[ SYNTAX ] Control Structures in Shell

IF_ELSE_FI

Pgm : Find whether the given number is odd or even

Code:

read n
if [ `expr $n % 2` = 0 ]
then
echo Even
else
echo Odd
fi

You can also use ‘test’ keyword instead of ‘[ ]’.


FOR LOOP

Pgm : To print Natural Numbers

Code:

read n
for((i=1;i<=n;i++))
do
echo $i
done

There is another way to use for loop.

Code:

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

WHILE LOOP

Pgm : Print Natural Numbers

Code:

read n
i=1
while [ $i –le $n ]
do
echo $i
i=`expr $i + 1`
done

CASE

Print Number in words

Pgm :

Code:

read n
case $n in
1) echo "One";;
2) echo "Two";;
3) echo "Three";;
4) echo "Enter number between one and three";;
esac

also this works


read n
case $n in

hi) echo "One";;
hello) echo "Two";;
you) echo "Three";;
me) echo "Enter number between one and three";;
esac

Am gettin the o/p for if-else-fi only if I give either of these formats

if [ 'expr $n % 2 = 0' ]
or
if [ 'expr $n % 2 eq 0' ]

dont use quote ( ' ) to enclose expr. use acute ( ` ).

Okay . I get the o/p now

for((i=1;1<=n;1++))

The above gives an error ./forloop.sh: line 2: ((: 1++ : syntax error: operand expected (error token is " ")

for (( i = 1 ; i <= n ; i++ )) Whereas this gives me the o/p

What is the problem with the previous for loop?

sorry .. it was i++ only.. typed wrongly.

why are two semi colons ( ; ; ) used in CASE control structure

The two semicolons are used to denote the end of that particular case .

why two brackets are used in for loop.

because it is the syntax...

single bracket has some other significance. so to differentiate between that and for loop arguments, double brackets are used here.