Intoduction to shell programs - control structures

shell programs based on Control Structures 

1. IF Statement

Syntax:

if [ expression ]

then

Statement(s) to be executed if expression is true

fi

Description:-

The if...fi statement is the fundamental control statement that allows Shell to make decisions and execute statements conditionally.

If expression is a shell command then it would be assumed true if it return 0 after its execution. If it is a boolean expression then it would be true if it returns true.

Example:

#!/bin/sh

a=10

b=20

if [ $a == $b ]

then

echo "a is equal to b"

fi

if [ $a != $b ]

then

echo "a is not equal to b"

fi

This will produce following result:

a is not equal to b

program:

#!/bin/bash
echo -n " enter a value :"
read a
echo -n " enter b value :"
read b

if [ $a -gt $b ]
then
echo "$a is greater then  $b"
fi

if [ $a -lt $b ]
then
echo "$a is less then $b"
fi

if [ $a -ge $b ]
then
echo "$a is  greater then or equal to  $b"
fi

if [ $a -le $b ]
then
echo "$a is less then oe equal to  $b"
fi

if [ $a -eq $b ]
then
echo "$a is equal to $b"
fi

if [ $a -ne $b ]
then
echo "$a is not equal to  $b"
fi

Output:

2. THE if…else Statement

Syntax:

if [ expression ]

then

Statement(s) to be executed if expression is true

else

Statement(s) to be executed if expression is not true

fi

Description:- The if...else...fi statement is the next form of control statement that allows Shell to execute statements in more controlled way and making decision between two choices.

Example:

If we take above example then it can be written in better way using if...else statement as follows:

#!/bin/sh

a=10

b=20

if [ $a == $b ]

then

echo "a is equal to b"

else

echo "a is not equal to b"

fi

This will produce following result:

a is not equal to b

Program:

#!/bin/bash
echo -n " enter a value :"
read a
echo -n " enter b value :"
read b

if [ $a == $b ]
then
echo "a is equal to  b"
else
echo "a is not equal then b"
fi

Output:

3. If…elif…fi

Syntax:

if [ expression 1 ]

then

Statement(s) to be executed if expression 1 is true

elif [ expression 2 ]

then

Statement(s) to be executed if expression 2 is true

elif [ expression 3 ]

then

Statement(s) to be executed if expression 3 is true

else

Statement(s) to be executed if no expression is true

fi

Description:- The if...elif...fi statement is the one level advance form of control statement that allows Shell to make correct decision out of several conditions.

Example:

#!/bin/sh

a=10

b=20

if [ $a == $b ]

then

echo "a is equal to b"

elif [ $a -gt $b ]

then

echo "a is greater than b"

elif [ $a -lt $b ]

then

echo "a is less than b"

else

echo "None of the condition met"

fi

This will produce following result:

a is less than b

program:

#!/bin/bash
echo " enter a value:"
read a
echo " enter b value:"
read b
echo " enter c value:"
read c

if [ $a -gt $b -a $a -gt $c ]
then
echo "a is big"
elif [ $b -gt $c ]
then
echo "b is big "
else
echo "c is big"
fi

output:

4. Case…esac

Syntax:

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.

case word in

pattern1)

Statement(s) to be executed if pattern1 matches

;;

pattern2)

Statement(s) to be executed if pattern2 matches

;;

pattern3)

Statement(s) to be executed if pattern3 matches

;;

esac

Description:- Shell support case...esac statement which handles exactly this situation, and it does so more efficiently than repeated if...elif statements.

Example:

#!/bin/sh

FRUIT="kiwi"

case "$FRUIT" in

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

;;

"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.

Example verified in ubuntu :

echo "enter 1st value"

read x

echo "enter 2nd value"

read y

echo "enter 1 adding"

echo "enter 2 sub"

echo "enter 3 mul"

echo "enter 4 quo"

echo "enter 5 rem"

echo "enter 6 exit"

echo "enter * invalid number"

echo "enter u r choice"

read s

case $s in

1)p=`expr $x + $y`

echo "sum =$p"

;;

2)p=`expr $x - $y`

echo "sub=$p"

;;

3)p=`expr $x \* $y`

echo "mul=$p"

;;

4)p=`expr $x / $y`

echo "quo =$p"

;;

5)p=`expr $x % $y`

echo "rem=$p"

;;

6)exit

;;

*)echo "wrong choice"

;;

esac

program:

#!/bin/bash
echo "enter 1st value"
read x
echo  "enter 2nd value"
read y
echo "enter 1 adding"
echo "enter 2 sub"
echo "enter 3 mul"
echo "enter 4 quo"
echo "enter 5 rem"
echo "enter 6 exit"
echo "enter * invalid number"
echo "enter u r choice"
read s
case $s in
1)p=`expr $x + $y`
echo "sum =$p"
;;

2)p=`expr $x - $y`
echo "sub=$p"
;;

3)p=`expr $x \* $y`
echo "mul=$p"
;;

4)p=`expr $x / $y`
echo "quo =$p"
;;

5)p=`expr $x % $y`
echo "rem=$p"
;;

6)exit
;;

*)echo "wrong choice"
;;
esac

output:

5. While

Syntax:

while command

do

Statement(s) to be executed if command is true

done

Description:- Here Shell command is evaluated. If the resulting value is true, given statement(s) are executed. If command is false then no statement would be not executed and program would jump to the next line after done statement

Example:

Here is a simple example that uses for loop to span through the given list of numbers:

#!/bin/sh

a=0

while [ $a -lt 10 ]

do

echo $a

a=`expr $a + 1`

done

This will produce following result:

0

1

2

3

4

5

6

7

8

9

program:

#!/bin/bash
echo "enter any  number"
read n
fact=1
while [ $n -ne 0 ]
do
fact=`expr $fact \* $n`
n=`expr $n - 1`
done
echo "factorial is : $fact"

output:

6. Loop

Syntax:

for var in word1 word2 ... wordN

do

Statement(s) to be executed for every word.

done

Description:- Here var is the name of a variable and word1 to wordN are sequences of characters separated by spaces (words). 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.

Example:

Here is a simple example that uses for loop to span through the given list of numbers:

#!/bin/sh

for var in 0 1 2 3 4 5 6 7 8 9

do

echo $var

done

This will produce following result:

0

1

2

3

4

5

6

7

8

9

program:

#!/bin/bash
fact=1
echo "enter a number"
read n
for (( i=$n; i>=1; i-- ))
do
fact=`expr $fact \* $i`
done
echo "the factorial of $n is $fact"

output: