basiclinuxinfo

7/27/2019 BasicLinuxInfo

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SYSTEM ORGANISATION

The functioning of Linux is manned in three levels

1. Tools and applications2. Shell

. erne

Tools and application programs which speak our

language.(Programming language )

Kernel interacts with actual hardware in machine

language.

e s e , or e comman n erpre er as s ca e , sthe mediator, which interprets the commands that we

ive and then conve s them to the kernel which

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ultimately executes them.


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Kernel Shell Relationship

Other

A lication

UNIX

Commands

User

System SW

Database Compilers

Hardware

Kernel

Internet Tools

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Theshellis an ordinary program which interprets the commands

that are typed at the terminal, andtranslates them into commandsthat are understood by the kernel.

system directories, such as/bin, /etc, or /usr/bin.

Theprocess of command interpretationby the shell can be

explained as follows :

reads the command

searc es or an ocates t e e w t t at name n t edirectories containing utilities.

loads the utilit into memor

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executes the utility


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The Linux based shells in the freely available distributions are:Bash

as s an acronym or ourne ga n e . s an

enhancements to the Bourne shell and is the default shell for most

Linux systems. In Red Hat Linux, sh is the symbolic link for the

Bash shell which is stored under the /bin directory.

Pdksh

Pdksh stands for Public Domain Korn Shell and is an

enhancement of the Korn shell. On Linux systems, ksh is the

symbolic link to the Pdksh shell.

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.

Tcsh

Tcsh stands for Toms C shell also known as the TC shell

and an enhancement of the C shell. The symbolic link

.

ASH

s usua y su a e on mac nes a ave very m e

memory.

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LINUX COMMANDS

Remembera few things that apply to all Linuxcommands

nux comman s s ou a ways e en ere n

small case letters (case sensitive).

Between the command name and the o tions that

may be available with the command there must

always be a space or a tab. The option is

- . Two or more options available with a command

can usually be combined.

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SYSTEM?

When you try to access your system, Linux will display a

prompt that looks something like this:

Login: perfect

Password: *****

Once the correct login name and password have been

supplied, you find some welcome messages from the suppliers

of the Linux version installed on the host machine, followed bya command prompt. The command prompt is a $(dollar) if you

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The passwd command is used to change passwords.

passw

Current password:******

New password:*******

Retype New password:********

The systemdoes not let you choose the old password, nor

can the password be wholly alphabetic or wholly numeric.

It is desirable that password contains at least two alphabetic

characters and at least one numeric or special character within

the first eight characters.

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The man command displays pages from the Linux

reference manual that is installed alon with the Linux

OS.

To get detailed information about the ls command, you

can use e o owng comman .

$ man ls

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FILE COMMANDS

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DIRECTORY PATH

The pwd (print working directory) command is used todisplay the full path name of the current directory.

Example:

$ pwd/home/perfect

$ _

/home/perfect is the directory in which the user is

currently working.

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. ..

Linux adopts a widely used convention . (a single period)stands for the current directory while . . (a double period)

Whenever you create any sub-directory, these invisible

directories are created automatically by the system.ou can remove ese spec a rec ores, nor can you wr e

into them.

They provide a shorthand notation for referring to the current

and parent directories.

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DIRECTORY

The cd (change directory) command changes the currentdirectory to the directory specified.

c a so c r n some s e s c ange rec ory

cd changes to user's home directory

'

cd .. goes up one directory level

cd ../.. goes up two directory levels

cd /full/path/name/from/root changes directory to absolute

path named (note the leading slash)

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The mkdir(make directory) command is used to createdirectories.

S ntaxmkdir[options] directory

Common Options

,

-m mode access permissions.

Examples

if your present working directory is /home/frank the followingwould be equivalent:

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The rmdir(remove directory) command removes thedirectory specified.

yn ax

rmdirdirectory

Exam les

To remove the empty directory /home/frank/data while in

/home/frank use:

or

$ rmdir /home/frank/data

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DIRECTORY

The ls command is used to display the names of the files

and sub-directories in a directory.

S ntax

ls [options] [argument]

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O tions of ls Options

-a lists all files, including those beginning with a dot

(.)

- , , ,

size, lastmodification (time).

-rWe can reverse the order of the presentation. -F indicates type o entry with a trailing symbol:

9 * executable

9

9Nothing ordinary file

-R Recursively itwill search for the particular file.

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specified file.

What it will doCommand/Syntax

cat [options] file concatenate (list) a file

more (or less or pg)[options] file

page through a text file

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e ca comman can e use o ver ca y conca ena e e

contents of more than one file.

Example: $ cat data1 data2

A sample fileanother sample file$ -

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Common Options

-i interactive rom tand wait for confirmation before

proceeding)

-

Complete path names can be specified with the cp

.

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The rm( remove) command is used to delete files or

directories.

Syntax

Example

.

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Common Options

9 -i interactive (prompt and wait for confirmation

9 -r recursively remove a directory, first removing the

files and subdirectories beneath it

9 - -

If the file to be deleted is not located in the current or

,

given.

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The mv command is used to move a file or directory fromone location to another or to change the name of a file or

directory.

Syntax

mv [options] old_filename new_filename

Examples

$ mv old_filename new_filename

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MOVING AND RENAMING FILES

Common Options

-i interactive rom tand wait for confirmation before

proceeding)

-fdont prompt, even when copying over an existing

target file (overrides -i)

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MULTIPLE FILES

Large files are sometimes impossible to edit with aneditor, and need to be split up.

$ split filename

What split does is that it creates a group of filesxaa,xab,xac and so on that could go up to xaz, and then

again from xba,..xbz. In this way you can have

* , .

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MULTIPLE FILES

Split by default break up the file 1000 lines per file.$ split 72 data1.txt

Contains 72 lines per file.

We can even select our own primary filename by

specifying it as the last argument in the command line

$ split chap small

This creates files smallaa, smallab,smallac, etc.,

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SIDE BY SIDE

The paste command takes two tables and combines them,side by side, wide table as output.

By default paste assumes a tab as the delimiter. Any other

delimiter must be specified.

Thed option specifies the delimiter that must be inserted.

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aste COMMAND

Example :

$ paste d$: file1 file2 file3

.

of file two and add : to it.It returns to perform the same

function on all the lines of the file, till the end of file. The paste utility expands a tables width, increasing the

number of columns. The cat utility expands a tables length,

.

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KNOWING FILE TYPES

Even though there are basically three types of files

(ordinary,directory, and device) we may often need to

. ,

may contain plain text, a C program or executable

code.$ file emp.lst

emp.lst : ascii text

progs directory,

$ file progs/*

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The nl command is used to add line numbers to a file.e ne num ers recor s t e re atve pos ton o a ne n

a file. Example :

.

1. #include

2.int main().

4. Printf(\nHai);

5. Return 0;

.

Thei option is used to specify the value, other than thedefault 1, by which line numbers must be incremented.

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e may requre o en er a num er o enames n e

command line. Example:

$ ls -l chap chap01 chap02 chap03 chap04 chap05

The above listing all files beginning with chap.

string(chap), the lengthy command line, using this

repeatedly, doesnt look elegant.

We can have a single pattern consisting of the stringchap, along with one or two special characters.

The shell does this throu h wildcard characters.

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The shell offers the facility to perform an operation on a setof files without having to specify all the names of the files

on which the operation is to be performed.

This is made possible by the use of certain special

characters in the command in place of the actual file

names.

w car c arac ers.

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The followin are the lists of wildcards available, with a

description Table. * -- Matches none or one character

or a string more than one

character.

-- .

[ ] -- Matches exactly one of a

s ecified setof characters.

Using this, the above example can be solved by as follows

$ ls l chap*

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Examples:

COMMAND OUTPUT

$ ls a* a ab abcd ac abc abcde

$ ls a? ab ac

Suppose there are following files in present in workingdirectory:

$ ls a? No output

$ ls a* art

$ ls *.* No such file and directory$ ls [!abc]art part tart

$ ls [b-d u-z]* art art1 art2 art3

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FILE ACCESS PERMISSIONS

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Determining the FAPs for a file

e ccess ermss ons re er o e permss ons assoc a e

with a file with respect to owner, group and other users of

the file.

available for files and directories.

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Access Denoted Action Action permitted on

file

read r Allows you todis la co t and

Allows you to list thecontents of the

compile the file directory.

write w Allows you to

edit, rename or

Allows you to create a

new file andmove the file toanother location

subdirectory withinthis directory.

execute x Allows you to Allows you to move toexecu e e eprovided the filealso has the readermission

e rec ory us ng ecd command.Todelete a file executeermission is re uired

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in the directory.


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PERMISSIONS

The access permissions associated with a file ordirectory can be changed using the chmod command.

Only the owner of a file can change the permissions

associated with it. Syntax:

chmod mode file/s

mode can be a symbolic or obsolute way.

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..

Symbolic mode :

r- read w - write x - execute

- - -

- means remove

+ means grant

= =read and write)

Example :

- .

Here g means group - means read permission has

been removed.

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ABSOLUTE MODE

The absolute mode of specifying FAPs uses a

series of digits to represent the actual permissions.

rea -- 4

write -- 2

execute -- 1

The figure below depicts a sample representation of

absolute numbers.

r w x r w - r - -7 6 4

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.

Example :$ chmod 764 data1.C

It means file access permission has been changedby the command chmod read, write, execute

permissions for the user, read and write for the

group owner, and only read for other users

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nux un ers an s e mpor ance o e secur y an

then sets up certain default permissions for the files youcreate.

nux uses e vara e ca e umas o ec e edefault permissions.

The current value ofumask can be easily determined byus ypng umas

$ umask022

The default permissions for all files should be 666.

