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INTRODUCTION

In this 21st century, computers play a dominating part in our lives. The facilities providedby these machines are really breathtaking and overwhelming and internet is one of themost significant one. Today communications and gathering information on each andevery aspect has a brand new meaning.

I think every-one has heard of or rather they know what computer hardware or softwareis? But did everyone ever wonder what an oper ati ng systemis?An operati ng system (the OS) is a life giver to a computer- providing it with basicintelligence to work. Technically speaking, it is a program that is loaded into thecomputers memory when the system is booted, and it remains there. It interacts with twoagencies:

Application that run under its auspices.

Users who pass commands to it using a command language interpreter. This toois a program; it translates commands to instructions that can be understood by theoperating system.

In a way an operating system runs programs and controls all resources that the programrequires. It also provides services for the user to handle mail, files and processes.

Besides DOS and WINDOWS, theres an operating system called UNIX. It arrived inthe year 1969 much before Dos and Windows and has stayed long enough to give us theinternet. It has practically everything an operating system should have, and severalfeatures that other operating system never had. Beginners with some experience inWindows think ofUNIX in terms of it, quite oblivious to the fact that the similarities areonly superficial. UNIX is a giant operating system and is way ahead of other systems insheer power. Even UNIX introduced a number of profound and diverse concepts

previously unknown to computing. But some people demand that UNIX is not userfriendly. This is quite a bit true but it in fact challenges you to unravel its mysteries. Inother words we can tell that UNIX system doesnt offer fixed a set of services. Here youhave to use your imagination in devising improvised tools from the existing ones. This iswhat makes UNIX so challenging and exciting.

Now let us throw some light on LINUX operating system. Linux was a phenomenonwaiting to happen. The computer industry suffered from a rift. In the 1980s and 1990s,

people had to choose between inexpensive, market-driven PC operating systems fromMicrosoft and expensive, technology-driven operating systems such as UNIX. Freesoftware was being created all over the world, but lacked a common platform to rallyaround. Linux has become that common platform.

LINUX is a free operating system with many similarities to UNIX. It is the result ofinitial work done by Linus Torvalds, a student of the University of Helsinki. He tried toimprove the Minix operating system and succeeded in improving it. He uploaded hisimproved version of the Minix operating system onto the internet in 1991. Soon,

programmers throughout the world began downloading Linux and started modifying and

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fine-tuning it. This process of modification and improvement is an ongoing one. This isthe major factor that has made Linux stand out among the prominent operating systems inthe world today. Linus was responsible only for creating the system kernel. The kernel is

the core of the operating system, which is responsible carrying out the system tasks.LINUX is now being referred to as a collection of software, which includes the kernel,making it a complete operating system.

LINUX has been catching the attention of almost everyone. Its leap from freeware to theenterprise platform has been a great achievement for Linux. A new breed of computer

professionals is discovering a miracle called Linux. First of all let us look at the reasonsfor the sudden upsurge in the popularity of Linux.

Free: Linux is available for free on the internet. We dont have to pay anythingto download it. A lot of distributions of the Linux are available for freedownload. A Linux distribution comes to us along with software that is worth

thousands of dollars, at a minimal cost or no cost whatsoever. Open Source: Open source means we get not only the executables but also the

source code. The user can access the source code and make improvements withit.

Well Documented: The HOW-Tos, as they are called, are extensive and verysimple. They are not written by programmers but by common people like us. Sothey are very easy to understand. They can guide us from the simplest of thingsto the most customized installations.

Customizable: Linux users have the option of opting for using other softwareother than those bundled with the installation kit. These other software mayeither be commercial software or free software available off the net.

Hardware: Linux runs on almost any hardware, be it 386, 486, Pentium MMX,Pentium I I, Sparc, Dec Alpha or even Motorola 68000 series.

Multi-Tasking: Linux is a multitasking system. What this means is, a singleuser can run multiple programs at the same time. Each task is called a process.

Multi-User: This means that more than one person can use the system at thesame time. The multi-user concept stems directly from the multitasking aspect.

Multiple Virtual Consoles: We have multiple virtual terminals on the server.This means that there are different screens each of which represents a separateterminal, enabling different users to log in at the same time on the samemachine.

TCP/IP Networking: TCP/IPnetworking is built into the kernel itself. Linux is

among the best operating systems in terms of networking. It includes programslike telnet, f tp, rlogin, r sh, and other such programs.

High Level Security: One of the main advantages of Linux is that it provides avery high level of security by using user authentication. It also stores passwordin an encrypted form. Linux also includes file system security that enhances theexisting security.

GUI: Linux these days has a very good Graphical User Interface(GUI) KDE,Window Maker and Afterstepall providing excellent GUI.

Multiple Processor Support: Linux supports multiple processors.

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Web Server: Linux can be used to run a web server such as Apache to serveapplication protocols such as HTTPorF TP.

Stable Operating System: Linux is a complete operating system that is stable.

This means that the malfunctioning of an application is not likely to bring thesystem down.

LINUX ARCHTECTURE

The Linux operating system is categorized into three levels. They are the Kernel, the shelland the various tools and applications as shown in the figure below.

The kernel carries out interactions with the hardware and co-ordinates system

critical tasks. The shell is a program comprising of a command interpreter. It interprets the user

requests and passes them on for execution. The tools and application add enhanced features and capabilities to the Linux

operating system.

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Some of the Linux directories that may interest you include the following:

/bin-Contains common Linux user commands, such as ls, sort, date, and chmod.

/boot-Contains bootable Linux kernel and boot loader configuration file(GRUB).

/dev-Contains files representing access points to devices on your system. Theseinclude terminal devices (tty*), floppy disks (fd*), hard disk (hd*), RAM (ram*),cd-rom (cd*).

/etc-Contains several system configuration files.

/home-Contains directories assigned to each user with a login account.

/mnt-Provides a location for mounting devices such as remote file systems andremovable media.

/proc-Provides a mechanism for the kernel to send information to processes.

/root-Represents root users home directory.

/sbin-Contains administrative commands and daemon processes. /tmp-Contains temporary files used by applications.

/usr-Contains user documentation, games, graphical files, libraries, and a varietyof other user and other administrative commands and files.

/var-Contains directories of data used by various applications.

/opt-Typically contains extra and third party software.

