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VIDYABHARTI TRUST COLLEGE OF BBA & BCA. UMRAKH

SUB: UNIX AND SHELL PROGRAMMING

IMPORTANT COMMANDS

Q Explain the Mode of vi editor.

A The vi has following modes:

1.Command mode

2.Input mode (Insert mode) and

3.ex mode (Last Line Mode)

Command Mode :

• This is the mode where you can pass command to act on text, using most of the keys of

the keyboard.

• Commands used to move the cursor, to delete or change part of the test, etc.

• As soon as the command is entered, it is executed – the Return key is not required.

• Commands are also known as hot key.

• The command must be valid command otherwise give unpredictable result.

Text (Input) Mode :

• To enter into this mode, first press “I”.

• When the vi editor is in the text mode, any key that is pressed by a user is considered

text.

• In this mode, the characters types by the user.

• The characters are inserted into the text at the cursor.

• To add the text in a document, we should place the cursor at the desired location.

• To place the cursor we must be in command mode.

• Then we switch to the text mode and edit the text.

Last line(ex) Mode :

• This mode used to handle files like saving file and perform substitution.

• Pressing a “:” in the command mode invokes this mode.

• We can then enter an ex mode command followed by “enter”.

• After running command, we are back to the default command mode.

Q Explain booting sequence with init process. [ Extract point as per mark ]

TYBCA (SEM:5) PREPARED BY AMIT PATEL Page 1


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IMPORTANT COMMANDS

A There are 6 high level stages of a typical Unix booting process.

1) System startup(Hardware )

2) Boot loader Stage 1 (MBR loading)

3) Boot loader Stage 2 (GRUB loader)

4) Kernel

5) INIT

System Startup :

• This is the first stage of booting process.

• After you power on/Restart your machine, The first instruction it will run is to pass control

to BIOS (Basic Input/output System) to do POST(Power On Self-Test). Once the control goes

to BIOS it will take care of two things

1) Run POST operation.

2)Selecting first Boot device.

Run POST Operation :POST is a processes of checking hardware availability. BIOS will have a

list of all devices which are present in previous system boot.

Selecting First Boot Device :Once the POST is completed BIOS will select the first boot

device and gives back the control to Processor(CPU).

Stage 2: MBR Loading

• Once the BIOS gives control back to CPU, it will try to load MBR of the first boot device MBR.

• MBR starting at the first sector on the first hard drive, loads its contents into memory, then

passes control to it.

Stage 3: GRUB Loader

• In this stage GRUB (Grand Unified Boot loader) which is located in the first 30 kilobytes of

hard disk immediately following the MBR is loaded into RAM for reading its configuration

and displays the GRUB boot menu (where the user can manually specify the boot



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IMPORTANT COMMANDS

parameters) to the user.

• GRUB loads the user-selected (or default) kernel into memory and passes control on to the

kernel. If user do not select the OS, after a defined timeout GRUB will load the default

kernel in the memory for starting it.

Stage 4: Kernel

• Once the control is given to kernel, it act as a mediator of hardware and software

components.

• Kernel once loaded into to RAM it always resides on RAM until the machine is shutdown.

• Once the Kernel starts its operations the first thing it do is executing INIT process.

Stage 5: INIT

• When init starts, it becomes the parent or grandparent of all of the processes that start up

automatically on your Unix system.

• The first thing init does, is reading its initialization file, /etc/inittab.

• This instructs init to read an initial configuration script for the environment, which sets the

path, checks the file systems, and so on.

• Basically, this step takes care of everything that your system needs to have done at system

initialization: setting the clock, initializing serial ports and so forth.

• Then init continues to read the /etc/inittab file, which describes how the system should be

set up in each run level and sets the default run level.

• A run level is a configuration of processes.

• Following are the available run levels

0 – System shutdown

1 – System administration mode.

2 – Multiuser, without NFS

3 – Full multiuser mode

4 – unused

5 – Graphical environment mode6 – reboot

• All UNIX-like systems can be run in different process configurations, such as the single user

mode, which is referred to as run level 1 or run level S (or s). When system is booted , init

first enter into level 1 0r S.