The default permissions for all directories are 777.

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LINK FILE

When a file is copied, both the original and copy occupyseparate space in the disk.

Linux allows a file to have more than one name, and yet

maintain a sin le co in the disk. Chan es to one file

are also reflected in the other.

,

it has more than one name.

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we acc en a y e e e a e w a s nge n or a s nge

name there is no bringing it back as Linux has no fileundelete facility.

one e as or more n s, our e s sa e even one othe links gets deleted.

If one file is to be shared between several users insteado gvng eac user a separa e copy o e same e wecan create links of this file in each users directory. Thisavoids unnecessary duplication of the same file contents

.

The file is physically not removed until at least one linkis pointing to the file.

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LINK

There are two types of links available in Linux

1. Hard link

. ym o c n

Hard Link

Hard links are special files that allow a single file tohave multiple names. You can have a hard link file

only for a file and not for directory. These Links are

known as hard links because the create a direct link

to an inode.$ ln data1.dat # by default hard link

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Limitation of Hard link

9 We cant have two linked files in two file systems. Inother words if we have a file in the ext2 file s stemwe cant link this file to another file in the /home filesystem.

9 We cant l ink a director even within the same filesystem.

Unlike Hard Links, a symbolic link points to the filewhich actuall has the contents.

We can create a symbolic link withs option of ln.9 ln s data lndata

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Hard l inks can be created only for fi les not for

directories. Symbolic link can be created for both.

ym o c n can wor across eren e sys ems.

But Hard link can not work across different file systems.

If it is a S mbolic link, whenever we do ls l it tells it is alinked file. But if we do Hard link it tells it is a ordinary

file.

sym o c ne e can no e use a er e e ng e

original file whereas a hard linked file can be used after

deleting the original file.

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It recursively examines a directory tree to look for files

matching some criteria, and then takes some action on

t e seecte es.

#find / -name .bash_profile print

from the root directory.

The last section s ecifies the action rint to be taken

on the file.

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FINDING UNUSED FILES

#find . -mtime -2 -print

This command searches files from the current directory,

, , .

-n less than n

+n more than n n exactly equal to n

find uses thesize operator to locate large files.

# find /home size +2048 size 8192 -print

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COMPARING FILES

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COMPARING TWO FILES

We require to know whether two files are identical so

that one of them can be deleted.

$ cmp chap01 chap02

Chap01 chap02 differ: char9, line1

The two files are compared byte by byte and the

the first line) is echoed on the screen.

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COMPARING TWO FILES

The l option :

-l -- gives a detailed list of the byte number and differing

Bytes in octal for each character that differs in both files.

$ cmp l note1 note2

3 143 145

5 178 190

9 172 175

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The comm command is used to give more information on

file differences.The command compares two sorted files

and generates a three-column output.

1st

column -- lists the lines that appear only in the firstfile.

2nd column lists lines that appear only in the second

file.

3rd column -- lists lines that appear in both files.

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Example :

$ cat file1 $ cat file2 $ comm file1 file2apple grape apple

mango mango grape

orange pear mango

ear rose oran epear

rose

comm 12 file1 file2 #will dis la onl column 3

mangopear

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diff COMMAND

The diffcommand is another file-comparison

command. It is used to compare two versions of the

same file.

The output is a list of editing actions that must be

added to the first file in order to generate the second

edited file.

y u , , ,

changing a line, and so on.

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Example

$ diff file1 file2

0a1 # Append after line 0 of firstfile

>Command options # this line

2c3,4 # Change line 2 in first file

options of file # these two

> list of functions # lines

4d5 # Delete 4 of first file< in the chapter. # containing this line

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There may be instances where the lines in a file are

duplicated.

The duplicates must be removed. The uniq command

.

A file must be sorted before using the uniq command.

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COMMAND

Options :

-u outputs only those lines that are not repeated.

-d outputs only one copy of each repeated line.

-c generates output in default style but with each

line receded b a countof the number of itemsit occurred.

-n the first n fields together with any blanks beforeeach are i nored

+n the first n characters are ignored.

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OTHER LINUX COMMANDS


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OTHER LINUX COMMANDS

date command shows the date and time to the

nearest second

Fri May 16 08:25:53 IST 2002

clear

screen.

who

who command is used to report who are all logged

in, terminal name and what time they logged in.

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PIPES AND FILTERS

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STANDARD FILES


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STANDARD FILES

A computer consists of mainly three parts, the input,

processing, and output devices.

Linux treats all input and output devices as special files.

These files are categorized as, Standard Input Files

Standard Out ut Files

Standard Error Files

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STANDARD In ut Files


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STANDARD In ut Files

Standard Input Files

In Linux when a user executes a command that

requires input, the shell interprets the command and

.

The keyboard is referred to as the standard input file.

,

input files

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STANDARD OUTPUT FILES


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STANDARD OUTPUT FILES

Standard Output File

for the output of any command that it executes. The

monitor is referred to as the standard output file. The file

descriptor 1, is assigned to the standard output file.

