Unix Shell Programming Basics

Shell Program

A shell program or shell script is a collection of Shell commands to perform a particular task .

Examples:

• Unix Shell scripts (ksh, csh, bash, sh)

• Python
• perl
• VBScript
• Java Script
• Apple Script etc.

Uses of Shell Programming

• To combine lengthy and repetitive sequences of commands into a single, simple command.
• To create new commands using combinations of pre-existing commands.
• To automate the process of setting up and running applications.

Shell Program structure

• Any text editor can be used to type shell program.
• #! /bin/sh
  # this is a comment
  body of program
  exit 0 
• #! /bin/sh indicates to other users to run the script using /bin/sh 
• #! /bin/sh is called as shebang or hashbang 
• Comment line starts with #  
• exit 0 represents the end of script

Steps involved in Shell Script

Step-1: Type shell program in a text file 
Step-2: Save file with .sh extension
Example: FileName.sh 
Step-3: Make FileName.sh Executable for all users using 'chmod 755 FileName.sh' 
Step-4: Run script using ./FileName.sh

Shell Script Examples

#! /bin/sh

#Script Name: Run.sh

#Purpose: to run hello.c file

gcc -o hello hello.c

./hello

exit 0

#! /bin/sh

#Script Name: UpdateUpgrade.sh

#Purpose: To update & upgrade debian #based system

apt-get update

apt-get upgrade

reboot

exit 0

Note: You need super user privileges to run UpdateUpgrade.sh script

Assignments-1

• Write a shell script to display your name on screen (Time: 1 Min) 
• Write a shell script to remove CDAC directory, whose absolute path is /home/reniguntla/CDAC (Time: 2 Min) 
• Write a shell script to display all lines contains “sh” in file UpdateUpgrade.sh (Time: 2 Min)

Shell Script Variables

• By deafault all variables in shell script are strings.
  
  
• #! /bin/sh
  #Script Name: Variable.sh
  #Purpose: To demonstrate variables
  x=Hello
  echo "$x"
  y="Hi"
  echo "$y"
  z="How are you?"
  echo "$z"
  exit 0
• Output: Hello

                      Hi

                      How are you?

Quoting of variables

#! /bin/sh

#Script Name: Quote.sh

#Script purpose: To demonstrate variable with #quotations

x=Hello

echo $x

x=”Hello”

echo $x

x=”Hello Mr.L”

echo $x

echo '$x'

exit 0

OutPut: Hello

              Hello

             Hello Mr.L

             $x

Assignments-2

• Write a shell script to print your name and student id on screen using variables? (1 Mins) 
• Write a shell script to swap 2 numbers using temporary variable? (2 Mins)

Shell Script Binary Operators 

 

Shell Script Binary Operators Example

#!/bin/sh

#Script Name: BinaryOp.sh

#Purpose: Demonstration of binary operators

a=10

b=20

val=`expr $a + $b`

echo "a + b : $val"

val=`expr $a - $b`

echo "a - b : $val"

val=`expr $a \* $b`

echo "a * b : $val"

val=`expr $b / $a`

echo "b / a : $val"

val=`expr $b % $a`

echo "b % a : $val"

exit 0

OutPut: a+b:30

               a-b:-10

               a*b:200

               b/a:2

               b%a:0

Shell Script Relational Operators

Shell Script Boolean Operators

Note: Explore string operators yourself

Shell Script File Test Operators

 

Reading Input From User

To read input we use read command 

 

#! /bin/sh

#Script Name: InputRead.sh

#Purpose: To demonstrate read command

echo “Enter your name?”

read MyName

echo “Your name is $MyName”

exit 0

OutPut: Enter your name ?

               Reniguntla

               Your name is Reniguntla

Assignments-3

Write a shell script to read your name, roll number, marks in 4 subjects. Display your name, roll number, average of 4 subjects & percentage on screen. (3 Min)

Shell Script Parameters

All command line parameters / arguments of a shell script can be accessed via $1, $2, $3,..., $9, ${10}, ${11}...

$# holds the number of parameters passed

$@ Holds A list of all parameters in a single variable

$* also holds all parameters

$0 holds the name of the script

Shell Script Parameters Example

#!/bin/sh

#Script Name: ScriptParam.sh

#Purpose: Script parameters demo

echo "There are $# parameters"

echo "The parameters are $@"

echo "The script name is $0"

echo "The first parameter is $1"

echo "The second parameter is $2"

exit 0

OutPut: /.ScriptParam.sh reni sam

             There are 2 parameters

             The parameters are reni sam

             The script name is ./ScriptParam.sh

             The first parameter is reni

            The second parameter is sam

Assignments-4

Write a shell script to delete file1.txt & to give RWX permissions to all users on file2.txt. Take file1.txt and file2.txt as shell script parameters. (3 Min)

If-then-fi

Syntax: if [ condition ]

             then

                Statements

            fi

Example:

