? __________
Before testing quotas for with non-root users, add read-write permissions on /data
chmod o+rw /data
In extreme cases it may be easier to reboot and let the init scripts build the aquota.user (or aquota.group) file. If nothing is showing with the quotas, repquota, or quotastats tools make sure you have read-write access for everyone on /data [chmod a+rw /data ]
6. (OPTIONAL) The instructor computer has a NFS share. Find out which directory is shared and edit /etc/fstab to mount this share on /mnt/nfs. Use the noauto option fot the share not to mount at boot time.
7. SWAPPING bootloaders
a. Uninstall LILO from the MBR (or the floppy)
lilo –u
b. Modify the grub.conf sample on p. 22 to reflect your system
c. Install GRUB on the floppy with grub-install /dev/fd0
The Linux Filesystem
1. The Filesystem Structure
A
filesystem is similar to a tree structure. The root of the tree is
always represented on top and the leaves below.
As
mentioned earlier, once partitions have been created each partition
must be given a mount point. This is typically done at
installation time. To help us understand where things are kept, let
us look at the Linux file system hierarchy.
The
top of a Linux file system hierarchy starts at root (/). This is
similar to C:\ under DOS except that C:\ is also the first device,
whereas the root directory can be mounted anywhere.
Figure 1: The base directories
The base directories are the first subdirectories under the root
directory. These are installed by an rpm package usually
called filesystem.
|
rpm -ql filesystem
|
During the booting process the kernel first mounts the root (/) partition.
In order to mount and check any further partitions and filesystems a
certain number of programs such as fsck, insmod or
mount must be available.
The directories /bin, /sbin, /etc
and /lib must be subdirectories of root (/) and not
mounted on separate partitions.
Base
directories:
Contain binaries needed
to boot up the system and essential commands.
Location for device or special files
Host specific configuration files
Shared libraries for
binaries in /bin and /sbin. Also contains kernel modules
Mount point for external filesystems
Kernel information. Read-only except for /proc/sys/
Contains the Linux
kernel, the system maps and the “second stage” bootloaders.
The directories for
users. Initially contains the contents from /etc/skel/
The directory for user root
Temporary files
User Specific Resource. Mainly static and shareable content
- /usr/local or /opt (optional)
Add-on software applications. Can also contain shared libraries for add-on
software.
- /var/www, /var/ftp/ or /srv (Suse)
Location for HTML pages and anonymous FTP directories.
Variable data, such as spools and logs. Contains both shareable (eg. /var/spool/mail) and non-shareable (eg. /var/log/) subdirectories.
2. Formatting and File System Consistency
In order to organise data on a disk partition one needs to create a file
system. At installation time you will be asked which type of file
system must be used.
Many file system types are supported. The ext2 file system type is
the default and is also known as “Linux Native”.
A different file system type must be used for SWAP. The file system for
Swap is of type swap and cannot be anything else.
The Second Extended File System
Let's take a closer look at the ext2 (second extended) file system.
The ext2 consists of blocks of size 1024 bytes =1 KB
(default). Without entering into too much detail, there are three
types of blocks:
Superblocks:
Repeated every 8193 blocks. Contains information about block-size, free
inodes, last mounted time, etc …
Inodes:
Contains pointers to data blocks. The first 12 blocks of data are directly
accessed. If the data exceeds 12KB, then indirect inodes act as
relays.
Each inode is 256 bytes and contains the name, user, group, permissions
and time stamp of the associated data.
Data Blocks:
These are either files or directories and contain the actual data.
Formatting tools
The file systems supported by the kernel allow one to read from a
preformatted disk. To create these file systems while running a Linux
system one also needs to install the associated formatting tools.
The formatting tool for ext2 is mkfs.ext2 or mke2fs.
Similarly the formatting tool for the xfs file system type
from Silicon Graphics will be mkfs.xfs and may have to be
installed separately.
The mkfs tool acts as a front for all these file system types. The
syntax is:
mkfs –t <fstype>
Notice that the ext3 is an ext2 file system type on which a
journaling system has been added (see the exercises for details).