• In this mode, only the system administrator can connect to the system. It is used to perform

maintenance tasks without risks of damaging the system or user data.

• Another run level is the reboot run level, or run level 6, which shuts down all running

services according to the appropriate procedures and then restarts the system.

Q Define process. Which command is used to know the status of process? Explain it



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IMPORTANT COMMANDS

with giving example and its option. What are the 3 phases to create process?

A • Process is an instance of a running program.

•

It is said to be “Born” when the program starts execution and remains alive as long as theprogram is active.

• After execution is complete , the process is said to “Die”.

• “ps” commandis used to see the status of currently running process in system.

• NOTE: ps command is explain above question please add it here.

Process Creation Phases

• There are three distinct phases in the creation of process using three important system

calls:

fork( ) ,

exec( ) and

wait( )

Fork( ) • A process in unix is created with the fork system call, Which creates a copy of

the process that invoke it.

• The process that invokes the fork system call is parent process and the newly

created process is the child process.

• fork() allocates entry in process table and assigns a unique PID to the child

process.

• child gets a copy of process image of parent: both child and parent share the

same code following fork(), different data.

• But fork() returns the PID of the child to the parent process and returns 0 to

the child process

• For example, when you enter a command at the prompt, the shell first creates a

copy of itself. The image is practically identical to the calling process, except for



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IMPORTANT COMMANDS

a few parameters like PID. The forking mechanism is responsible for the

multiplication of processes in the system.

Exec( ) • The parent then overwrite the image that it has created with the copy of the

program that has to be executed.

• This is done with the exec system call and the parent is said to exec the

process.

Wait( ) • The parent then executes the wait system call to keep waiting for the child

process to complete.

• When the child process has completed execution, it send a termination signal

to the parent.

• The parent is then free to continue with other function.

Q Explain Special variable used in Shell programming

A • We can not use certain non-alphanumeric characters in your variable names. This is

because those characters are used in the names of special Unix variables.

• Special variables are reserved for specific functions.

• For example: $ character represents the process ID number, or PID, of the current shell:

$echo $$

• Above command would write PID of the current shell − 29949

• There are following types of special variable:

0 The filename of the current scrit.

$n These variables correspond to the arguments with which a script was invoked. Here n

is a positive decimal number corresponding to the position of an argument (the first

# The number of aruments su lied to a scrit.

$* All the arguments are double quoted. If a script receives two arguments, $* is

$@ All the arguments are individually double quoted. If a script receives two arguments,



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IMPORTANT COMMANDS

? The exit status of the last command executed.

$$ The process number of the current shell. For shell scripts, this is the process ID

under which the are executin.

! The rocess number of the last backround command.

Following script uses various special variables related to command line −

#!/bin/sh

echo "File Name: $0"

echo "First Parameter : $1"

echo "First Parameter : $2"

echo "Quoted Values: $@"

echo "Quoted Values: $*"

echo "Total Number of Parameters : $#"

Output: $./test.sh OM SAI

File Name : ./test.sh

First Parameter : OM

Second Parameter : SAI

Quoted Values: OM SAI

Quoted Values: OM SAI

Total Number of Parameters : 2

Q Explain Associative Array With Example. (Mar/Apr-2015)

A • An associative array is an array which uses strings as indices instead of integers.

• Syntax: array[string] = value

• The subscript is often called the key and is associated with the value assigned to the

corresponding array element.



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• The key and values are stored internally in a table .

• The array element are not stored in sequential order and when the content of the array are

displayed, they may not be in order we expect.

• The first array associate name with age. In this array, the name identify each person’s age.

• The second array associates department number with the sales for that department. The

department number in this array are string.

• The associate array structure has several design constraints that we must remember when

we use them

1.Index must be unique, means each index can be associated with an array value only

once.

2.The association of the index with its values is guaranteed.

3.There is no ordering imposed on the indexes. It means if we create an associate array

and print it, there is no guarantee that the element will be printed based on the order in

which the array was created.

