It will provide brief knowledge regarding different types of constructs such as conditional statement execution and looping etc, which is crucial for shell Scripting.

1. UNIX
Programming Construct
Presentation By
Nihar R Paital

2. Nested ifs
 You can write the entire if-else construct within either the body of the if statement of
the body of an else statement. This is called the nesting of ifs.
 Ex :
chos=0
echo "1. Unix (Sun Os)"
echo "2. Linux (Red Hat)"
echo -n "Select your os choice [1 or 2]? "
read chos
if [ $chos -eq 1 ] ; then
echo "You Pick up Unix (Sun Os)"
else
if [ $chos -eq 2 ] ; then
echo "You Pick up Linux (Red Hat)"
else
echo "What you don't like Unix/Linux OS."
fi
fi
Nihar R Paital

3. Multilevel if-then-else
#!/bin/ksh
echo Enter a number :
read n
if [ $n -gt 0 ]; then
echo "$n is positive"
elif [ $n -lt 0 ]
then
echo "$n is negative"
elif [ $n -eq 0 ]
then
echo "$n is zero"
else
echo "Opps! $n is not number, give number"
fi Nihar R Paital

4. Loops in Shell Scripts
Bash supports:
– for loop
– while loop
In each and every loop,
(e) First, the variable used in loop condition must be initialized, then execution of
the loop begins.
(b) A test (condition) is made at the beginning of each iteration.
(c) The body of loop ends with a statement that modifies the value of the test
(condition) variable.
Nihar R Paital

5. for Loop
 1) for x in 10 20 30 40 50
do
echo $x
done
 2) Ex: Arrays with for loop
#!/bin/ksh
y="shell scripting Training"
for x in ${y[*]}
do
echo $x
done
Nihar R Paital

6. Ex: for statement
 3) for x in `ls`
do
echo $x
done
 4) for x in `ls` `cat forloop`
do
echo $x
done
Nihar R Paital

7. while loop
 Syntax:
while [ condition ]
do
command1
command2
command3
..
....
done
Nihar R Paital

8. Ex: while statement.
x=1
while [ $x -lt 10 ]
do
echo $x
x=`expr $x + 1`
done
Nihar R Paital

9. until statement
Syntax:
until control command
do
<commands>
Done
Ex:
x=1
until [ $x -gt 10 ]
do
echo $x
x=`expr $x + 1`
done
Nihar R Paital

10. case statement.
The case statement is good alternative to Multilevel if-then-else-fi
statement. It enable you to match several values against one variable.
Its easier to read and write.
Syntax:
case $variable-name in
choice1) commands ;;
choice2) commands ;;
....
....
Esac
The $variable-name is compared against the cases until a match is found. The
shell then executes all the statements up to the two semicolons that are next to
each other. The default is *) and its executed if no match is found. For e.g. write
script as follows:
Nihar R Paital

11. Ex: case statement
echo enter value for x
read x
case $x in
1)ls;;
2)cal;;
3)date;;
*)echo invalid
esac
Nihar R Paital

12. Useful Shell Scripting commands.
 break
– To come out of a loop.
 continue
– To jump to the start of loop.
 exit
– To prematurely terminate a program.
 #
– To interpret the rest of line as comments.
Nihar R Paital

13. Nihar R Paital