Example 1: Making a jfs filesystem
mkfs
–t jfs /dev/hda12
Example 2: Making a ext2 filesystem
mke2fs
/dev/hda11 [or mkfs –t ext2 /dev/hda11]
File System Consistancy
If the file system is damaged or corrupt, then the fsck utility
should be run against the partition (the minimum requirement is that
the file system be mounted as read-only).
fsck
acts as a front that automatically detects the file system type of a
partition. Then as with mkfs, the tools fsck.ext2,
fsck.ext3 will be named accordingly.
You can explicitly specify a file system type with the following syntax:
fsck –t <fstype> <device>
Example:
Checking a reiserfs filesystem on the /dev/sdb10 device:
fsck
–t reiserfs /dev/sdb10
fsck.reiserfs /dev/sdb10
3. Monitoring Disk Usage
Using mount and df:
Both of these tools work on
a device level, as opposed to a directory level. The mount and
umount tools maintain the list of mounted filesystems in
/etc/mtab.
Typing mount with
no options will show all filesystems currently mounted. The output is
similar to /etc/mtab. Notice that the kernel also keeps track
of mounted filesystems in /proc/mount.
In addition to showing
all mounted devices the df tool will also show Used
and Available disk
space. By default this is given in blocks of 1K.
|
df -h
|
|
Filesystem
Size Used Avail Use% Mounted on
|
|
/dev/hda9
289M 254M 20M 93% /
|
|
/dev/hda2
23M 7.5M 14M 35% /boot
|
|
none
62M 0 61M 0% /dev/shm
|
|
/dev/hda5
1.4G 181M 1.1G 13% /share
|
|
/dev/hda7
787M 79M 669M 11% /tmp
|
|
/dev/hda3
4.3G 3.4G 813M 81% /usr
|
|
/dev/hda6
787M 121M 627M 17% /var
|
|
//192.168.123.2/share 12G
8.8G 3.7G 71% /mnt/smb
|
Using
du:
This tool will display disk usage. This is done on a
per directory basis. Notice that du cannot display available
space since this information is only available at a device level.
4. File Permissions
Changing
permissions and owners
From the previous figure we see that permissions can be acted upon with
chmod. There are 3 owners for each files and directories:
The symbolic values for the owner fields:
u: a valid user with an entry in /etc/passwd
g: a valid group with an entry in /etc/group
o: other
Example:
-rw-rw-r-- 1 jade sales 24880 Oct 25 17:28 libcgic.a
Changing Permissions:
|
chmod g=r,o-r libcgic.a
chmod g+w libcgic.a
|
Changing user owner and group owner:
|
chown root libcgic.a
chgrp apache libcgic.a
|
|
NOTICE
|
|
A usefull option for chmod,
chown and chgrp is
–R which recursively
changes owners and permission through a directory.
|
Symbolic and octal notation
Permissions can be read=r, write=w and execute=x. The octal values of these
permissions are listed in the next table.
Table 2: Octal and symbolic permissions.
|
Symolic
|
octal
|
|
read
|
4
|
|
write
|
2
|
|
execute
|
1
|
Permissions apply to the user, the group and to others. An item has a set of 3
grouped permissions for each of these categories.
Table 3: How to read a 755 or -rwxr-xr-x permission
|
user
|
group
|
other
|
|
rwx
4+2+1=7
|
r_x
4+1=5
|
r_x
4+1=5
|
The standard permission
UNIX
system create files and directories with standard permissions as
follows:
Standard
permission for:
Files 666 -rw-rw-rw-
Directories 777 -rwxrwxrwx
Umask
Every user has a defined umask that alters the standard permissions.
The umask has an octal value and is subtracted from the octal
standard permissions to give the files permission (this
permission doesn't have a name and could be called the file's
effective permission).
On systems where users belong to separate groups, the umask can have a
value of 002.
For systems which place all users in the users group, the umask
should be 022.
This becomes clearer if you look at the following:
Permission arithmetics:
permission = standard permission – umask
SUID permissions
It is possible for root to give users permission to execute programs
they would usually be unable to. This permission is the SUID
permission with a symbolic value s or a numerical value 4000.
For example root can write a shell script that executes a program and set
the SUID of the script with chmod 4777 script or chmod u+s script.
Examples:
|
chmod 4755 /bin/cat
chmod u+s /bin/grep
|
SGID permissions
The SGID is a similar permission set for group members. The symbolic
value is s and the octal value of 2000.