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STANDARD ERROR FILES


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STANDARD ERROR FILES

Standard Error File

Shell utilities display error messages on the monitor. The

descriptor 2, is assigned to the standard error file.

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Redirection changes the assignments for the standard

input, standard output, and the standard error.

The three types of redirection are:

Input redirection Output redirection

Error redirection

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INPUT REDIRECTION


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INPUT REDIRECTION

The following example illustrates the usage of input

redirection:

Here, the less than symbol,


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The following example illustates the usage of output

redirection:

Here, the greater than symbol, >, implies redirection of

output to the file, test2. The output of the cat command is

wr en o a s e, es .

However, if the file already exists, its contents are deleted

before the output is written to it.

If you want to append the output to the file, test2, thecommand is:

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ERROR REDIRECTION

The following example illustrates the usage of error

redirection:cat test 2>erro -mes

Assume that the file, test, does not exist in the currentdirectory. So a user tries to execute this command, Linux

will enerate an error messa e since the execution isunsuccessful.

This error message will be written to the file named error-mes .

Unlike the file descriptors for the standard input andstandard output, the file descriptor for the standard errorfile 2 is com ulsor .

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FILTERS

A filteris a program that takes its input from the

standard input file, processes( or filter) it, and sends its

The grep filter

The grep stands for globally search for regular

.

Syntax

grep regular_expression [filename]

regular_expression which has to be searched in the

file. file name is optional ,without it grep expectsstandard input.

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rep In the grep command regular expressions are always given in

.

Example:

grep New[abc] Matches any one of a set of

.

grep New[a-c] Matches any one of a range of

characters.

grep ew a c e pa ern o owng mus occur athe beginning of each line.

grep New[^a-c] The pattern must not contain any

.

grep New[abc]$ The pattern preceding it must occur atthe end of each line.

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grep ew. a c a c es any one c arac er.


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The grep filter also has options that alter the output of

the command. These are : - ,

with its line number. The number is printed at the

beginning of the line.

-c s prn s ony a coun o e nes a ma c apattern.

-v This prints out all the lines that do not match the

pattern specified by the regular expressions.

Options must be specified before the regular expression.

O tions can also be combined.

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wc The wc filter is used to count the number of lines, words,

and characters in a disk file or in the standard input.

Syntaxwc [-lwc] [filename/s]

Example:

$ wc test

The file test has 2 lines, 7 words, 9 characters Options :

-l -- Displays the number of lines

-w -- Displays the number of words -c -- Displays the number of characters

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cut FILTER

The cut filter is useful when specific columns from the

output of certain commands(such as ls, who) need to be

Syntax

cut [options][filename/s]

-f -- Displays the specified

columns

- _ --

characters. -d--Specifies the column delimiter

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cut FILTER

Example :

Here the cut command has been used to extract only

the names of the users that exist in the /etc/passwd file.

The field separator is a :.

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tr

The tr filter can be used to translate one set of characters

to another. his filter can also be used to s ueeze re eated

occurrences of a character into one.

Thes option is used to squeeze several occurrences

of a character into one character.

Example:tr s < tem orar

Here tr command takes input from temporary file and soption of tr works on every record, squeezing repeatedspaces into a single space.

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tr FILTER

$ tr : < /etc/passwd

character with a space, for the file /etc/passwd and

displays the output on the standard output.

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sort FILTER

The sort filter arranges each line of the standard input

in ascending order. S ntax :

sort [options] [filename]if filename is not there, it will take the input from a

standard input file. Options : -r -- This sorts the input in the reverse order. -f -- It arranges in the order of the ascii values of

the characters.

-n -- Arranges the input in ASCII sequence,numbers would also be arranged accordingly.

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sort

Example :

sort n12239121 d

Output:12121

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SORTED ORDER

Specify the file name along with sort.

Example :

$ sort names # similar to sort < names>

The above command will display the lines in the file in

sorted order.

$ sort newnames oldnames

The above command will display the lines in the files,

newname and oldnames in a sorted order.

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SORTED ORDER

The +pos1 pos2 Option:

In certain cases, a user may want to display each line in afile in the order of a specific column, and not in the usual

sort order. By default, sort assumes that the columns are

separated by a space or a

o sorta file on a column the osition of the column in thefile has to be specified. This is done as follows: The number of column separators (space or < Tab) that

have to be ignored to reach that column must be,

number. The column separator at which the sort filter has to stopmust be s ecified as os2, where os2 is the

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appropriate number.


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SORTED ORDER

For example, to specify sorting on the third column,

the user will have to specify +2 and 3.

$ sort +2 3 names

To sort the file, names, on the third column, within

which on the fourth column, the following command

can be used

$ sort +2 3 +3 4 names

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COMMAND

Combination of Options :

+4n 5 -- Numeric sort on

the fifth column

-rn or nr -- Combination of r and n

nr or rn -- everse numerc sor on

the fifth column.