/net- Standard mount point for entire remote file systems.

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BASIC COMMANDS

Commands have the following syntax:o command options arguments

Each item is separated by a space Options modify a command's behavior

o Single-letter options usually preceded by - Can be passed as -a -b -c or -abc

o Full-word options usually preceded by -- Example: --help

Arguments are filenames or other data needed by the command Multiple commands can be separated by ;

How to see date time & calendar?

date - display date and timecal - display calendar

How to edit date and time?

#date -s mm/dd/yyyy

#date s hh:mm:ss

Linux Help tools

The whatis command:

Displays short descriptions of commands

Uses a database that is updated nightly

Often not available immediately after install

# whatis cal

cal (1) - displays a calendar

# makewhatis (This command is used to built the database forcefully that ismaintained by whatis)

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The Man command:

Provides documentation for commands

Almost every command has a man "page"

Pages are grouped into "chapters"

Collectively referred to as the Linux Manual

# man command

Manual sections:

1) User commands2) System calls

3) Library calls

4) Special files

5) File formats

6) Games

7) Miscellaneous

8) Administrative commands

The info Command:

Similar to man, but often more in-depth

Run info without args to list all page

info pages are structured like a web site

Each page is divided into "nodes"

Links to nodes are preceded by *

# info

The Help command:

Displays usage summary and argument list.

Used by most, but not all commands.

# date --help

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How to make a directory?

# mkdir

How to delete a directory?

# rmdir

(the above command removes only empty directories)

# rm r (the above command removes a directory and its contents)

How to create a file?

# cat > (After creating a file with cat command u will get an empty space for writing

some text, write some text and then save the file by pressing ctrl-d)

How to create an empty file?

# touch

How to display the contents of the file created by cat command?

# cat

-b (number each line of output)

-s (squeeze multiple adjacent blank lines into a single blank line)

-A (Show all characters, including the control and non-printingcharacters)

How to remove a file?

# rm

-i (interactive)

-r (recursive)

-f (forceful)

# rm -r

# rm i

# rm -f

# rm ri

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# rm -rf

How to copy a file?

# cp

-i (interactive copy, ask before overwriting a file)

-r (recursive, recursively copy an entire directory tree)

-p (preserve permissions, ownerships and time-stamps)

-a ( archive, copies files and directories recursively while preserving

permission)

How to list directory contents?

# ls

# ls a (includes so called hidden files and directories whose name begin with adot)

# ls -l (long listing ,for some more details)

# ls -R(rescurses through subdirectories, listing their contents too)

# ls -d(this command list directory name and not their contents)

# ls -C

# ls -c

How to move a file or a directory?

# mv

-i (interactive)

-f (forceful)

How to rename a file?

# mv

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How to display the terminal id in which you are logged in?

# tty

How to see the present working directory?

# pwd

How to display which are the users logged in and which terminal they are

occupying?

#who

How to see who you are?

#whoami

How to change directory?

cd changes directories

to an absolute or relative path

cd /home/Anindya/projects

cd Anindya/projects

To a directory one level up

cd . . to your home directory

cd

to your previous working directory

cd ~

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GETTING STARTED WITH VI EDITOR

The vi Editor

An editor is a program used to edit files. Files can be of different types such as text files,graphics files or others, depending on what they contain.

A text editor is a program that is used to edit files that contain text only. For example, aletter or source code of a program. Linux offers editors such as pico, vi, and manyothers.

The vi editor is one of the most powerful and the earliest editors available in Linuxand UNIX environments. It offers many powerful features that are not available in othereditors. Its features are so extensive that very few people are even aware of all theavailable capabilities.

The vi Editor

There are three basic modes of operation in vi. The three modes are: Command mode. Insert or input mode.

The last line mode.

At any time, vi will be in one of these modes.

The command mode is one in which user is able to execute commands, for example, toadd text, to delete text. This can be compared to the menu commands of notepad.

In the input mode or insert mode, the user can enter the data or edit the data.

The last line mode is one in which the user is able to enter special commands whichtypically start with the : (colon).

When vi is invoked, it comes up in command mode. For the user to be able to enterany text, the user must switch to input mode. The user can come back to the commandmode by pressing the Escape key.

The vi Functions

The functions of the vi editor include: Inserting and deleting text. Reading and writing files.

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Searching for text. Copying and moving text.

LIST OF VI COMMANDS

Commands to Insert Text

Command Purpose

i Invokes insert mode and inserts before character.

I Same as i but insertion is at the beginning of the line.

o Opens blank line before the current line.

O Same as o but puts blank line after current line.a Same as i but appends after cursor.

A Same as a but appends at the end of the line.

Commands for Deleting Text

Command Purpose

x Deletes one character at the cursor position.

nx Deletes the next n characters from the cursor position.

dw Deletes one word at the cursor position.

ndw Deletes n words at the cursor position.d0 Deletes from the beginning of the line till the cursor position.

d$ or D Deletes from the cursor position till the end of the line.

dd Deletes one line at the cursor position.

ndd Deletes the text n lines from the cursor position.

u Undoing a change.

Commands to Replace Existing Text

Command Purpose

r Replace a single character at the current cursor position.R Invoke insert mode and go on replacing till is pressed.

s Deletes the current character and inserts character until

is pressed.

S Removes all the characters in the current line and allows the

user to enter data.

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Commands for Saving and Exiting

Command Purpose

:q Quit vi.

:q! Quit vi without saving changes.

:w Save and continue working in vi.

:wq Save the changes and exit from vi.

:x Save and exit from vi.

:ZZ Save the changes and exit from vi.

Commands to Copy Text

Command Purposeyy Copy a line into temporary storage.

nyy Copy n number of lines into temporary storage.

dd Move a line into temporary storage.

ndd Move n lines into temporary storage.

p Move the text from temporary storage to the line below the

current position of the cursor.

P Move the text from temporary storage to the line above the

current position of the cursor.

Commands for Pattern Searching

Command Purpose

/g Search for a pattern g forwards.

?g Search for pattern g backwards.

/ or n Repeat the last search forward.

? or N Repeat the last search backward.

Commands for Scrolling

Command PurposeCtrl-f Scrolls a screen forward.

Ctrl-b Scrolls a screen backward.

Ctrl-u Scrolls half a screen backward.

Ctrl-d Scrolls half a screen forward.