Setting SGID on a directory enables members of the group owner to create
files with the appropriate group ownership (no need to use newgrp
to change the effective group)
Examples:
|
chmod
2755 /home/data
chmod g+s /bin/wc
|
The sticky bit
The sticky bit permission with value 1000 has the following
effect:
Applied
to a directory it prevents users from deleting files unless they are
the owner (ideal for directories shared by a group)
Applied
to a file this causes the file or executable to be loaded into
memory and causes later access or execution to be faster. The
symbolic value for an executable file is t while for a non
executable file this is T.
Examples:
|
chmod 1666 /data/store.txt
chmod o+t /bin/bash
|
5. Exercises
Filesystem
1. Create 2 new
partitions (larger than 50M) on the /dev/hda device using
fdisk.
HINT: To create a new
partition type n. The partition type defaults to 83
(Linux)
To write the new
partition table type w.
The partition table
needs to be read: REBOOT the computer !
2. Format the first
partition using the ext2 filesystem type and the second with
reiserfs.
HINT: The mkfs
tool is a front for mkfs.ext2 or mkfs.reiserfs, etc.
The syntax is
mkfs –t <fstype>
<device>
3. Make directories in
/mnt and mount the new partitions
mkdir /mnt/ext2
mkdir /mnt/reiserfs
4. Check the status of
your system:
Use mount to
verify which devices are mounted. The permissions set in fstab
are visible too.
Use df to check
the total number of blocks used. The –k option will convert
the number of blocks in kilobytes (the default block size for ext2)
Run fsck on one
of the newly created filesystems. The fsck utility is a front
for fsck.ext2, fsck.ext3, fsck.reiserfs, etc.
The syntax is:
fsck <device>
5. Going further:
Changing from ext2 to ext3 :
Notice that there are no
tools to create ext3 formated partitions. In fact the ext3
format is the same as the ext2 format with a journal added.
These are the steps:
mke2fs /dev/hda10
tune2fs –j /dev/hda10
At this stage the system
has added a .journal file on the /dev/hda10 partition,
making it an ext3 formated partition. This process is
non-destructive and reversible. If you mount an ext3 as an
ext2 filesystem, the .journal file will be erased. You
can add it again with tune2fs …
File permissions
1. Login as a user (non root). Create a file using touch and
verify that it has the effective permission 664.
2. Change the umask to 027. If you create a new file what is
it’s effective permission? _________
Where is the value of umask set? Depening the systems this can be
/etc/profile or /etc/bashrc
3. Add 2 users to your system.
useradd user1
useradd user2
Add passords with passwd user1 and passwd user2
4. Create a group called sales.
groupadd sales
5. Add the users to the group sales
gpasswd -a user1 sales
gpasswd -a user2 sales
6. Create a directory /news
owned by the group sales and
read-writable for this group.
mkdir -m 770 /news ; chown .sales /news
7. Set the GID to the /news directory.
chmod g+s /news
What are the symbolic permissions (eg.
-rwxr_xr_x) on /news?
[use ls -ld /news
] ______
Verify that a group member doesn’t need to type “newgrp
sales” in order to create files with the right permissions. Can members of the group sales
modify any files in this directory?
8. Add the sticky-bit permission on the /news
directory. Verify that only user-owners can modify the files in the
that directory. What are the permissions like on /news? ______________
9. Set the sticky-bit on the binary mozilla.
chmod o+t 'which mozilla`
Start mozilla twice and verify that the second time it will execute faster.
10. As root set SUID root xeyes. Login as a non root user. Check that
this binary runs with UI root.
chmod u+s `which xeyes`
Log in as another user and run xeyes. Then do:
ps
aux | grep xeyes
(the binary should be running as root)
The Command Line
Overview
A basic way to interact with a computer system is to use the command line. The shell
interprets the instructions typed in at the keyboard. The shell
prompt (ending with $ or # for user root) indicates that it is ready for user input.
The shell is also a programming environment which can be used to perform automated tasks.
Shell programs are called scripts.
|
Most Common shells
|
|
The Bourne shell
|
/bin/sh
|
|
The Bourne again shell
|
/bin/bash
|
|
The Korn shell
|
/bin/ksh
|
|
The C shell
|
/bin/csh
|
|
Tom's C shell
|
/bin/tcsh
|
Since the bash shell is one of the most widely used shells in the Linux world the LPI
concentrates mainly on this shell.