-fr or -- Sortin in the reverse

order of ascii characters

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Commands can be combined in such a way that

the standard output of one command can be sent

pipes.

Example :

$ ls > tempfile

$ more tempfile

by using pipes.$ ls | more

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The following command will display the lines

containing #inlcude

$ ls l | grep #include text.*

To display the names of all ordinary files in the

current directory, the command will be,

$ ls l | grep - | tr s | cut d f9

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ee

The intermediate output in a pipe is not saved on the

disk. The intermediate output can be saved in a file using

this tee command.

If the file already exists, the contents of the file are

overwritten. The -a option can be used to append the

new contents to an existing file.

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Example :

$ cat temp | tee temp1 temp2,

temp, on the screen, and also writes to the files temp1 and

temp2

$ sort numbers | tee sort-numbers | more

The above command, the file, numbers is sorted, data is

-,

numbers, and stores the data in this file. It also sends thedata as input to the more utility.

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The ps command is used to display the attributes of a

process.,

user at that terminal:

$ ps

PID TTY TIME CMD

476 tty03 00:00:01 login

659 tty03 00:00:01 sh

684 tty03 00:00:00 ps

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OPTIONS OF


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OPTIONS OF ps

The options:

a Lists out the processes of all users, but doesnt

display the system processes.

-u We can know the activities of a particular user.

-e Lists out all the system processes.

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A multi-tasking system lets a user do more than one job at a

time. Since there can be only one job in the foreground, the.

The & is the shells operator used to run a process in the

background.

J ust terminate the command line with an &, the command

will run in the background:

sor o emp. s emp. s

The shell immediately returns a number the PID of theinvoked command.

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no up :


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no up :

The nohup command, when prefixed to a command,

permits execution of the process even after user has.

In the following command, no output filename was

specified, nohup sends the default output to a file

nohup.out

$ nohup sort emp.lst &

Sending output to nohup.out

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no up :


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no up :

When you use ps command after using nohup you will

notice something very significant:

$ ps u per

UID PID PPID STIME TTY TIME COMMAND

per 101 1 14:52:08 03 0:13 sort emp.lst

sh died on logging out, but not its child(sort). What the

kernel has done here is to reassign the PPID of the sort

process that has a PID1.

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A PROCESS

Kill command is used to terminate process.

.

$ kil l 121 125 132

,simply kill the parent in order to kill all its children.

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NUMBERS


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NUMBERS

The system variable $! stores the PID of the last

background$ sort o emp.lst emp.lst &345

$ kill $!# kills the SORT command

Kill, by default uses the signal number 15 to Terminatethe process.

The process can be killed with signal number 9,sometimes known as the sure kill signal.

$ kill 9 121#kills process 121 with signal number 9

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PRIORITY


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PRIORITY

Processes in the Linux system are usually executed with

equal priori ty.,

operator to reduce the priority of jobs.

To run a job with a low priority, the command name

s ou e pre xe w nce:$nice wc l file1 &

Commands runs with a nice value of 20 in the Bash

shell.

$ nice n 15 wc l file1#Nice value becomes 35

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OTHER UTILITIES


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When we invoke bc (calculator) withoutarguments, the input has to be keyed in each lineterminated by pressing . After you have

finished your work use .$ bc12 + 517ibase=211001010202 # output in decimal, I.e., base 10

# To come out of calculator $ factor

7

2

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The head command is used to display the first linesof the file. The user can specify the number of lines

to be displayed by using the n argument.

The default number of lines that are displayed is 10.$ head 2 dolls

The tail command, as the name suggests is used todisplay the last lines of a file.

The de ault number o lines that are displayed is

again 10.$ tail -4 dolls

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v


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v

A vi session begins by invoking the command vi with (or

without) a filename: When we o en a file with vi the cursor is ositioned at

the top left-hand corner of the screen. We are said to bein the command mode.

o enter text ou have to leave the command mode andenter the input mode.

We have to save our file or switch to editing another file.Commands can be entered in the last line of the screento act on text. This mode is known as ex mode or line

mode.

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After opening the file, the user is in command mode, if

you press

i insert

r or R replace

you wan o sw c ac o comman mo e or ex mo efrom insert mode press esc key.

, .

This text goes into a place in memory known as buffer.

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COMMAND FUNCTION

zz Writes the buffer to the

file and quits vi

:wq Writes the buffer to the

file and quits vi

:w fi le name Writes the buffer to file

and :q filename(new) and quits vi

:w! fi lename overwrites the existing file

and :q filename with the contents

of the buffer and quits vi

:q! Quits vi whether or not changesmade to the buffer were written

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to a file.


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o e a e o use oc comman s we mus remem er wothings:

1. All the block commands work in ex command.

2. Line numbers should be associated with the textbefore we issue any block commands.

To set the line numbers in the ex mode t e set number andpress enter.