Ctrl-L Refreshes the screen.

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Basic Cursor Movement

Command Purpose

k Up one line

Down one line

h Left one character

l Right one character

Right one word

b Left one word

Setting Basic Options in VI

Command Purpose

:set List a small number of configuration items

:set all Lists the vast panoply of configuration items

:set sm This option causes cursor to jump momentarily to the

matching left curly brace to right curly brace

:se nosm Turns off the above mentioned behavior

:se ai Causes new line to inherit the indentation level of the previous

line

:se noai Turns off the above mentioned behavior

:se nu Causes line numbers to display

:se nonu Turns off the above mentioned behavior

:se ic Causes searches to be case-insensitive

:se noic Turns off the above mentioned behavior

:se textwidth=65 Causes to wrap when text width exceeds 65 characters

:se textwidth=0 Turns off the behavior

:se wrapmargin=15 Causes text to wrap when it reaches 15 characters from the

right margin.

:se wrapmargin=0 Turns the above mentioned behavior off

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More on Cursor Movement

Command Purpose

e Move to the end of current word

$ Move to end of current line

^ Move to beginning of current line

+ Move to beginning of next line

- Move to beginning of previous line

G Go to last line of the file

:n Go to line with this number

) Move to the next sentence

( Move to the previous sentence

} Move to the next paragraph

{ Move to the previous paragraphH Move to the top line of the screen

M Move to the middle line of the screen

L Move to the last line of the screen

% Move to matching brackets ( { [ ] } )

Some Advanced Commands

Command Purpose

:r newfile Inserts new file contents into buffer

:r !date Inserts result of date into buffer:1,20w xfile Writes lines 1-20 of buffer to xfile

:.,$w yfile Writes current line through end of buffer to yfile

:1,20w >> zfile Appends lines 1,20 to zfile

:n Switches to next open file

:N Switches to previous open file

:1,20y Copies 1 to 20 lines of a file

:put Pastes the copied lines below the current cursor position

:1,5d Deletes lines numbered 1 to 5

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STANDARD I/O AND PIPES

Linux provides three I/O channels to the processes:

Standard input- Keyboard by default.

Standard output- terminal window by default.

Standard error- terminal window by default.

Redirecting Input and output

Shell redirection operators allow I/O channels to be redirected to/from a file.

Pipes allow output channels to be redirected to the input of other program.

Common Redirection operators

Command > file Direct standard output of command to file.

Command >> file Append standard output of command to file.

Command < file Send file as input to command.

Command 2>- Redirect error message from command to file.

Command 2>>- Append error messages from command to file.

Piping

command 1 | command 2- Pipe the standard output of command 1 into thestandard input of command 2.

REDIRECTING OUTPUT

To demonstrate I/O redirection we will use the find command

$ find /etc -name passwdThis command will search for all the lines named passwd in /etc and its subdirectories.By default both the standard output and standard error are displayed on the screen.

Example:

$ find /etc -name passwd

/etc/passwd

find: /etc/default: Permission denied

/etc/pam.d/passwd

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Redirecting Standard Output

Redirect Standard output with >

Example:

$ find /etc name passwd > findresultfind: /etc/passwd: Permission denied

$ cat findresult

/etc/passwd/etc/pam.d/passwd

But the error message is still displayed on the screen.

Overwriting Vs. Appending

If the target exists, the file will be overwritten

To append data to an existing file use >> to redirect instead of>

Example:

Redirect output and create/overwrite the output file

$ find /etc -name passwd > outputfind: /etc/default: Permission denied

View the output file

$ catRedirect output and append to the output file

$ find /etc -name passwd >> outputfind: /etc/default : Permission deniedfind: /etc/cups/certs: Permission denied

View the output file

$ cat output/etc/passwd/etc/pam.d/passwd/etc/passwd/etc/pam.d/passwd

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Redirect output and overwrite the output file

$ find /etc name passwd > output

View the output file

$ cat output/etc/passwd

/etc/pam.d/passwd

Redirecting Standard Error

$ find /etc name passwd 2> finderrors/etc/passwd/etc/pam.d/passwd

View the file

$ cat finderrorsfind: /etc/default: Permission deniedfind: /etc/cups/certs : Permission denied

Redirecting some more stderr to the file$find /tmp name passwd 2>> finderrors

View the file

$ cat finderrorsfind: /etc/default: Permission deniedfind: /tmp/orbit-root: Permission denied

Redirecting both Standard output and Error

$ find / -name passwd 2> errs > results

Redirect both stderr and stdout to one file

$ find /etc name passwd > alloutput 2>&1

View the output file$ cat alloutput/etc/passwdFind: /etc/default: Permission denied/etc/pam.d/passwd

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Redirecting Input

$ cat filename.txt

Hello, World!

$ cat < filename.txtHello, World!

Using Pipes

$ ls /usr/lib | less

$ ls C | tr a-z A-Z

$ cat file.txt | xargs rm f

$ ls l | less

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USERS GROUPS AND PERMISSIONS

Users are of two types:

Normal user

Root User or Super User or System Administrator

How to create a user?

# adduser

Or

# useradd

The lists of options that can be used with useradd command are:

How to set Password to your user account?

# passwd

How to delete a user without deleting its home directory?

# userdel

How to delete a user along with its home directory?

# userdel r

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How to create a group?

# groupadd

How to set passwd to your group?

# gpasswd

How to insert a user to a group?

Inserting a user as a primary member of a group

# usermod g

Inserting a user as a secondary member of a group

# usermod G

How to delete a group?

# groupdel

How to change the group of a file or a directory?

# chgrp

How to change ownership of a file or a directory?

# chown :

Permissions

There are three types of permission:

Read

Write

Execute

Default Permission

Default permission for directories is 777 minus umask

Default permission for files is the directory defaults execute permission (i.e. 666 minusumask)

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Non-privileged users umaskis 002

So a non-privileged users file will have permission 664 after subtracting the umask andthe directory permission of a non-privileged user will be 775 after subtracting the umask.

Roots umask is 022

So roots file will have permission 644 after subtracting the umask and the directorypermission root will be 755 after subtracting the umask.

To whom we are assigning the permissions:

Users(u): users who owns the file

Group(g): users in the files group

Other(o): Other users

Changing Permission- Symbolic Method

To change access modes:

chmod [-R] mode file

Where mode is:

u, g or o foruser, group and other.

+ or forgrant ordeny.

r, w, x forread, write and execute.Examples:

ugo+r: Grant read access to user, group and other users

o- x: Deny write and execute to others.

Examples

# chmod u+w : grant write access to owner of the file ordirectory.# chmod u+w, go- : grant write access to owner anddenies it to the group and others.

# chmod u=rw : sets user permissions to read and write,with execute turned off, regardless of the current permission.

# chmod +r : makes the file world readable

Changing Permission- Numeric Method

Uses a three-digit mode number

first digit specifies owners permissions

second digit species group permissions

third digit represents others permissions

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Permissions are calculated by adding

4 for read

2 for write

1 for execute

Examples

# chmod 664 file: grants read/write to the owner and group and read-only to others.

# chmod 660 file: grants read/write to the owner and group and no permission for others.

# chmod 600 file: grants read/write permission to the owner and no permission for groupand others.

# chmod 755 dir: grants full permission to the owner, read and execute to group andother.

#chmod 770 dir: grants full permission to the owner and group and no permission toothers.

# chmod 700 dir: grants full permission to the owner and no permission to group andothers.

User and Group Information Files

Authentication information is stored in plain text files:

/etc/passwd: This file contains seven colon delimited fields. They are as follows-user name, password placeholder, uid number, gid number of the users primarygroup, GECOS field, home directory, and shell to be run when a user logs in.

/etc/group: This file contains four colon delimited field. They are as follows:group name, group password placeholder, gid number, and a comma separated listof group members.

/etc/shadow: This file is referenced when someone logs in. The file contains amapping of a user name to a password.

/etc/gshadow: This file contains the information about group password.

Changing Your Identity

To change your password, run passwd

$ passwd

To start a new shell as a different user:

su

su

su

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su -

User Information commands

Find out who you are$ whoamiFind out the group you belong to$ groups$ id

Find out who is logged in$ users

$ who

$ w

Login/reboot history$ last

Special Permission

Special permissions: a fourth permission set (in addition to user/group/other)

Applicable in four cases:

suid for an executable (S, s or 4)

sgid for an executable (S or s or 2)

sgid for a directory (S or s or 2)

sticky bit for a directory (T or t or 1)Special permission for executables

suid: command run with permission of the owner of the command, not executoror the command

sgid: command runs with group affiliation of the group of the command

Examples:

$ chmod 4551 passwd

$ chmod 2551 same-gnome

Special permission for directories Sticky bit: files in directories with the sticky bit set can only be removed by the

owner and root, regardless of the write permission of the directory.

sgid: Files created in directories with the sgid bit set have group affiliations of thegroup of the directory.

Examples:$ chmod 2551

$ chmod 1551

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LINUX FILESYSTEM IN DEPTH

A Linux file system makes no difference between a file and a directory, since a directory

is just a file containing names of other files. Programs, services, texts, images and so onare all files. Input and output devices and generally all devices are considered to be filesaccording to the system.

Most files are just simple files called regular files, they contain normal data for exampletext files, executable files or programs, input for or output from a program and so on.While it is reasonably safe to suppose that everything you encounter on a Linux system isa file, there are some exceptions.

Directories: files that are list of other files.

Special files: the mechanism used for input and output. Most special files are in/dev.

Links: a system to make a file or directory visible in multiple parts of the systemsfile tree.

Sockets: a special file type, similar to TCP/IP sockets providing inter-processnetworking protected by the file systems access control.

Named pipes: acts more or less like sockets and form a way for processes tocommunicate with each other, without using network socket semantics.

The table given below gives an overview of the characters determining the file types:

Symbol Meaning- Regular file

d Directory

l Soft Link file

c Character Special file

b Block Special file

s Socket

p Named pipe

Files can also be identified with the help of their colors. The table given below gives anoverview of the colors determining the file types:

Color File Type

Blue Directories

Red Compressed Archives

White Text Files

Pink Images

Cyan Links

Yellow Devices

Green Executables

Flashing Red Broken links

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Partitions and File system

Disk drives are divided into partitions.

Partitions are formatted with filesystem. A filesystem is a data structure written tothe media that allow users to store and access files.

Default filesystem in Linux is Third Extended Linux Filesystem (ext3) or SecondExtended Linux Filesystem (ext2).

ext2 or ext3 filesystem keeps a list of files in the filesystem in a table called an inodetable. An individual entry in the inode table is called an inode. The inode isreferenced by its number, the inode number, which is unique within a filesystem.The inode contains metadata about files. Among the data stored in the inode is:

The file type

File permission

Link count: the number of file names associated with the inode number.

The user ID number of the file owner.

The group ID number of the associated group.

Time stamp, including last access time, last modification time, and the inodechange time.

Location of the data on the hard disk.

Other metadata about the file.

Directories

We know that a directory is a container for files and other subdirectories. In fact adirectory is a mapping between the file names that humans use to reference files andinode numbers used by the computer to reference files.When a file name is being referenced by a command or application, Linux referencesthe directory in which the file resides, determines the inode number associated withthe file name, looks up the inode information in the inode table, if the user has

permission, and returns the contents of the file.

cp and inodes

When a file is copied to a new name in the same directory, the directory and the inodetable gets a new entry.

$ ls li penguin

246674 -rw-rw-r-- 1 digby digby 26 Sep 25 20:56 penguin

$ cp penguin tux

$ ls li tux

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246575 -rw-rw-r 1 digby digby 26 Sep 25 20:56 tux

mv and inodes

When a file is moved, the underlying file, either as inode entry or data on the harddisk, does not move. What moves is the entry in a directory. When a file is movedwithin the same filesystem, only the directories itself are affected:

$ ls li tux

246575 -rw-rw-r-- 1 digby digby 26 Sep 25 20:56 tux

$ mv tux fedora

$ ls li fedora

246575 -rw-rw-r-- 1 digby digby 26 Sep 25 20:56 tux

The inode number remains the same. The data on the filesystem is not moved. Theinode is not changed, except that the inode change time is updated.

rm and inodes

The rm command has a broad effect. The entry in the directory is effectively deleted;the inode number is made available; the block locations that the file was using are

placed on the free list.The underlying data is actually not removed, however. The data will be overwritteneventually when the blocks are reused by some other file, but the data is otherwiseunmodified.