4.An array index cannot be sorted. The data value can be sorted.

Creation of Associative Array

• To gain insights about array let us create and access the elements of the array.

$ awk 'BEGIN { fruits["mango"]="yellow";

fruits["orange"]="orange"

print fruits["orange"] "\n" fruits["mango"]

}'

• On executing the above code, you get the following result:



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IMPORTANT COMMANDS

orange

yellow

• In above example we have declared array namely fruits whose index is fruit name and value

is colour of the fruit. To access array element we have used array_name[index] format.

• The special for loop is used to read through an associated array when string are used as

subscript or the subscript is not consecutive numbers.

Q Explain string function of awk.

A Name Description

tolower • This function convert string into upper case to lower case.• Non alphabetic characters are left unchanged.

Syntax: tolower(string)

• For example:

tolower("MiXeD cAsE 123")

Output: mixed case 123

Toupper • This function convert string into upper case to lower case.• Non alphabetic characters are left unchanged.

Syntax: toupper(string)

• For example:

tolower("MiXeD cAsE 123")

• Output: MIXED CASE 123.

Length • It determines the length of its argument.

Syntax: length(string)

• If no argument is present, it assumes the entire line as argument.

• Following statement is used to locate the records whose length exceeds 57

characters.

Awk –F “|” ‘length > 57 ‘ emp.txt

• For example in emp.txt file 2nd field contain the name of employee. To list out

those employee whose name is less than 10 character.

Awk –f “|” ‘length($2) < 10’ emp.txt

• Locate line in file which contain line between 100 and 150 character



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IMPORTANT COMMANDS

Awk –f “|” ‘length > 100 && length < 150’ emp.txt

Substr • It extract a substring from a string. It has two format

Syntax: substr(string, position)substr(string, position, length)

• If length is specified, it returns up to length character for position.

• For example: we have a field which contain string “abcde”, we can use this

function to find out whether “b” is present or not

x = substr(“abcde”,2)

Output: bcde

x = substr(“abcde”,2,2)

Output: bc

Split • It Divide string into pieces separated by fieldsep and store the pieces in array

and the separator strings in the seps array

Syntax: split(string, array)

split(string, array, field seperator)

• In first format, the field in a string are copied into an array. The first piece is

store in array[1], the second in array[2] and so on. The end of each field is

identified by a field separator character.

• In second format, the field separator is specified as the parameter.

• Before splitting the string, split() deletes any previously existing elements in

the arrays array.

• For example: split("cul-de-sac", a, "-")

splits the string ̀cul-de-sac' into three fields using ̀-' as the separator. It sets

the contents of the array a as follows:

a[1] = "cul"a[2] = "de"

a[3] = "sac"

Index • This searches the string in for the first occurrence of the string find, and

returns the position in characters where that occurrence begins in the string

in. Consider the following example:

Syntax: index(in, find)

• For example:

$ awk 'BEGIN { print index("peanut", "an") }'

3

• If find is not found, index returns zero.

• Remember that string indices in awk start at one.



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Q Explain System Variable.(Oct/Nov-2014)

A • Without these built-in variables it’s very much difficult to write simple AWK code.

• These variable are used to format output of an AWK command, as input field separator and

even we can store current input file name in them for using them with in the script.• AWK built-in variables are listed below in table

Variable Description

NR Current count of the number of input records.

NF Keeps a count of the number of fields.

FILENAME The name of the current input file.

FNR No. of records in current filename.

FS Contain the “field separator” character.RS Stores the current “record separator” or Row Separator.

OFS Stores the “output field separator”.

ORS Stores the “output record separator” or Output RS.

NR Variable

• This variable keeps the value of present line number.

• This will come handy when you want to print line numbers in a file.

$ cat emps

Tom Jones 4424 5/12/66 543354

Mary Adams 5346 11/4/63 28765

$ awk '{print NR, $0}' emps // same line cat –n emps.