:nd Deletes nth line

:m,n,d Deletes lines from m to n

:m,n,w>>filename Appends lines m to n to a file

:r filename Reads the contents of thefilename at current cursor

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position.

SEARCH TEXT OR PATTERN IN vi


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SEARCH TEXT OR PATTERN IN vi

vi is equipped with a mechanism to search strings in a

file.Command function

/Pattern searches forward in the bufferfor the next occurrence of theattern of text.

?Pattern searches backward in thebuffer for the first occurrenceof the attern of text.

n repeats the last search

N repeats the search commandin the o osite direction

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,


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,

If you want to delete a particular l ine, keep the

cursorin the corresponding line and press

If you want to delete 4 lines from the file, keep the

cursor in the first line and press 4dd.

an means w ou e e ng , v copes eseobjects into buffers without removing them from the

file.

For pasting, keep the cursor below in the line which

you want to have the copied buffer, and press p.( ornp)

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SYSTEM ADMINISTRATION

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The startup sequence begins when you turn on the

power. he boot ro ram checks for the existence and inte rit

of any hardware on the system. The program thendetermines which partition of the disk is active and bootsthe UNIX kernel.

The termbooting means taking a copy of the kernelprogram on the hard disk and placing it inmemory(RAM), where it can function.

The kernel then starts spawning further processes, the

most important of which is init. This process always hasthe PID 1.

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init is responsible for keeping the system indifferent states, also called runlevels. Each run level

is normally a single digit(0 to 6). Normally thesystem wou e n any one o t ese run eve s:

0 System shutdown

1 System administration mode

2 Full multi user state

3 Network multiuser

5 Shut down same as 0

6 Shutdown and reboot mode

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shutdown script to bring the system down.

File systems are unmounted, processes are killedand messages are broadcast to users with wall

command.

level to determine whether the system is to be placed

in single user mode, rebooted or simply powered

own.

By default this state is 0, which means a totalshutdown.

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The user can also become a super user status with the su

command if she knows the root password. For example,

$ su

password: ********

# pwd

/home/local # prompt changes, but directory

The # prompt indicates that the user is a super user.

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The super user can invoke the date command with a

numeric argument to set the system date. This argument is usually an eight-character string of the

formMMDDhhmm and optionally followed by a two digit

.# date 09180934

Thu Se 18 09:34:00 IST 1997

The system date can be set only by the administrator.

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df (disk free) reports the amount of free space available on

the disk. The output always reports for each file systemseparate y:

# df

-/home (/dev/home ): 1448 blocks 382 i-nodes

Thet o tion includes the above out ut, as well as the total

amount of disk space in the file system.

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The du (disk usage) command reports usage by a

recursive examination of the directory tree.s s ow u s s e usage o ome sa es per ec

# du /home/sale/perfect

534 /home/sale/perfect/data

1234 /home/sale/perfect/reports

By default, du lists the usage of each sub-directory of

its argument, and finally produces a summary.

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Compress is quite fast and produces .z file.

$ compress file1.txtuncompress e1.txt.z

To view a compressed file, the zcat command can be

. . gzip command is comparatively slow but remarkably

efficient. The compressed files have the extension .gz.

$ gzip sales.dbf # produces sales.dbf.gz

$ gunzip sales.dbf.gz

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CHANGING ANY PASSWORD


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The passwd command is used by a user to change thepassword, the old password is first prompted for, and thenthe new one accepted.

However, when you invoke the command as a super user,the system behaves in a more lenient and helpful manner:

# asswd

INFO: Changing password for root

New password: ********

-

To change the particular user password,

#passwd perfect

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dd was extensively used in copying file systems but today


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dd was extensively used in copying file systems, but today

its role is mainly restricted to copying media, like floppies and

tapes.= ev o = emp s= cn =

The keywords are if=(input filename), of=(output filename)

and bs=(block size).

The above command copies a 1.44MB floppy to a temporary

file temp, using a block size of 147456, which is exactly one-

y . .

dd only uses raw divides, ie., those in /dev/rdsk or those filesin /dev beginning with r, like /dev/rfd0135ds18, or /dev/rct0

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HANDLING DOS DISKETTES


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It has now become quite common to see both DOS andUNIX systems on the desktop.Unix provides a family ofcommands with which one can read from and write ontofloppy diskettes.

mcopy - Copies files from or to from1.44MB floppy drive

mdir - Lists files in DOS diskette inDOS style

mt e - Concatenates files in DOSdiskette

mdel - Deletes file on DOSdiskette

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DISKETTES & BACKUPS


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Before you use a floppy for backup purposes, we need to

format it first. This is done with the fdformat with the raw

# fdformat /dev/fd0H1440

The two most popular programs for backups are cpio and

ar. The cpio command can be used to copy files to and from a

backup device. It uses standard input to take the list of

filenames, and then copies them with their contents and

headers, into a single archive that is written to the standardoutput.