Links

A link is nothing more than a way of matching two or more file names to the same setof file data. There are two ways to achieve this:

Hard link: Associate two or more files with the same inode. Hard links sharethe same data blocks on the hard disk, while they continue to behave as

independent files. Soft link or symbolic link: A file that is a pointer to another file. A symbolic

link contains the path to the target file instead of a physical location on thehard disk. Since inodes are not used in this system, soft links can span across

partitions.

How to create a soft link file?

$ ln -s

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$ ln s package package1

$ ls li package package1

30338 -rw-rr-- root root 1729 Sep 25 11:43 package

33276 lrwxrwxrwx digby digby 8 Sep 26 11:05 package1->package

The content of the symbolic link is the name of the file that it references. The size of thesymbolic link is always the number of the characters in the path name.

How to create a hard link?

$ ln

Checking Free Space

df- Reports disk space usage

Reports total kilobytes, kilobytes used, kilobytes free per filesystem.

-h: displays sizes in easier to read units.

$ df h

du- Reports disk space usage

Reports kilobytes used per directory.

Includes subtotals for each subdirectory.

-s option reports only single directory summary

Also takes h option

Removable Media

Mounting means making a foreign filesystem look like a part of the main tree. Beforeaccessing a media must be mounted. Before removing a media must be unmounted. Bydefault non-root users may only mount certain devices. Mountpoints are usually under/media directory.

Mounting CDs and DVDs

Automatically mounted under GNOME/KDE

Otherwise must be manually mounted.

# mount /media/cdrom

# mount /media/cdrecorder

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Eject command unmounts and ejects the disk

Otherwise must be manually unmounted

# umount /media/cdrom

# umount /media/cdrecorder

Mounting USB Media

Detected by the kernel as SCSI devices.

/dev/sdax or /dev/sdbx or similar.

Automatically mounted in GNOME/KDE

Icon created in Computer Window

Mounted under /media/Device ID

Device ID is built into device by vendor.

Mounting Floppy Disks

Must be manually mounted or unmounted# mount /media/floppy

# umount /media/floppy

DOS floppies can be accessed with the help of mtools

Mounts and unmounts device transparently

Uses DOS naming conventions

# mdir a:

# mcopy /home/file.txt a:

Formatting Floppy Disks

Two types of format needed to prepare a floppy

A low level format# fdformat /dev/fd0H1440

A filesystem format, one of:# mkfs t ext2 /dev/fd0

# mke2fs /dev/fd0

# mkfs t vfat /dev/fd0

# mformat a:

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File Archiving

Why Archive files?

Archiving places many files into one target file. Easier for backup, store and transfer.Tar is a standard Linux Archiving command.

Creating an Archive

# tar cvf archive_name files

# tar cvf home.tar /home

c: create a new archive

v: display verbose message

f: archive_name is the name of a new file

Options do not need a leading dash

Inspecting Archives

# tar tf archive_name.tar

# tar tvf archive_name.tar

First command displays a list of all files in the archive

Second command displays a long listing of each file in the archive.

Extracting an Archive

# tar xvf archive_name.tar

Extract to the current directory by default.

Target can be specified with C dir

File Compression

Why do we compress file?

Results in smaller file size

Text files can be compressed over 75%

Binary files usually dont compress much, if any.

tar archives are often compressed

Compression Utilities

gzip, gunzip

bzip2, bunzip2

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gzip or gunzip

Standard Linux compression utility

Over 75% compression for text files

bzip2 or bunzip2

Newer Linux compression utility

Generally achieves better compression than gzip

How to compress and decompress a file using the compression utilities?

$ gzip v

$ gzip d

$ gunzip

$ gunzip

$ bzip2 v

$ bzip2 d

$ bunzip2

$ bunzip2

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INTRODUCTION TO STRING PROCESSING

Head

Displays first few lines (default 10 lines) of text in a file.

# head

Uses n or-lines parameter to change number of lines displayed

# head n 15

Tail

Displays the last few lines (default 10 lines) of text in a file.

# tail

Uses n or -lines to change number of lines displayed.

# tail n 15

Use f to follow the end of a text file as it changes

# tail f

Word Count

Count words, lines, bytes and characters

Can act upon a file or STDIN

Uses l for only line count

Uses w only for word count

Uses c for only byte count

Uses m for character count (not displayed)

# wc

# wc l

# wc w

Sort

Sorts text to stdout- original file unchanged

# sort file(s)

Common options

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-r performs a reverse sort

-n performs a numeric sort

-fignores case of characters in string

-u removes duplicate lines in output

-t c uses c as a field separator

-k x sorts by c-delimited field x

Uniq

Removes successive duplicate lines in a file

Can use in conjunction with sort to remove all duplicates

Sort u achieves the same effect

Use c to count number of occurrences of duplicate data

CutDisplays specific column of the field data

$ cut f4 results.dat

-fspecifies field or column

-d specifies field delimiter

$ cut f3 d: /etc/passwd

-c cuts by characters

$ cut c2-5 /usr/share/dict/ ords

Paste

Paste combines files horizontally. It takes a line from each file and pastes them togetherto standard output separated by a tab. Use the d option to change the output delimiter.

$ paste d: file.txt file2.txt > file3.txt

tr (Character Translator)

$ tr a-z A-Z > file3.txt

Version Comparison with diff

Compares two files for differences

$ diff file1.txt file2.txt

33c33

< x = y+2;

-------

> x = y+4;

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33c33 indicates lines where files differ. indicatesline in the second file.

Spell Checking with Spell

Interactive spell-checker

Easy way to check spelling in a file

$ aspell check

$ aspell l < filename

aspell l will non-interactively list the misspelled words in a file read from standardinput.

Look

Lookcommand is a quick spelling dictionary lookup. It comes in handy when you needthe spelling of a word of which you know the first few letters.

$ look exer

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FINDING AND PROCESSING FILES

Basic find Examples

$ find name a.txt

Search for files whose names are a.txt

$ find iname a.txt

Case-insensitive search for files named a.txt, A.txt etc.

$ find user anindya group anindya

Searches for files which are owned by the user anindya and the group anindya

$ find / -name .txt

In the above example find command will search for the files that are names .txt, notfiles that contained in their names the string .txt. / indicates the searching will becarried out in every directory.