1 Tom Jones 4424 5/12/66 543354

2 Mary Adams 5346 11/4/63 28765

NF Variable

• NF gives you the total number of fields in a record.

• Awk NF will be very useful for validating whether all the fields are exist in a record.



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• The final value of a row can be represented with $NF.

$ awk '{print $2,$NF;}' employee.txt

Thomas $5,000

Jason $5,500Sanjay $7,000

• In the above example $2 and $5 represents Name and Salary respectively. We can get the

Salary using $NF also, where $NF represents last field. In the print statement ‘,’ is a

concatenator.

• How to print last field without knowing the number of field in file?

$ awk '{print $NF;}' employee.txt

$5,000

$5,500

$7,000

In the above example $NF represents last field.

FILENAME

• This variable contain file awk command is processing.

Example: Print filename for each line in a given file.

awk ‘{print FILENAME, NR, $0}’ abc.txt

abc.txt 1 Jones 2143 78 84 77

abc.txt 2 Gondrol 2321 56 58 45

FNR Variable

• This variable keeps count of number of lines present in a given file/data.

• This will come handy when you want to print no of line present in a given file.

• This command is equivalent to wc -l command.

Example: Print total number of lines in a given file.



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IMPORTANT COMMANDS

awk ‘END{print FNR}’ db.txt

5

FS Variable

• This variable is useful in storing the input field separator.

• By default AWK can understandonly spaces, tabs as input and output separators.

• But if your file contains some other character as separator other than these mention

one’s, AWK cannot understand them.

• For example Unix password file which contain ‘:’ as a separator. So in order to mention

the input filed separator we use this inbuilt variable.

Print first column data from db.txt file.:

awk ‘{print $1}’ db.txt

John,29,MS,IBM,M,Married

Barbi,45,MD,JHH,F,Single

If you see entire file is displayed which indicates AWK do not understand db.txt file separator

“,”. We have to tell AWK what is the field separator.

$ awk ‘BEGIN{FS=”,”}{print $1}’ db.txt

OFS Variable

• This variable is useful for mentioning what is your output field separator which

separates output data.

• Example: Display only 1st and 4th column and the separator between at output for

these columns should be $.

$ awk ‘BEGIN{FS=”,”;OFS=” $ “}{print $1,$4}’ db.txt

Om $ IBM

Sai $ JHH

Ram $ BofA

RS Variable

• Row Separator is helpful in defining separator between rows in a file. By default AWK



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takes row separator as new line. We can change this by using RS built-in variable.

• Example: I want to convert a sentence to a word per line. We can use RS variable for

doing it.

$ echo “Om Sai Ram” | awk ‘BEGIN{RS=” ”}{print $0}’

OmSai

Ram

ORS Variable

• This variable is useful for defining the record separator for the AWK command output.

By default ORS is set to new line.

•

Example: Print all the company names in single line which are in 4th column.$ awk -F’,’ ‘BEGIN{ORS=” “}{print $4}’ db.txt

IBM JHH BofA DELL SmartDrive

Q ‘expr’ command .

A • expr is a command line Unix utility which evaluates an expression and outputs the

corresponding value.

• The Bourne cell doesn’t have any computing features. So it rely on external command “expr”

for that purpose.

• Syntax: expr (expression)

• expr evaluates integer or string expressions, including pattern matching regular expressions.

• It is not working with floating point value.

• The operators perform following function

• Perform arithmetic operation on integers

• Manipulating string.

Arithmetic Operator used with expr

Operator Description

+ Addition - Adds values on either side of the operator



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- Subtraction - Subtracts right hand operand from left hand operand

* Multiplication - Multiplies values on either side of the operator

/ Division - Divides left hand operand by right hand operand

% Modulus - Divides left hand operand by right hand operand and returns

remainder

• “Operand must be enclosed on either side by whitespace.

• Observe that multiplication operator has to be escaped to prevent thte shell from interpreting

it as the filename metacharacter.

