Lab2.  Shell Programming

 Shell programs are a file that holds the one or more unix or linux commands.

A .Input and Output

echo : this command is used for standard output

Use echo command to display text or value of variable.

echo [options] [string, variables...]
Displays text or variables value on screen.
Options
-n Do not output the trailing new line.
-e Enable interpretation of the following backslash escaped characters in the strings:
\a alert (bell)
\b backspace
\c suppress trailing new line
\n new line
\r carriage return
\t horizontal tab
\\ backslash

For e.g. $ echo -e "An apple a day keeps away \a\t\tdoctor\n"

Ex1.

clear
echo "Knowledge is Power"

ex2. Script to print user information who currently login , current date & time

clear
echo "Hello $USER"
echo –e "Today is \c ";date
echo –e "Number of user login : \c" ; who | wc -l
echo "Calendar"
cal

B. Variables:

In shell programming declaring a variable before using it is not necessary. The value of the variable can be seen using $ sign before its name:

$ cat > myprog

message=”Hello world”

echo $message

^D

$ ./myprog

Hello world

$

read : this command is used for standard input

$ cat > myprog

echo input number

read number

echo your number is: $number

^D

$chmod 700 myprog

$ ./myprog

input number

5

your number is: 5

$

ex2.

cat > myprog2

echo input a character

read ch

echo "your number is: $a"

^D

C. Arithmetic Operations

Operations: / ,- ,+ ,% ,* ,= ,>= ,<= ,< ,> ,& ,| ,( , ), a,o,!

              expr op1 math-operator op2

Examples:
$ expr 1 + 3
$ expr 2 - 1
$ expr 10 / 2
$ expr 20 % 3
$ expr 10 \* 3
$ echo `expr 6 + 3`

ex.

typeset i result

a=100; b=50

result=$a  \*   $b

echo $result

ex.

a=3.749

b=22.34

echo "$a \* $b" | bc

ex.

x=`expr 3 \* 5`

echo $x

D. Testing

test : test command is used for testing operations. If test result is true, it returns true.

test –d  myfile : test the file called myfile whether it is a directory or not

test –f  myfile : test the file called myfile whether it is a file or not

test –r  myfile : test the file called my file whether it has  read permission or not

test –w  myfile : test the file called my file whether it has  write permission or not

test –x  myfile : test the file called my file whether it has execution permission or not

test str1=str2  : is str1 equal to str2

test str1!=str2 : is str1 not equal to str2

test n1 –eq n2 : is n1 equal to n2

test n1 –le n2  : is n1 less than or equal to n2

test n1 –ge n2 : is n1 greater than or equal to n2

test n1 –ne n2  : is n1 not equal n2

test n1 –lt  n2  : is n1 less than n2

E. If Statements

Syntax of the if statement:

if  condition

then

     command1

     command2

elif  

then

     command3

     command4

else

      command5

fi

example:

if test –r file1

then

     echo read permission

else

     echo no read permission

fi

example:

echo "select a number between 0 and 20"

read number

if test $number -lt 10

then

        echo "Selected number is less than 10"

elif

        echo "Selected number is greater than or equal to 10"

fi

F. Case Statement

Syntax of the case statement:

case variable-name in

option1 )

     command1

     command2

;;

option2 )

     command3

     command4

;;

*)

     command5

;;

esac

example:

clear

echo "1. Clear screan"

echo "2. List logined users"

echo "3. List the files"

echo -n "Enter your choice:"

read choice

case $choise in

1)

    clear   ;;

2)

    who

    ;;

3)

    ls -al  ;;

*)

    echo "wrong choice"

esac

G. For statement

for  variable name  in  list

do

       execute one for each item in the list until the list is

       not finished (And repeat all statement between do and done)

done

for i in 1 2 3 4 5 do echo "Welcome $i times" done

 

ex: Lists all words in the file called mylist

do

         echo $i

done    

example: List all files in the current directory

do

     echo $i

done

H. While Statement

while [ condition ]
do
     command1
     command2
     command3
      ..
       ....
done

i=1

while test $i –le 10

do

            echo “i= $i”

            i=`expr $i + 1`

done

Example:

clear

choice=0;

while $choice -ne -1

do

    echo "1. Clear screan"

    echo "2. List logined users"

    echo "3. List the files"

    echo -n "Enter your choice:"

    read choice

    case $choise in

    1)

        clear   ;;

    2)

        who

        ;;

    3)

        ls -al  ;;

    *)

        echo "wrong choice"

    esac

done

Question: Write a script to find out biggest number from the given three numbers. Numbers are supplies as command line argument. (Hw: Print error if sufficient arguments are not supplied.)