$ find / -name *.txt

The above command will find all files on the system that have .txt as the end of their

name.

Find and Logical Operators

Criteria are ANDed together by default

Can be ORd or negated with o and not

Parentheses can be used to determine logic order, but must be escaped in bash

$ find user anindya not group anindya

$ find user anindya o -group anindya

$ find not \(-user anindya o -user arya\)

Find and Permissions

Can match ownership by names or id

Can match octal or symbolic permissions

$ find / -user anindya o uid 500

$ find perm 755 matches if mode is exactly 755

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$ find perm +222 matches if anyone can write

$ find perm -222 matches if everyone can write

$ find perm +o+w matches if the other members can write

$ find not perm ug+r matches the files where neither the user nor the group have

read access

Find and Numeric Criteria

$ find size 1024k matches files which are exactly 1 megabyte

$ find size +1024k matches files which are more than 1 megabyte

$ find size -1024k matches files which are less than 1 megabyte

Find and Access Time

-atime when file was last read

-mtime when file data last changed

-ctime when file data or metadata was last changed

Value given is in days

$ find / -atime 5

$ find / -atime -5

$ find / -atime +5

$ find / -mtime 5

$ find / -mtime -5

$ find / -mtime +5

$ find / -ctime 5

$ find / -ctime -5

$ find / -ctime +5

The Gnome Search Tool

Actions -> Search for Files.

Graphical tool for searching by name, content, owner/group, size, modificationtime.

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THE BASH SHELL

Bash Introduction

Bourne Again Shell

Successor to sh, the original Unix shell

Developed for the GNU project

The de facto standard Linux shell

Backward compatible with Bourne shell (sh)- the original UNIX shell

Command Line Shortcuts (File Globbing)

When typing commands, it is often necessary to issue the same command on one or morethan one file at the same time. The use of wildcards or metacharacters, allow one pattern

to expand to multiple filenames by a process called globbing.Globbing is wildcard expression:

* - matches zero or more characters.

?- matches any single character.

[a-z]- matches a range of characters.

[^a-z]- matches all except the range.

Examples:

If a directory contains the following files: joshua.txt, James.txt, alex.txt, Angelo.txt,gonk.mp3, zonk.mp3

Type the following commands:

$ rm *mp3

$ rm gonk.mp3 zonk.mp3

The result is that all files in the directory that have names ending in mp3

In addition to wildcards, ranges of characters can be specified within straight-braces (?[?and ?]?). Red Hat Linux uses UTF-8 encoding of characters, which means that each

capital letter comes directly after the corresponding lower-case letter. As a result acommand like:

$ ls [a- ]*.txt

Would list alex.txt, Angelo.txt and joshua.txt but not James.txt.

echo can be used to test the expansion of metacharacters before using them in adestructive command like rm.

$ echo ?o*

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Command Line Shortcuts (The Tab Key)

Type tab to complete the command lines. For the command names it will complete a

command name. For an argument, it will complete a file name.

Examples:

$ xte

$ xterm

$ ls myf

$ ls myfile.txt

Command Line Shortcuts ( History)

bash stores a history of commands youve entered, which can be used to repeatcommands. Use history commands to see the list of remembered commands.

$ history

In addition to basic command recall with the arrow keys, the bash history mechanismsupports a variety of advanced ways of retrieving commands from the history list.

!!- repeats the last command

!char- repeats last command that started with char

!num- repeats a command by its number in history output.

!?abc- repeats last command that contains abc

!-n- repeats a command entered n commands back.

Command Line Expansion (The tilde)

The tilde (~) expansion feature is borrowed from the C shell and makes it easy toreference files and directories inside your or someone elses home directory.

Tilde ( ~ )

May refer to your home directory.

$ cat ~/.bash_profile

May refer to another users home directory.

$ ls ~julie/public_html

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Command Line Expansion (Variable and string)

Parameter/Variable: $

Substitute the value of a variable in a command line

$ cd $HOME/public_html

Curly braces: {}

A string is created for every pattern inside the braces regardless if any file exists.

$ rm hello.{ c, o}

Curly braces expand to create string patterns they describe. Commas separate the variousstring patterns and an initial comma means that the null string is one of the requested

patterns. For example:

$ echo {a, b}

a b

$ echo x{a, b}

xa xb

$ cp file.txt{,-save}

Curly braces are useful for generating patterned strings. For example:

$ mkdir p work/{inbox,outbox,pending}/{normal,urgent,important}

$ ls work

inbox outbox pending

$ ls work/inbox

important normal urgent

Command Line Expansion (Command and Math)

Command output - `` or $( )

$ echo Hostname : `hostname`

Arithmetic - $[ ]

Substitute the result of arithmetic expression in a command line

$ echo Area : $[ $x * $y]

$ echo Area: `expr $x \* $y`

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Command Editing Tricks

Ctrl-a moves to beginning of line

Ctrl-e moves to end of line

Ctrl-u deletes to beginning of line

Ctrl-k deletes to end of line

Ctrl-arrow moves left or right by word

GNOME-terminal

Applications -> System Tools-> Terminal

Graphical terminal emulator that supports multiple tabbed shells

Ctrl-Shift-t creates a new tab

Ctrl-Pgup/PgDn switches to next/prev tab

Ctrl-Shift-c copies selected text

Ctrl-Shift-v pastes text to the prompt.

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CONFIGURING THE BASH SHELL

The shell is configured through a variety of mechanism:

Local Variables

Aliases and functions

The set and shopt command

The shell can also configure other commands or applications throughenvironment variables.

Variables

A variable is a label that has a value.

Set with VAR=VALUE

Used to configure the shell or other programs Two Types: local and environment

Local variables are used only by the shell

Environment variables are passed onto other commands

Display variables with set and env

set- displays all variables

env- displays environment variables

$ HI=hello

$ echo $HI

hello

The PS1 Local Variable

The PS1 variable sets the prompt. The prompt can vary each time it is displayed by usingspecial escaped sequences. These include:

\d the date

\h short hostname

\t the current time

\u user name

\ the current working directory

\! the history number of the current command

\$ - shows $ if you are a non-privileged user and a # if privileged user.

$ PS1=\u@\h:\ \$

digby@kennel: /tmp $

Aliases

Aliases let you create shortcuts to commands. Use alias by itself to see all set aliases. Usealias followed by a alias name to see alias value.

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$ alias c=clear

$ alias

$ c

The above created alias will be lost if you log out and log back in. To ensure that the newalias available each time the user logs in, type the following commands as given below:

$ vi .bashrc

Locate the line that contains the text: # User specific aliases and functions. Add your aliasto the line immediately below:

$ alias c=clear

Save the file and exit. Test your change by logging out, logging back in on the terminal.

Other Shell Configuration Command

Set and shopt are less common but powerful commands to configure elements of theshell. The set commands can configure many different aspects of the shell.

Examples:

set b : Reports termination of background commands immediately, rather thanwaiting for next prompt.

set u : Unsets variables generate an error

set o noclobber : Prevents overwriting of files with > and >& operators

set +o noclobber : Enables overwriting of files.

Common Environment Variables

HOME Path to users home directory

LANG Identification of default language rules to use

PWD Users current working directory

EDITOR Default editor programs should invoke for text editing LESS Options to pass to the less command

The TERM Environment Variable

TERM environment variable sets the terminal type

reset command used to reset a terminal should the screen become corrupted.

The PATH Environment Variable

The PATH environment variable is a colon separated list of locations wherecommands can be found

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hich command showing location in the PATH of an executable. The whichcommand searches through the directories listed in the PATH environmentvariable looking for a matching executable. When it finds one, it prints the path tothe executable.

$ echo $PATH

/usr/local/bin: /usr/bin: /bin: /usr/X11R6/bin: /home/digby/bin

$ which less

/usr/bin/less

Shell Startup Scripts

Most of the configuration items that we have seen in this unit are only valid in the

given shell. But typically, we will want settings to be established every time we starta shell, rather than having to type in all of our variables, aliases, and other commandson a per shell basis. To accomplish this, we use startup scripts, scripts that run when ashell is created. The system administrator sets up some startup scripts, but individualusers can control startup by editing scripts in their home directories.

Login Shell

A critical concept to understand when it comes to startup scripts in the idea of thelogin shell. A login shell is a shell that someone started by logging in the system. Anon-login shell is a shell started up in some other way, perhaps by a user or a

program issuing the bash command.

Startup Scripts: Order of Execution

Login shells

/etc/profile

/etc/profile.d

~/.bash_profile

~/.bashrc

/etc/bashrc

Non-login shells

~/.bashrc /etc/bashrc

/etc/profile.d

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/etc/profile

The /etc/profile shell script is the first startup script to run when a login shell isstarted. It only runs for login shells, non-login shells do not invoke this script.This script will set a series of variables including PATH, USER, LOGNAME,MAIL, HOSTNAME, HISTSIZE and INPUTRC. It will also run scripts in the/etc/profile.d directory.

/etc/profile.d

The /etc/profile.d directory contains additional setup shell scripts. Two copies ofeach script are listed in this directory, a Bourne shell style script with a .sh suffix,and a C shell style script with a .csh suffix. These do such things as set an alias for

ls so that it displays colors by default. The script in this directory are controlled bythe system administrator, they cannot be modified by non-privileged users.

~/.bash_profile and ~/.bashrc

The ~/.bash_profile and ~./bashrc files are controlled by the individual user. Theuser may put variables, aliases, or any other startup command in these files. The~/.bash_profile file is only called by login shells. The ~/.bashrc file is called by

both the login shells and the non-login shells.

~./bash_logout

The seldom used ~/.bash_logout file runs when a user logs out. Although mostusers do not use this file, it may be handy to do such housekeeping chores asmaking backup files, deleting temporary files or just displaying the logout dateand time.

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STRING PROCESSING USING REGULAR EXPRESSION

Regular expressions are a pattern matching engine. Regular expressions are as

abbreviated regex. Unlike the shells file name generation pattern matching system,regular expressions are designed for power over ease of use.Among the tools that use regular expressions are:

grep analyze the contents of files a line at a time, returning lines that match apattern.

sed, the stream editor returns the contents of a file or stream of data,performing the specified search and replace instruction.

less, this uses regular expressions in search commands.

vi, which uses regular expressions for searches or search and replace.

awk, a data oriented programming language.

Regular expressions are greedy. That is if a regular expression can match a smaller stringor a bigger string it will match the largest string possible. Regular expressions can bedivided into three broad categories: ildcard characters, modifiers and anchors.

Wildcard Characters

Wildcard characters stand for another single character.

. any single character

[abc123] any single character in the set.

[0-9] any single character in the range

[^abc123] any single character not in the set

[^0-9] any single character not in the set.

Modifiers

Modifiers determine the number of the previous character.

* Zero or more of the previous character.

\+ one or more of the previous character.

\? Zero or one of the previous character.

\ {i \} exactly i of the previous character.

\ {i, \} i or more of the previous character.

\ {i, j\} i to j of the previous character.

AnchorsAnchors match the beginning or end of a line or word.

^ the beginning of a line

$ the end of a line

\< the beginning of a word

\> the end of a word

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grep

Prints line of a file where a pattern is matched. Options used with the grep command are:

-v: return lines that do not contain the pattern.

-n: precede return lines with line number.

-c: only return a count of lines with the matching pattern.

-l: only return the name of files, starting with the named directory.

-r: performs a recursive search of files, starting with the named directory.

-i: performs a case in-sensitive search.

Examples:

$ grep john /etc/passwd.

$ ls | grep .c

$ grep ^g /etc/passwd

$ grep v ^g /etc/passwd

$ grep bash$ /etc/passwd

$ grep c fish /root/package

$ grep ^.$ /root/package

$ grep ^.{16 \}$ /root/package

Sed (Stream Editor)

Reads a file or stream of data, writes out the data, performing search and replaceinstruction. Uses regular expressions in search string.

Stream Editing with regular expression

Consider a file called cats containing the following words:

cat

catalog

concatenate

polecat

Cat

Now try out the following commands given below and see the output that they generate.

$ sed s/cat/dog/cats

$ sed s/[Cc]at/dog/ cats

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$ sed s/\/dog/ cats

$ sed s/\dog/ cats

$ sed s/\/& and dog/ cats

In the above written commands the cats file will sent to standard output with the stringcat being replaced by the string dog. By default, sed makes a maximum of onechange per line. To instruct sed to make multiple changes per line, the g commandstanding for global, should be appended at the end of the search and replace pattern.