• “expr” often used with command substitution to assign a variable. For example, we can set a

variable z to the sum of two number

$ x=6 y=2; z= ̀expr $x + $ỳ

$ echo $z

8

For example:$ expr 1 + 2

Output : 3

String Operation using expr

• “Expr” can perform three important string functions.

• Determine the length of the string.

• Extract a substring.

• Locate the position of a character in a string.

Length of the String

• The regular expression “.*” signifies to “expr” that it has to print the number of

characters matching the pattern. Means give the length of the string.$ expr “omsairam” : ‘,*’

8



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IMPORTANT COMMANDS

• Here expr counted the number of occurances of any character (.*) .

• Here space is require on either side of “:”.

Extracting the Substring

• “expr” can extract a string enclosed by the escaped characters $ and $.

•

If we wish to extract 2 digit year from a 4 digit string, we must create a pattern groupand extract it this way

$ stg=2015

$ expr “$stg” : ̀ ..$..$̀

• Here ‘..$..$’ tells that first two character in the value of “stg” have to be ignored and two

characters have to be extracted from the third character position.

Locating Position Of Chracter

• “expr” can also return the location of the first occurrence of a character in a string.

• To locate the position of the character “d” in the string value of the “stg”, we have to

count the number of characters which are not “d” ( [̂d]* ) followed by d.

$ stg = abcdefgh ; expr “$stg” : ̀[̂d]*d̀

4

Q FOR LOOP with example.

A • The for loop operate on lists of items. It repeats a set of commands for every item in a list.

• Syntax:

for var in word1 word2 ... wordN

do

Statement(s) to be executed for every word.done

Done

In the above syntax:

• for, in, do and done are keywords

• Here var is the name of a variable.

• word1 to wordN are sequences of characters separated by spaces (words). If it is missing

in the for statement, then it takes the positional parameter that were passed into the

shell.

• Each time the for loop executes, the value of the variable var is set to the next word in the

list of words, word1 to wordN.• For example, if the list of values contains 5 items, the for loop will be executed a total of 5 times, once

for each item in the list. The current item from the list will be stored in a variable “varname” each time

through the loop. This “varname” can be processed in the body of the for loop.

#!/bin/sh

for var in 1 2 3 4 5

do

echo –n $var



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done

This will produce following result –1 2 3 4 5

• There are two types of bash for loops available. One using the “in” keyword with list of

values, another using the C programming like syntax.

Bash For Loop using C like syntax

• The second form of the for loop is similar to the for loop in “C” programming language,

which has three expressions (initialization, condition and updation).

• Syntax

for (( expr1; expr2; expr3 ))

do

Body of the loop.

done

In the above bash for command syntax,

• Before the first iteration, expr1 is evaluated. This is usually used to initialize variables for

the loop.

• All the statements between do and done are executed repeatedly until the value of expr2 is

TRUE.

• After each iteration of the loop, expr3 is evaluated. This is usually used to increment a loop

counter.

Q Explain Test Command



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IMPORTANT COMMANDS

A • When we use if to evaluate expressions, we need the test statement because the true or false

values returned by expressions can’t be directly handled by if.

• “test” use certain operators to evaluate the condition on its right and returns either a true or

false exit status, which is then used by if for making decision.

• In the if statement instead of explicitly mentioning the word test, we have placed the

condition within [ ] which is allowed.

• Here we must provide space around the operators and operands.

• “test” command can carry out following types of test

• Numerical Test

• String Test

• File Test

Numerical Test :-

• All relation operator is used as numeric comparison operator with test.

• It always begin with – (hyphen) followed by two-letter string and enclosed on either side by

whitespace.

Operator Description

eq Equal to

-ne Not Equal to

-gt Greater Than

-lt Less Than

-ge Greater than or equal to

-le Less than or equal to

File Test :-

• Test command has several options for checking the status of a file which listed below in

table.

• Using this we can find out whether the specified file is an ordinary file or a directory or a

whether it grants read, write or execute permissions.

•

It also use with negates test !

O erator Descrition



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IMPORTANT COMMANDS

Following script check inputted filename is regular file or not.

