2. Conditional loop
• conditional loop is a way for computer
programs to repeat one or more various
steps depending on conditions set either
by the programmer initially or real-time
by the actual program.
• Basically a loop has a conditional
statement or a command and body of that
loop which has some list of commands or
statements to be executed repeatedly.
3. While Loop
• The while loop is used to execute a set of
commands repeatedly until some
condition occurs.
• It is usually used when the value of a
variable has to be manipulated repeatedly.
5. Steps to execute a while loop
1. Execute command.
2. If the exit status of command is nonzero,
exit from the while loop
3. If the exit status of command is zero,
execute list.
4. When list finishes execution, return to
step 1.
6. For example,
The output looks like,
x=0 0
while [ $x –lt 10 ] 1
do 2
echo $x 3
x=`echo “$x + 1” | bc` 4
done 5
6
7
8
9
7. Example(2)
c=1 Output,
while [ $c -le 5 ] Welcome 1 times
do Welcome 2 times
echo "Welcome $c Welcome 3 times
times" Welcome 4 times
(( c++ )) Welcome 5 times
done
8. Nested while loop
• It is a loop in which a while loop will be a
part of the body of another while loop.
• There is no restrictions for the number of
nested while loops.
• But it will b better to avoid more than 5
nested loops.
9. Syntax is,
while command1 ; # this is loop1, the outer loop
do
list1
while command2 ; # this is loop2, the inner
loop
do
list2
done
list3
done
10. Example,
x=0
while [ "$x" -lt 10 ] ; # this is loop1
do
y="$x"
while [ "$y" -ge 0 ] ; # this is loop2
do
echo "$y c"
y=´echo "$y - 1" | bc´
done
echo
x=´echo "$x + 1" | bc´
done
11. The output will be,
0
10
210
3210
43210
543210
6543210
76543210
876543210
9876543210
12. For loop
• The for loop is used to execute a set of
commands repeatedly for each item in a
list.
• One of its most common uses is in
performing the same set of commands for
a large number of files.
13. The common syntax is,
for name in word1 word2 ... wordN
do
list
done
It can also be written as,
for name in word1 word2 ... wordN ; do list ; done
14. For a simple for loop,
for i in 0 1 2 3 4 5 6 7 8 9 The output is,
do 0
echo $i 1
done 2
3
4
5
6
7
8
9
15. another example,
for i in `cat 1.txt` Output is,
do
echo $i F
done S
T
The contents in 1.txt,
F
S
T
16. Example(3)
alphabet="a b c d e" Output,
count=0 a
for letter in $alphabet
b
do
count=`expr $count + 1` c
echo "Letter $count is d
[$letter]" e
done
17. Command line arguments
• The arguments used to pass to the shell
scripts while interpreting are called
command line arguments.
• $0 indicates the name of the script.
• $1 indicates the 1st argument of that script.
• $2 indicates the 2nd argument.
• $$ used to denote the process ID.
• $# used to count the number of arguments.
• $@ denotes all arguments.
