decision loops - statement iteration

1. Lesson 12
• Decision with Loops
• Statement Iteration
• While
• Until
• For
• Select
• Nesting loops
• Loop Control
• Infinite
• Break
• Continue

2. Decision Making with loops
Iteration Statements
Loops
Powerful programming tool that enables execution of a set of commands
repeatedly.
• The while loop
• The for loop
• The until loop
• The select loop
Nesting Loops
All loops support nesting
– You can put one loop inside another similar or different loops.
This nesting can go up to unlimited number of times based on your requirement.

3. while loop
while loop iterates “while” the expression is true
enables to execute a set of commands repeatedly until some condition occurs.
It is usually used to manipulate the value of a variable repeatedly.
• It’s a looping structure. Executes a set of commands while a specified condition is true.
• The loop terminates as soon as the condition becomes false.
• If condition never becomes false, loop will never exit.
Syntax:
while command
do
Statement(s) to be executed if command is true
done
• 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.

4. while loop
#!/bin/sh
a=0
while [ $a -lt 10 ]
do
echo $a
a=`expr $a + 1`
done
example display the numbers zero to nine
0
1
2
3
4
5
6
7
8
9
example result
• Each time this loop executes, the variable a is checked to see
whether it has a value that is less than 10.
• If the value of a is less than 10, this test condition has an exit
status of 0.
• In this case, current value of a is displayed, then a is incremented
by 1.

5. while loop
#!/bin/bash
COUNTER=0
while [ $COUNTER -lt 10 ]; do
let COUNTER+=1
done
example
#!/bin/bash
while read line; do
echo $line
done < /etc/passwd
example

6. while loop
$ vi while.sh
#!/bin/bash
echo –n “Enter a number: ”; read x
sum=0
i=1
while [ $i –le $x ]; do
sum=$[sum+i]
i=$[i+1]
done
echo “the sum of the first $x numbers is: $sum”
example

7. while loop
$ vi menu.sh
#!/bin/bash
clear ; loop=y
while [ “$loop” = y ] ;
do
echo “Menu”; echo “====”
echo “D: print the date”
echo “W: print the users who are currently log on.”
echo “P: print the working directory”
echo “Q: quit.”
echo
read –s choice # silent mode: no echo to terminal
case $choice in
D | d) date ;;
W | w) who ;;
P | p) pwd ;;
Q | q) loop=n ;;
*) echo “Illegal choice.” ;;
esac
echo
done
example

8. until loop
until loop will execute the loop while the expression evaluates to false
while loop is perfect to execute a set of commands while some condition is true.
until loop is when u need to execute a set of cmds until a condition is true.
• Similar to the while structure.
• until structure loops until the condition is true. “until this condition is true, do this”.
Syntax:
until command
do
Statement(s) to be executed until command is true
done
• If the resulting value is false, given statement(s) are executed.
• If command is true then no statement would be not executed and program would jump
to the next line after done statement.

9. until loop
#!/bin/sh
a=0
until [ ! $a -lt 10 ]
do
echo $a
a=`expr $a + 1`
done
example
0
1
2
3
4
5
6
7
8
9
example result

10. until loop
#!/bin/bash
COUNTER=10
until [ $COUNTER -lt 1 ]; do
let COUNTER-=1
done
example

11. until loop
$ vi countdown.sh
#!/bin/bash
echo “Enter a number: ”; read x
echo ; echo Count Down
until [ “$x” -le 0 ]; do
echo $x
x=$(($x –1))
sleep 1
done
echo ; echo GO !
example

12. for loop
for loop
Operates on lists of items. used when you are looping through a range of
variables. It repeats a set of commands for every item in a list.
statements are executed with var set to each value in the list
Syntax:
for var in word1 word2 ... wordN
do
Statement(s) to be executed for every word.
done
• 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, value of the variable var is set to the next word in
the list of words, word1 to wordN.