$ sed s/[Cc]at/dog/g cats

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PROCESS

A Process can be defined as program in execution.

Creation of a process

Traditionally in UNIX like operating system process began through a process calledfork and exec. That is when one command starts another, the child process firstforks, the kernel copying over pages of memory from the parent process to a newlocation for the child process. The child than execs, executing the new commandoverwriting the data. For example when shell executes the ls command the kernelwould duplicate the shells pages of memory and then execute the ls command,overwriting the duplicate data. This was wasteful for system resources.

LINUX replaced this model for process initialization with three other methods, themost commonly used being copy-on-write. With copy-on-write, when one processstarts a subprocess, the original process does not duplicate its pages of memory whenit forks, but rather it points the child to the same pages in memory as the original

process, but making the memory area as read-only for both parent and child. Wheneither process wishes to write the data, as will happen when the child execs, a copy-on-write fault will occur and the kernel will create new pages in memory for that

process.

Manually execing Processes

It is also possible for a command to exec without forking. For example when logginginto the X window system, if the .xsession file exists, the .xsession file will executethe commands in a shell. The last command will be something like:

exec metacity

This means the shell in which the .xsession command was running will be replaced inmemory with the metacity command. Were the command to fork and exec, the shellthat ran .xsession would remain on the system performing no useful function until themetacity command exited.

Process Ancestry

init is the first process started at boot time- always has PID 1. Except init everyprocess has a parent. A process can be both parent and a child at the same time.

Viewing Process Ancestry

$ pstree

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Process States

The following process states are defined in Linux:

Runnable (R): Process is on the run queue. It is waiting for its turn to run or itis executing.

Sleeping (S): Process is not executing, nor it is ready to run. It is waiting foran event to occur or a signal to arrive to wake it up.

Stopped (T): Process is not executing because it has been stopped.

Uninterruptible Sleep (D): Process is sleeping and cannot be woken up untilan event occurs. It cannot be woken up by any signal. Typically the result ofan I/O operation.

Zombie (Z): Just before a process dies, it sends a signal to its parent and waitsfor an acknowledgement before terminating. Even if the parent process doesnot immediately acknowledge this signal, all resources except for the processidentity number (PID) are released. Zombie processes are cleared from thesystem during the next system reboot and do not adversely affect system

performance.

Viewing Processes

The ps command provides several styles of output depending on the options used.Without options it displays information about processes specific to the active terminal.Some of the options that can be used with ps command are:

-a: displays all processes, not including processes not controlled by aterminal.

-x: includes processes not controlled by a terminal, such as daemon processes.

-l: displays a long listing, which includes more information about a process.

-u: displays the user name of the process.

$ ps alx

Sending Signals to Processes

Signals can be specified by their name such as KILL, or by their number, such as 9. killcommand can send many signals, but processes only respond to the signals they havebeen programmed to recognize. Default signal is TERM.

Syntax:

kill[signal] pid(s)

kill[signal] %jobid

Example:

$ kill 3428

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$ kill -9 3428

$ kill TERM 3428

$ kill KILL 3428

In addition to kill there is a killall command that can be used to send a signal to a groupof commands, such as all getty processes. It should be used with caution since killingsome processes may have a detrimental effect on the system. Heres an example thatsends a KILL signal to all running processes named galleon-bin:

$ killall KILL galleon-bin

Terminating Processes

Processes ordinarily terminate on their own when they have completed their task.Interactive applications may need the user to issue a quit command. Many processes canterminated with Ctrl-c, which sends an interrupt (INT) signal to the process. The processis shutdown cleanly, that is child processes are terminate at first and any pending I/Ooperations are completed. The same is true if a process is sent to terminate (TERM)signal via the kill command. If a process will not respond to a TERM signal, the KILLsignal can be used. However, the processes may not be ended cleanly. The KILL signalshould be used only if a process will not respond to a Ctrl-c or TERM signal. UsingKILL signal on a routine basis may cause zombie processes and lost data.

Altering Process Scheduling Priority

Every running process has a scheduling priority. A ranking among running processesdetermining which should get the attention of the processor. The formula for calculatingthis priority is complex, but users can affect the priority by setting the niceness value,one element of this complex formula. The niceness value is a number ranging from -20(highest priority) to 19 (lowest priority). It defaults to a value zero.

Syntax:

nice n priority programname

Non-privileged users may not set niceness value to less than zero, that is they may notrequest a higher than normal priority for their processes. This is a function reserved forthe superuser.

Altering priorities of running programs

Syntax:

renice priority p PID

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$ renice -15 p 3426

The p option is not strictly necessary, however if u wish to change the priority of an

entire process group, use the g option. Also useful if the u option, which can be used tomodify the priority of all of the processes of a particular user:

$ renice 15 u student

Interactive Process Management Tools

Display real-time process information

Allowing sorting, killing and renicening

Command-line: top

GUI: gnome-system-monitor

Running a Process in the Foreground

When a process is started from a terminals command-line, it is normally running in theforeground. The process can be stopped, restarted in the background, or terminated. Thisis called ob-control.

Running a Process in the Background

Running a command in the background allows another process to run concurrently on thesame terminal. Launch a program as a background process by appending an &(ampersand to the end of a command.

$ firefox &

When a process is started in the background a new bash subshell is created. The bashprogram is then replaced with the command being executed. Background processes canbe managed like any other process.

Suspending a Process

Foreground jobs can be suspended temporarily halted without being killed. Suspend aforeground process with Ctrl-z. Suspended jobs can be resumed in the foreground (fg)and can also be resumed in the background (bg).

Listing Background and Suspended Jobs

Jobs display all processes running in the background or that are suspended.

$ jobs

[1]+ Stopped man bash

[2]- Running find / -name joe > output 2>&1 &

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The number in the brackets is a job number, used to kill jobs or bring them back to theforeground. Job numbers are referred with %. The plus or minus sign next to the jobnumber tells which job is the default, that is the job that is referenced if no job number is

given.

When a command is suspended or backgrounded it can be brought back to the foregroundwith fg. Suspended jobs can be resumed in the background with bg.

Syntax:

fg %jobnumber

bg %jobnumber

Scheduling Periodic Processes