#!/bin/sh

Echo “Enter the file name:”;

Read fname;

If test -f fname then

echo “It is a file”

Else

echo “ You enter is not a file

name”

Fi

We can use test and [ ]

interchangeably.

String Test :-

• There are following string operator used with expr to perform operation on string.

O erator Descrition



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IMPORTANT COMMANDS

• Following script check two string are equal or not.

#!/bin/sh

Str1 = “Good Morning”

Str2 = “Good Bye”

If test “$str1” = “$str2” ; then

echo $?

Since two string are assigned to variable str1 and str2, we have taken care to enclose the string

within a pair of double quotes.

Q Shift Command

A • Shift transfer the contents of positional parameters to its immediate lower numbered one.

• This is done as many time as the statement is called.

• When called once, $2 becomes $1, $3 becomes $2, and so on.

For example 1: $ date

Wed Aug 19 04:10:30 IST 2015$ echo $1 $2 $3

Wed Aug 19

For example 2:

$ Shift

$ echo $1 $2 $3

Aug 19 04:10:30

Q Explain SET utility

A • We have used the set command to set upto 9 words.



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• For Example: $ set Good morning Amit Patel. Hello, How are you. Hope you are fine.

$ echo $1 $2 $3 $4 $5 $6 $7 $8 $9 $10 $11

Output: Good morning Amit Patel. Hello, How are you. Hope Good0 Good1

• If we observed the output of last two word – Good0, Good1. This occurred in the output

because at a time we can access only 9 positional parameters. When we tried to refer $10 it

was interpreted by the shell as if you wanted to output the values $1 and 0. Same is true for

$11 as $1 and 1.

• To access word after 9th word, we have to use shift.

• For example:

$ Shift 9

$ Echo $1 $2 $3

Output: you are fine.

Q Explain UMASK command.

A • Definition: User mask is a 3 digit octal system variable which is used initially set

permission for a directory of files.

• Defined initially by system administrator when user account is created.

• It contains the octal settings for the permissions that are to be removed from the default

when a directory or file is created.

•

You can change the settings by creating mask entry in you login file.

• When new directory or file is created, the number in the mask is used to set the default

permission.

• Default Permission :

For Directory: 777

For File: 666

MASK DIRECTORY PERMISSION FILE PERMISSION



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IMPORTANT COMMANDS

• To display the current user mask settings, use the umask command with no argument.

$ umask

000

• To set it, use the command with the new mask setting.

$ umask 022

$ umask

022

Q Explain at Command. (Mar/Apr-2015)

A • The AT command schedules commands and programs to run on a computer at a specified

time and date.

Syntax: at time date

• It takes as its argument the time the job is to be executed and display the “at>” prompt.

Input has to be supplied from the standard input.

For Example:

$ at 14:08

at> empdetails.sh [Ctrl –d ]

• The job goes to the queue and at 2:08 PM today, the script file empdetails.sh will be

executed.

• “at” doesn’t indicate the name of script to be executed when it execute program.

• “at” also offer key words now, noon, midnight, today and tomorrow It also accept “+” symbol

to act as an operator. The words that can be used with this operator include hours, days,

weeks, months and years.

For Example: at 15, at 5pm, at noon, at 9am tomorrow, at 3:08pm + 1 day

• You can schedule a job to be executed using relative time from now.

• Syntax: $ at now + COUNT UNIT



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• For example, following job will be execute 1 minute from now.

$ at now + 1 min

• The above example will read the commands from stdin, and it will execute the job after a

minute. When you give something wrong in time format, you will get the error ‘Garbled time‘.

• “at –l” is used to listed the job and “at –r” is used to remove any job from list.

Q Explain batch Command. (Mar/Apr-2015)

A • It is used to run multiple job without overloading the system.

• The command doesn’t take any arguments but uses an internal algorithm to determine the

execution time.

• After typing batch at the command prompt, enter any number of commands one on each

line. When you are done press CTRL-d.

Syntax: $ batch

command1

....

commandN

CTRL-d

• This allows you to schedule a number of commands at once without slowing down the

system by running them simultaneously. It will also continue to run the commands after you

logout usually sending email when the commands are finished.

• For example, suppose you want to sort three large files. Instead of sorting them

simultaneously on background

$ sort bigfile1 &

$ sort bigfile2 &

$ sort bigfile3 &

• Use the batch command to run them one at a time

$ batch

sort bigfile1

sort bigfile2



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sort bigfile3

• so that you do not slow down the computer as much.

• When using batch you will typically want to use redirection to save output normally sent to

the screen to a file.

Difference between at and batch

at batch

The AT command schedules commands and

programs to run on a computer at a specified

time and date

It is used to schedules job for later

execution as soon as the system load

ermits.It takes as its argument the time the job is to

be executed

For Example:

$ at 14:08

at> empdetails.sh [Ctrl –d ]

The job goes to the queue and at 2:08 PM

today, the script file empdetails.sh will be

executed.

The command doesn’t take any arguments

but uses an internal algorithm to determinethe execution time.

Q Explain crontab Command. (Mar/Apr-2015)

A • Once job submitted using this command have been executed, the job have been rescheduled

if they are to be carried out again.

• For example: If we want to backup all our “c” program files at the end of the day then we do

it as follow

$ at 5 PM cp *.c ./cbackup

• We must remember to issue this command every morning to be able to take backup every

evening without failing.

• To overcome this we have to use “crontab” command. The job can be carried out on a regular

basis using “crontab” command.

• Crontab utility is an effective way to schedule a routine background job at a specific time

and/or day on an on-going basis.

• Crontab format: MIN HOUR DOM MON DOW CMD



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IMPORTANT COMMANDS

• A nice value of 1 represents highest priority, and a nice value of 19 represent least priority for

a process.

• Higher nice value implies a lower priority.

• By default when a process starts, it gets the default priority of 0.

To Launch a Program with Less Priority :-

• Instead of launching the program with the default priority, you can use nice command to

launch the process with a specific priority.

• In this example, test.pl is launched with a nice value of 10.

$ nice -10 wc –l manual &

• Remember that -10 in the above command sets the priority of a process to 10. The – in nice

command stands for the hypen.

Change the Priority with option –n:-

• The process priority can be adjusted with the help of -n option.

• Increase the priority: $ nice -n -5 wc –l manual &

• Decrease the priority: $ nice -n 5 wc –l manual &

Q Explain kill Command.(Sep-Oct 2012)

A • Use the kill command to send a signal to each process specified by a pid. The default signal

is SIGTERM (terminate the process).

• We mostly use the kill command for terminating or killing a process. However we can also

use the kill command for running a stopped process.

• Syntax: kill [-s signal] pid OR kill –l

• PID means process id that kill command send a signal

• -s signal means sent the specified signal to process.

• -l means list all signals.

• Kill command use one or more PID as its arguments.

• For example: $ kill 105

• Terminates the job having PID 105. To know the PID run “ps” command.

• To kill multiple process, run following command

$ kill 121 122 125

• If all these process have the same parent, you may simply to kill the parent of all this

process to know the PPID.

To Killing the last background job:-

• The system variable “$!” store the PID of the last background job.



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IMPORTANT COMMANDS

• So we can kill the last background job without using the “ps” command to find out PID

$ sort –o emp.lst &

345

$ kill $!

• By default, ”kill” uses the SIGTERM signal (15) to terminate the process.

• Process can be killed with the SIGKILL signal (9).

• This signal cant be generated at the press of a key, we must use kill with signal name

preceded by the –s option

$ kill –s KILL 121 OR $ kill -9 121

•

To kill your login shell :$ kill -9 $$ OR $ kill –s KILL 0

SIGNAL USAGE

1 HUP (hang up)

2 INT (interrupt)

3 QUIT (quit)

6 ABRT (abort)

9 KILL (non-catchable, non-ignorable kill)

14 ALRM (alarm clock)

15 TERM (software termination signal)

• The signal number 1 is a hangup signal. I recommended using 1 signal because it should

kill the process and it can save the buffer (if supported). For example if it is an editor, save

the buffer. This is the default if you do not specify a signal number. Signal number 9, a kill

signal, is the surest way to kill a process.Q Explain ps command.