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c io COMMAND


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Backing Up Files

-o option is used to create an archive, which can then beredirected to a device file. This is how ou co files in thecurrent directory to a 1.44MB floppy

# ls | cpio ov > /dev/fd0

A complete archive or selected files can be restored withthei key option. To restore all the files that were backed

operator(


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tarnot only creates archives on tapes, but supports floppies aswell .

Exclusive features

- It doesnt use standard input to obtain its file list.taraccepts file and directory names as arguments.

- , . .,it operates recursively.

- It can create several versions of the same file in a.

- It can append to an archive without overwritingthe entire archive.

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FILES


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ac ng up es

taraccepts directory and filenames directly on the command

line.* .

The verbose option(-v) shows the number of blocks used byeach file.

.as high a value as the system permits. Higher block sizesspeed up I/O operations.

Restorin files

Files are restored with x(extract) option. When no file or

directory name is specified, it restores all files from the backupdevice.

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# tar xvf /dev/fd0 18

fdisk IN LINUX


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fdisk in Linux creates both partitions as well as filesystems.

# fdisk

The options:

-d delete a partition

-m print this menu

-n add a new partition

-p print the partition table

-q quit without saving changes-w write table to disk and exit

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FILE SYSTEM MOUNTING


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The interesting thing about a file system is that, once

created, it is logically separate entity with its own tree

.

These files systems unite to become a single file system at

.

The root file system becomes the main file system, and its

root director is also the director of the unified s stem.

This happens by aprocess

known asmounting

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mount COMMAND


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. ., ,

/mnt) must first be made available in the main file system.

/dev/fd0 loses its separate identity. It now becomes the

The type of fi le system is specified with thet option. If

system should be vfat filesystem.

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Unmounting is achieved with the umount

command, which requires either the file system or

# umount /mnt/floppy

# umount /dev/fd0

You cant remove a directory unless you are placedin a directory above it, you cant unmounta file

s stemunless ou are laced above it. If ou tr to

do that, this is what you will see :

#umount /dev/fd0 busy : Device busy.

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Almost everyday the administrator is required to

add or delete a user or group from the system.e comman o a e user s ,

# useradd

Password can be set for the user b the root b

giving the command,

# passwd

e comman o e e e e user s,

# userdel

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RPM (Redhat Package Manager)


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Open packaging system

UNIX systems .

Features include - installing, uninstalling,upgrading, querying, verifying all in one

line commands .

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Installing

# rpm -ivh pkg- 1.0- 2.i386. rpm If r m ives an error sa in acka e alread installed

and still want to install the package, use -- replacepkgsi.e.,# rpm -ivh --replacepkgs rpm_name

Similarly use --replacefiles for conflicting files use --nodeps to install package with dependency conflicts

Uninstallingrpm pkgname // not the rpm ile name

use --nodeps to uninstall package with dependencyconflicts.

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Print commands allows us to print files to standardou pu or o a ne prn er


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ou pu or o a ne prn er.

Following are the commands to submit, cancel, and

check the status of a print job.

Command/Syntax What it will do

jobs

lpr(lp) [options] file print to defined printer

lprm (cancel) [options] remove a print job from the

print queue.

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Shell Programming

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USER DEFINED VARIABLES

The variable length can be of any reasonable length andmay cons u e a p a e s, g s, an un erscores. owever


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may cons u e a p a e s, g s, an un erscores. oweverthe first character must be an alphabet or an underscore.

$ a=20$ echo a $ echo $a

a 20

$ str=Two words $ echo Enter our no

$ echo $str $ read noTwo words

,expr before the operation. For ex:

$ expr $a + $b `

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.

Like C program shell program also takes command line.


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$1 $2 $3 $4 $9 we represent command line

arguments like this in the program.-- o o argumen s

$$ -- pid of the current process

$0 -- Name of the shell program

$* -- List of all the command line arguments.$? -- exit status of the last executed command

Ex:vi tem

echo $# $3 $0 $$ $1

When we execute temp, ./temp hello how are you

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are emp e o

CONDITIONAL STATEMENTS

Ifcondition1 # if conditions is TRUE=0

en


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cmd list-1

elif condition2; thencmd list-2

else

cmd list-n

fi Ex: if [ $# -ge 2 ]

then

echo more than two arguments

elseecho no input

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fi

LOOPING(for)

case param1 in


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case param1 inpattern1) cmd-list1;;

pattern2) cmd-list2;;* e cm st;;

esac

forvar [ in list of vals]

o cmd-listdone

.do

cp $file new$file

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LOOPING (while)

While [ codition ]

do


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do

cmd-list

[ continue ]

done

x:while [ $# -gt 0 ]

do

echo $1

shift # $#=$# -1

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LOOPING(until)

until loop executes till the conditions becomes false..

til [ diti ]


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until [ condition ]

do -

[ break ]

[ continue ]

one Ex:

until [ $# -le 0 ]

do

echo $1shift

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done


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PROGRAMMING TOOLS

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Linux software development perhaps this is because the

Linux s stem was ori inall develo ed in C.