13. for loop
#!/bin/sh
for var in 0 1 2 3 4 5 6 7 8 9
do
echo $var
done
example span list numbers
0
1
2
3
4
5
6
7
8
9
example result

14. for loop
#!/bin/sh
for FILE in $HOME/.bash*
do
echo $FILE
done
example display all the files starting with .bash and available in your home. executing from my root
/root/.bash_history
/root/.bash_logout
/root/.bash_profile
/root/.bashrc
example result

15. for loop
#!/bin/bash
let sum=0
for num in 1 2 3 4 5
do
let “sum = $sum + $num”
done
echo $sum
example

16. for loop
#!/bin/bash
for i in $(ls); do
echo $i
done
example
#!/bin/bash
for i in $(seq 10); do
echo $i
done
example

17. for loop
#!/bin/bash
for x in paper pencil pen
do
echo “The value of variable x is: $x”
sleep 1
done
$vi for1.sh
#!/bin/bash
for x
do
echo “The value of variable x is: $x”
sleep 1
done
$ for1.sh arg1 arg2
The value of variable x is: arg1
The value of variable x is: arg2
if the list part is left off, var is set to each parameter passed to the script ( $1, $2, $3,…)
example

18. for loop
$ vi old.sh
#!/bin/bash
# Move the command line arg files to old directory.
if [ $# -eq 0 ] #check for command line arguments
then
echo “Usage: $0 file …”
exit 1
fi
if [ ! –d “$HOME/old” ]
then
mkdir “$HOME/old”
fi
echo The following files will be saved in the old directory:
echo $*
for file in $* #loop through all command line arguments
do
mv $file “$HOME/old/”
chmod 400 “$HOME/old/$file”
done
ls -l “$HOME/old”
example

19. for loop
$ vi args.sh
#!/bin/bash
# Invoke this script with several arguments: “one two three“
if [ ! -n “$1” ]; then
echo “Usage: $0 arg1 arg2 ..." ; exit 1
fi
echo ; index=1 ;
echo “Listing args with ”$*”:”
for arg in “$*” ;
do
echo “Arg $index = $arg”
let “index+=1” # increase variable index by one
done
echo “Entire arg list seen as single word.”
echo ; index=1 ;
echo “Listing args with ”$@”:”
for arg in “$@” ; do
echo “Arg $index = $arg”
let “index+=1”
done
echo “Arg list seen as separate words.” ; exit 0
example

20. C-like for loop
An alternative form of for structure is
for (( EXPR1 ; EXPR2 ; EXPR3 ))
do
statements
done
• First, the arithmetic expression EXPR1 is evaluated.
• EXPR2 is then evaluated repeatedly until it evaluates to 0.
• Each time EXPR2 is evaluates to a non-zero value, statements are executed and EXPR3
is evaluated.
$ vi for2.sh
#!/bin/bash
echo –n “Enter a number: ”; read x
let sum=0
for (( i=1 ; $i<$x ; i=$i+1 )) ; do
let “sum = $sum + $i”
done
echo “the sum of the first $x numbers is: $sum”

21. select loop
select loop (fuction is like a for loop with menu selection)
Easy way to create a numbered menu from which users can select options.
Useful for asking the user to choose one or more items from a list of choices.
Syntax:
select var in word1 word2 ... wordN
do
Statement(s) to be executed for every word.
done
• 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.
• For every selection a set of commands would be executed with-in the loop.
loop was introduced in ksh and has been adapted into bash. It is not available in sh.