1. The interactive shell
Shell commands are often of the form
command [options] {arguments}.
The the bash shell uses the echo
command to print text to the screen.
|
echo
“this is a short line”
|
The interactive shell is often refered to as the session leader
and will be the parent process
of any new process started from the shell which is then called
a child process.
There are two methods available to execute a new command: exec
and fork. By default a process will use the fork method. To force a process to use the
exec method the command is preceeded by the exec command:
Notice that when xeyes is terminated the parent process will also exit. A useful example is a
window manager started with exec in such a way that the X11 server will exit once the window manager is closed.
Full/Relative path
The shell interprets any string given on the command line as a command.
If the string is a full path to an executable then the
executable is started. If not (the command is a string) the shell
will scan directories defined in the PATH variable and attempt to run
the first command matching the string.
For example if the PATH variable only contains the directories /bin
and /usr/bin then the
string xeyes won't be
found since it is stored in /usr/X11R6/bin/xeyes so
the full path needs to be run
An alternative to typing the full path to an executable is to use a relative path. For example, if the user is in the directory where the xeyes program is stored then one can type
2. Variables
Shell variables are similar to variables used in any computing language.
Variable names are limited to alphanumeric characters. For example
CREDIT=300 simply assigns the value 300 to the variable named CREDIT.
|
1. initialise a variable:
|
Variable-Name=value (no spaces !!)
|
|
2. reference a variable:
|
$Variable-Name
|
Export, Set and Env:
There are two types of variable: local and global.
Local variables will be accessible only to the current shell. On the other
hand, global variables are accessible by both the shell and any child
process started from that shell.
The commands set and env are used to list defined variables
|
The set and env commands
|
|
set
|
Lists all variables
|
|
env
|
Lists all global variables
|
A global variable is global in the sense that any child process can
reference it.
Example:
Make the CREDIT variable a global variable. Test whether it's listed
with set or env.
|
export CREDIT
|
|
env | grep CREDIT
|
Start
a new shell (child process) and verify that CREDIT is accessible.
Can one start any shell and be sure that CREDIT is still declared?
Table 1.2 List of common predefined variables
|
PREDEFINED VARIABLES
|
MEANING
|
| DISPLAY |
Used by X to identify where to run a client application |
| HISTFILE |
Path to the users .bash_history file |
| HOME |
The path to the user's home |
| LOGNAME |
The name used by the user to log in |
| PATH |
List of directories from which programs can be executed |
| PWD |
The current working directory
|
| SHELL |
The
shell used (bash in most Linux distributions) |
| TERM |
The current terminal emulation |
Special
variables
The
next few variables are related to process management.
$! represents the PID value of the last child process
$$ represents the PID of the running shell
$? is 0 if the last command was executed successfully and 1 otherwise
|
3. Input, Output, Redirection
Any
UNIX process has the ability to open three standard file descriptors
which enable it to process input and output. These standard
descriptors can be redefined for any given process. In most cases the
stdin descriptor is the keyboard, and the two output
descriptors, stdout and stderr, is the screen.
|
Numerical
values for stdin, stderr and stdout
|
|
stdin
|
0
|
|
stdout
|
1
|
|
stderr
|
2
|
stdout redirection
program
> file
The
data flows from left to right.
This
will run the fdisk utility and output the result to the
partitions.txt file. No output is visible. Also notice that
the shell will read this line from the right. As a result, the
partitions.txt
file will be created first if it doesn’t exist and overwritten if
the ‘>’ operator is used.
The
‘>>’ operator will append output to a file.
stdin redirection
program
< file
In
this case data flows from right to left. The ‘<’
operator is only used for stdin and cannot be used for stdout.
If
the file instuctions contains on each line the letters p,
m, and q then the next example would cause fdisk
to print the partition table of /dev/hda, print the utility’s
help screen and finally quit:
|
fdisk
/dev/hda < instructions
|
stderr redirection
program
2> errorfile
stdin,
stdout and stderr are represented by 0, 1 and 2 respectively. This
allows one to select the stderr stream:
piped commands
program1
| program2
Pipes
are represented by the “|” symbol. The data stream goes from the
left to the right. The next figure illustrates how the stdout
for one process is redirected to the stdin for another
process.