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IMPORTANT COMMANDS

A • The ps command is used to display the attributes of a process. It has knowledge of the

kernel built into it.

• It reads through the kernel data structures or process table to fetch the process

characteristics.

• By default, ps list out the processes associated with a user at that terminal

$ ps

PID TTY TIME CMD

476 tty03 00:00:01 login

659 tty03 00:00:01 sh

684 tty03 00:00:001 ps

• Each line show the pid, the terminal with which the process is associated, the cumulative

processor time that has been consumed since the process has been started and processname.

OPTION DESCRIPTIONS

-f Full listing showing the PPID of each process.

-e or –A All process including system and user process.

-u usr Process of user “usr” only.

-a Process of all user excluding processes not associated with terminal.

-l Long listing showing memory related information.

-t term Process running on terminal “term”

• Example 1: Display a detailed listing of process also show parent of process.

$ ps -f

UID PID PPID C STIME TTY TIME CMD

root 476 1 0 17:51:58 tty03 00:00:01 /bin/login kumar

kumar 659 476 4 18:10:29 tty03 00:00:01 -sh

kumar 685 659 15 18:26:44 tty03 00:00:00 ps –f

• Description of all this column is listed in table

COLUMN DESCRIPTION

UID User ID that this process belongs to (the person running it).

PID Process ID.

PPID Parent process ID (the ID of the process that started it).



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IMPORTANT COMMANDS

C CPU utilization of process.

STIME Process start time.

TTY Terminal type associated with the process

TIME CPU time taken by the process.

CMD The command that started this process.

• Example 2: Display all user process but doesn’t display the system process.

$ps –a

PID TTY TIME CMD

662 tty02 00:00:00 ksh

705 tty04 00:00:00 sh

1005 tty01 00:00:00 ksh

1056 tty02 00:00:00 sort

1069 tty02 00:00:00 ps

• There are 3 user work here as terminal names displayed.

Example 3: Display the system process.

$ps –e

PID TTY TIME CMD

0 ? 00:00:00 sched

1 ? 00:00:01 init2 ? 00:00:00 vhand

? 00:00:01 bdflush

Example 4: Display the process run by user om.

$ps –u om

Q Explain Case Condition With Example In Shell. (Mar-Apr 2012).

A • You can use multiple if...elif statements to perform a multiway branch. However, this is not

always the best solution, especially when all of the branches depend on the value of a single

variable.



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IMPORTANT COMMANDS

• Unix Shell supports case...esac statement which handles exactly this situation, and it does

so more efficiently than repeated if...elif statements.

• Unix Shell's case...esac is very similar to switch...case statement we have in other

programming languages like C or C++

• The basic syntax of the case...esac statement is to give an expression to evaluate and several

different statements to execute based on the value of the expression.

• The interpreter checks each case against the value of the expression until a match is found.

If nothing matches, a default condition will be used.

• Syntax:

case word in

pattern1)

Statement(s) to be executed if pattern1 matches

;;

pattern2)


;;

pattern3)


;;

esac

• Here the string word is compared against every pattern until a match is found. The

statement(s) following the matching pattern executes. If no matches are found, the case

statement exits without performing any action.

• There is no maximum number of patterns, but the minimum is one.

• When statement(s) part executes, the command;; indicates that program flow should jump

to the end of the entire case statement. This is similar to break in the C programming

language.

#!/bin/sh

FRUIT="kiwi"

case "$FRUIT" in

"apple") echo "Apple pie is quite tasty."



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IMPORTANT COMMANDS

;;

"banana") echo "I like banana nut bread."

;;

"kiwi") echo "New Zealand is famous for kiwi."

;;

esac

This will produce following result − New Zealand is famous for kiwi.

Documents

LONG QA STUDENT COPY 2.doc