22. select loop
#!/bin/bash
select DRINK in tea cofee water juice appe all none
do
case $DRINK in
tea|cofee|water|all)
echo "Go to canteen"
;;
juice|appe)
echo "Available at home"
;;
none)
break
;;
*) echo "ERROR: Invalid selection"
;;
esac
done
example let the user select a drink of choice (A)

23. select loop
$./test.sh
1) tea
2) cofee
3) water
4) juice
5) appe
6) all
7) none
#? juice
Available at home
#? none
$
example result

24. select loop
$PS3="Please make a selection => " ; export PS3
$./test.sh
1) tea
2) cofee
3) water
4) juice
5) appe
6) all
7) none
Please make a selection => juice
Available at home
Please make a selection => none
$
example alternate let the user select a drink of choice (B)
change the prompt displayed by the select loop by altering the variable PS3

25. Nesting while Loops
while loop as part of the body of another while loop
Syntax:
while command1 ; # this is loop1, the outer loop
do
Statement(s) to be executed if command1 is true
while command2 ; # this is loop2, the inner loop
do
Statement(s) to be executed if command2 is true
done
Statement(s) to be executed if command1 is true
done

26. Nesting while Loops
#!/bin/sh
a=0
while [ "$a" -lt 10 ] # this is loop1
do
b="$a"
while [ "$b" -ge 0 ] # this is loop2
do
echo -n "$b "
b=`expr $b - 1`
done
echo
a=`expr $a + 1`
done
example add another countdown loop inside the loop used to count to nine

27. Nesting while Loops
0
1 0
2 1 0
3 2 1 0
4 3 2 1 0
5 4 3 2 1 0
6 5 4 3 2 1 0
7 6 5 4 3 2 1 0
8 7 6 5 4 3 2 1 0
9 8 7 6 5 4 3 2 1 0
example result
echo -n option let echo to avoid printing a new line character

28. Loop Control
Loop control is needed to stop a loop or skip iterations of the loop
two statements used to control shell loops
break statement
continue statement
The infinite Loop
• loops have a limited life and they execute once the condition is false or true
• loop may continue forever due to required condition is not met.
A loop that executes forever without terminating executes an infinite number of times.
Is called infinite loops.
#!/bin/sh
a=10
while [ $a -lt 10 ]
do
echo $a
a=`expr $a + 1`
done

29. Break statement
Break statement
Used to terminate the execution of the entire loop, after completing the execution
of all of the lines of code up to the break statement. It then goes to the code
following the end of the loop.
Syntax:
break
break command can also be used to exit from a nested loop using this format:
break n #n specifies the nth enclosing loop to exit from
.

30. Break statement
#!/bin/sh
a=0
while [ $a -lt 10 ]
do
echo $a
if [ $a -eq 5 ]
then
break
fi
a=`expr $a + 1`
done
example loop would terminate as soon as a becomes 5
0
1
2
3
4
5
example result

31. Break statement
$ vi break.sh
#!/bin/bash
LIMIT=19
echo
echo “Printing Numbers 1 through 20, but something happens after 2 … ”
a=0
while [ $a -le “$LIMIT” ]
do
a=$(($a+1))
if [ “$a” -gt 2 ]
then
break
fi
echo -n “$a ”
done
echo; echo; echo
exit 0
example

32. continue statement
continue statement
Similar to break command
except that causes the current iteration of the loop to exit, rather than entire loop.
That is It causes a jump to the next iteration of the loop, skipping all the remaining
commands in that particular loop cycle
This statement is useful when an error has occurred but you want to try to execute the next
iteration of the loop.
Syntax:
continue
an integer argument can be given to the continue command to skip commands from nested loops:
continue n #n specifies the nth enclosing loop to continue from
.

33. continue statement
#!/bin/sh
NUMS="1 2 3 4 5 6 7"
for NUM in $NUMS
do
Q=`expr $NUM % 2`
if [ $Q -eq 0 ]
then
echo "Number is an even number!!"
continue
fi
echo "Found odd number"
done
example
Found odd number
Number is an even number!!
Found odd number
Number is an even number!!
Found odd number
Number is an even number!!
Found odd number
example result

34. continue statement
$ vi continue.sh
#!/bin/bash
LIMIT=19
echo
echo “Printing Numbers 1 through 20 (but not 3 and 11)”
a=0
while [ $a -le “$LIMIT” ]; do
a=$(($a+1))
if [ “$a” -eq 3 ] || [ “$a” -eq 11 ]
then
continue
fi
echo -n “$a ”
done
example