The GNU C compiler, gcc, is one of the most versatile and

advanced compilers around.

Unlike the other C compilers, gcc supports all the modern C

standards currently in use such as the ANSI C standard

as well as many extensions specific to gcc itself.

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QUICK OVERVIEW

Example:

.

i l d di h


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#include

int main(){

printf(\nHello World!);

;

}

The gcc compiler must generate an object file from this

source code. The object file is essentially the machine code

equivalent of the C source.

141

,

printf() function, and code to return the value of 0.

cc continuation..

Linkerhas to link the object file to produce an executable.

The job of the linker is to take object files, merge them withd f lib i d d bl


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code from libraries, and produce an executable.

First and foremost, the code for printf() must be linked in. Where does the code for printf() come from?

Answer: Libraries

,

including an index. When searching code for the printf(), the linker looks at the

.

It finds the object file containing the printf() function and

extracts that object file and links it to the executable.

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cc BASIC USAGE

programmer would use the command:


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p g

$ gcc o hello hello.cen execu ng s s nge gcc comman , rs gcc a o

compile your source file, hello.c, into an object file, hello.o.

Next, it had to link hello.o against the standard libraries

and produce an executable. If you want only to compile a source file into an object file

and fore o the linkin rocess use the c switch with cc

as in:

gcc c hello.c //produces the object file hello.o

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e ave a program cons s ng o wo source es, oo.c anbar c


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bar.c.

We would use one or more header files containing functionec ara ons s are e ween e wo programs.

To compile these two source files and link them together toproduce the executable baz, we would use the command:

$ gcc o baz foo.c bar.c This is roughly equivalent to the three commands:

$ cc c foo.c

$ gcc c bar.c

$ gcc o baz foo.o bar.o

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An archive (or static library) is simply a collection of objectfiles stored as a single file.

hi i h d hi


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You can create an archive using the ar command.Archive

files traditionally use a .a extension rather than the .oextension used by ordinary object files.

Heres how you would combine test1.o and test2.o into asingle libtest.a archive:

$ar cr libtest.a test1.o test2.o The cr flags tell ar to create the archive.

-with gcc.

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LIBRARIES

an archive?


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9

The most fundamental difference is that when a shared,not actually contain the code that is present in the sharedlibrary. Instead, the executable merely contains a

To create a shared library, you must compile the objects thatwill make up the library using the -fPIC option to the compiler,like this:

9 $gcc -c -fPIC test1.c

The -fPIC option tells the compiler that you are going to be

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. .

Position-Independent Code (PIC)

9 PIC stands for position-independent code. The functions ina shared library may be loaded at different addresses in


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a shared library may be loaded at different addresses in

different programs, so the code in the shared object mustno epen on e a ress or pos on a w c sloaded.

Combine the object files into a shared library, like this:

$ gcc -shared -fPIC -o libtest.so test1.o test2.o The -shared option tells the linker to produce a shared library

rather than an ordinary executable.

Shared libraries use the extension .so , which stands for

shared object. For Static archives and Shared libraries, the filename alwa s

148

begins with lib to indicate that the file is a library.

_ _

When you link a program with a shared library, the linker places only

the name of the shared library.


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The system searches only /lib and /usr/lib , by default.

your s are rary es outs e t ese rectores, suppose t s

present in /usr/local/lib

$gcc -o app app.o -L.-ltest -Wl,-rpath,/usr/local/lib

Another solution to this problem is to set the LD_LIBRARY_PATH

environment variable.

rs w searc n _ _ vara e an en w

search in /lib and /usr/lib.

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MAKEFILES

gcc o output hello.c hello1.c


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gcc o output hello.c hello1.c

cc command in the above exam le would recom ile

each source file in turn before linking the executable. This

can be a large waste of time, especially if you only made

modifications to a single source file since lastcompilation.There would be no reason to recompile the

To overcome this problem, makefiles are used.

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WHAT MAKE DOES

steps.

If lot of source files make up the final executable we


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If lot of source files make up the final executable, we

chan e one and rebuild the executable without havin torecompile everything.

Example:

edime: main.o edit.o

gcc o edime main.o edit.omain.o: main.c

gcc c main.c

edit.o: edit.c

gcc c edit.c

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FILES

The lines containing commands have to begin with


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tabs(not spaces). And dont use a tab before any otherline.

We can place a hash mark(#) anywhere on a line to start

a comment. Everything after the hash mark is ignored.

If we put a backslash at the end of a line, it continues on

the next line.

154

When people use a filename or other string more than

i k fil th t d t i it t


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once in a makefile, they tend to assign it to a macro.

Example:

OBJECTS = main.o edit.o

e me:

gcc o edime $(OBJECTS)

, . .

you specify $(OBJ ECTS).

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