#! /bin/sh

#Scriptname: ifthenfi.sh

#purpose: To demonstrate if-then-fi

echo "Is it morning? Please answer yes(1) or no(0)"

read response

if [ $response -eq 1 ]

    then

         echo “good morning”

fi

exit 0

if-then-else-if

Syntax: if [ condition ]

             then

                 Statements

            else

                Statements

            fi

Example:

#! /bin/sh

#ScriptName: Ifthenelsefi.sh

#purpose: To demonstrate if-then-else-fi

if [ -d /home/faculty ]

then

    echo “/home/faculty is a directory”

else

    echo “/home/faculty is not a directory”

fi

exit 0

Assignments-5

Write a shell script to check whether read number is even or odd? (Time: 2 Min)

Write a shell script to check given file(take this a script parameter) is plain file or directory. (Time: 2 Min)

Case Statement

Syntax:

case variable in

Pattern ) statements ;;

Pattern ) statements ;;

......

esac

Example:

#! /bin/sh

#ScriptName: Case.sh

#purpose: To demonstrate case statement

echo “is it morning? Please anser yes or no”

read timeofday

case “$timeofday” in

yes | Y | y | YES ) echo “Hi”

echo “good morning” ;;

yo | N | n | NO ) echo “Hi”

echo “Good After Noon” ;;

* ) echo “sorry answer not recognized” ;;

esac

exit 0

For Loop

Syntax: for variable in values

            do

                Statements

           done

Example:

#! /bin/sh

#ScriptName: For.sh

#Purpose: To demonstrate for loop

for file in $(ls *.sh)

do

     echo “$file”

done

exit 0

Note: You can use loop controls break, continue in for loop with a value.

While loop

Syntax: while [ condition ]

             do

                 Statements

            done

Example:

#! /bin/sh

#ScriptName: While.sh

#Purpose: To demonstrate While loop

echo "enter password:"

read password

while [ "$password" != "secret" ]

do

    echo "try again"

    read password

done

exit 0

Note: You can use loop controls break, continue in while loop with a value.

Note: Explore do-while loop for yourself

Arrays in Shell Script

An array variable can hold multiple values at the same time and each value can access using index value.

Example:

#!/bin/sh
#ScriptName: Array.sh
#Purpose: To demonstrate Array in shell script
name[0]="Reni"
name[1]="Sam"
name[2]="Ashok"
name[3]="Mahesh"
#Displaying array item based on index number
echo "First Method: ${name[0]}"
echo "First Method: ${name[2]}"
# Different method to display all items in array
echo "First Method: ${name[*]}"
echo "Second Method: ${name[@]}"
exit 0
 

Function with No parameters & No return 
#!/bin/sh 
#ScriptName:Function1.sh 
#Purpose: To demonstrate function with no #parameters & no return 
# Function Hi definition 
Hipgdst () { 
   echo "Hi PGDST" 
} 
# Invoking function Hi 
Hipgdst 
exit 0

Function with  Parameters & Return Value
#!/bin/sh
#ScriptName:Function2.sh
#Purpose: To demonstrate function with parameters & return val
# Function add definition
add () {
   return `expr $1 + $2`
}
# Reading 2 nums for adding through func
echo "enter first number"
read a
echo "enter second number"
read b

# Invoking function add with a & b as #parameters
add $a $b
# Capture value returned by last command
sumval=$?
echo "Return value is $sumval"
exit 0


Shell Script Assignments

1. Write a shell script to check read number is prime or not? (use for / while loop) 

2. Write a shell script to determine whether the given word begins with lower case 

   or upper-case letter or whether it is starting with a special character using case statement. (use case) 

3. Write a shell script to read contents of a file (which has been provided 

   as a script parameter) line by line and display on screen. (use while loop) 

4. Write a shell script to read 10 values into an array and print them on screen using array index. 

5. Write a shell script to read 5 values into array1 and 5 values into array2 and concatenate 

   both arrays store in array3. 

6.  Write shell script functions to calculate +, -, *, / , % of two numbers which 

   are provided as parameters to the functions and and each function must return result. 

   Call all those functions and print return values. 

7. Write a single shell script to do the following
   a. Create a file profile.txt 
   b. Change permissions of profile.txt to _rwxr_xr_x 
   c. Read your name, e-mail id, roll number and write them to profile.txt 
   d. Read the contents of profile.txt using a loop and display on screen. 

8. Write a single shell script to do the following
   a. read a number 
   b. Define a function checkprime() which takes read number as parameter 

   & checks that number is prime or not & return the result. 
   c. Call checkprime() with parameters 12, 97, 111
   d. Display the return values of above function calls on screen 

9. Write a single shell script to 
   a. read a number N 
   b. read N numbers in to an array called array_all using loop 
   c. Navigate through array_all and check for even & odd numbers. Store 

   all even numbers in array even_array & odd numbers in array odd_array 
   d. display all even numbers then odd numbers on screen.