SHELL PROGRAMMING PART 5

Looping Statements | Shell Script

Looping Statements in Shell Scripting: There are total 2 looping statements which can be used in bash programming
1. while statement
2. for statement
To alter the flow of loop statements, two commands are used they are,
1. break
2. continue
Their descriptions and syntax are as follows:
 while statement
Here command is evaluated and based on the result loop will executed, if command raise to false then loop will be terminated
Syntax
 while command

 do

 Statement to be executed

done
 for statement
The for loop operate on lists of items. It repeats a set of commands for every item in a list.
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.
Syntax

 for var in word1 word2 ...wordn

 do
  Statement to be executed

done
Example Programs
for loops iterate through a set of values until the list is exhausted:

for.sh
#!/bin/sh
for i in 1 2 3 4 5
do
echo "Looping ... number $i"
done

Try this code and see what it does. Note that the values can be anything at all:

for2.sh
#!/bin/sh
for i in hello 1 * 2 goodbye
do
 echo "Looping ... i is set to $i"
done

This is well worth trying. Make sure that you understand what is happening here. Try it without the * and grasp the
idea, then re-read the Wildcards section and try it again with the * in place. Try it also in different directories, and
with the * surrounded by double quotes, and try it preceded by a backslash (*)
In case you don't have access to a shell at the moment (it is very useful to have a shell to hand whilst reading this
tutorial), the results of the above two scripts are:
Looping .... number 1
Looping .... number 2
Looping .... number 3
Looping .... number 4
Looping .... number 5
and, for the second example:
Looping ... i is set to hello
Looping ... i is set to 1
Looping ... i is set to (name of first file in current directory)
... etc ...
Looping ... i is set to (name of last file in current directory)
Looping ... i is set to 2
Looping ... i is set to goodbye
So, as you can see, for simply loops through whatever input it is given, until it runs out of input.
While Loops
while loops can be much more fun! (depending on your idea of fun, and how often you get out of the house... )

while.sh
#!/bin/sh
INPUT_STRING=hello
while [ "$INPUT_STRING" != "bye" ]
do
echo "Please type something in (bye to quit)"
read INPUT_STRING
echo "You typed: $INPUT_STRING"
done

What happens here, is that the echo and read statements will run indefinitely until you type "bye" when prompted.
Review Variables - Part I to see why we set INPUT_STRING=hello before testing it. This makes it a repeat
loop, not a traditional while loop.

The colon (:) always evaluates to true; whilst using this can be necessary sometimes, it is often preferable to use a
real exit condition. Compare quitting the above loop with the one below; see which is the more elegant. Also think
of some situations in which each one would be more useful than the other:
while2.sh
#!/bin/sh
while :
do
echo "Please type something in (^C to quit)"
read INPUT_STRING
echo "You typed: $INPUT_STRING"
done

Another useful trick is the while read f loop. This example uses the case statement, which we'll cover later. It
reads from the file myfile, and for each line, tells you what language it thinks is being used. Each line must end
with a LF (newline) - if cat myfile doesn't end with a blank line, that final line will not be processed.

while3a.sh
#!/bin/sh
while read f
do
case $f in
 hello) echo English ;;
howdy) echo American ;;
gday) echo Australian ;;
bonjour) echo French ;;
"guten tag") echo German ;;
*) echo Unknown Language: $f
;;
esac
done < myfile

On many Unix systems, this can also be done as:

while3b.sh
#!/bin/sh
while f=`line`
do
.. process f ..
done < myfile
But since the while read f works with any *nix, and doesn't depend on the external program line, the former
is preferable. See External Programs to see why this method uses the backtick (`).
Had I referred to $i (not $f) in the default ("Unknown Language") case above, there would have been no warnings
or errors, even though $i has not been declared or defined. For example:
$ i=THIS_IS_A_BUG
$ export i
$ ./while3.sh something
Unknown Language: THIS_IS_A_BUG
$
So make sure that you avoid typos. This is also another good reason for using ${x} and not just $x - if x="A" and
you want to say "A1", you need echo ${x}1, as echo $x1 will try to use the variable x1, which may not exist,
or may be set to "B2," or anything else unexpected.
I recently found an old thread on Usenet which I had been involved in, where I actually learned more ... Google has
it here..

A handy Bash (but not Bourne Shell) tip I learned recently from the Linux From Scratch project is:
mkdir rc{0,1,2,3,4,5,6,S}.d
instead of the more cumbersome:
for runlevel in 0 1 2 3 4 5 6 S
do
mkdir rc${runlevel}.d
done
And this can be done recursively, too:
$ cd /
$ ls -ld {,usr,usr/local}/{bin,sbin,lib}
drwxr-xr-x 2 root root 4096 Oct 26 01:00 /bin
drwxr-xr-x 6 root root 4096 Jan 16 17:09 /lib
drwxr-xr-x 2 root root 4096 Oct 27 00:02 /sbin
drwxr-xr-x 2 root root 40960 Jan 16 19:35 usr/bin
drwxr-xr-x 83 root root 49152 Jan 16 17:23 usr/lib
drwxr-xr-x 2 root root 4096 Jan 16 22:22 usr/local/bin
drwxr-xr-x 3 root root 4096 Jan 16 19:17 usr/local/lib
drwxr-xr-x 2 root root 4096 Dec 28 00:44 usr/local/sbin
drwxr-xr-x 2 root root 8192 Dec 27 02:10 usr/sbin

We will use while loops further in the Test and Case sections.
Example 1:
Implementing for loop with break statement
filter_none
brightness_4
#Start of for loop

for a in 1 2 3 4 5 6 7 8 9 10

do
 # if a is equal to 5 break the loop

if [ $a == 5 ]

then

break

fi

# Print the value

echo "Iteration no $a"

done

Output
$bash -f main.sh
Iteration no 1
Iteration no 2
Iteration no 3
Iteration no 4
Example 2:
Implementing for loop with continue statement
filter_none
brightness_4
for a in 1 2 3 4 5 6 7 8 9 10

do

# if a = 5 then continue the loop and

# don't move to line 8

 if [ $a == 5 ]

then

continue

fi

echo "Iteration no $a"

done

Output
$bash -f main.sh
Iteration no 1
Iteration no 2
Iteration no 3
Iteration no 4
Iteration no 6
Iteration no 7
Iteration no 8
Iteration no 9
Iteration no 10
Example 3:
Implementing while loop
filter_none
brightness_4
a=0
# -lt is less than operator

#Iterate the loop until a less than 10

while [ $a -lt 10 ]

do

# Print the values

echo $a

# increment the value

a=`expr $a + 1`

done

Output:
$bash -f main.sh
0
1
2
3
4
5
6
7
8
9
Note: Shell scripting is a case-sensitive language, which means proper syntax has to be followed while writing the scripts.

Example
Here is a simple example of loop nesting. Let's add another countdown loop inside the loop that you used to count
to nine −

#!/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

This will produce the following result. It is important to note how echo -nworks here. Here -n option lets echo
avoid printing a new line character.
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
Here is a simple example that uses the while loop to display the numbers zero to nine −

#!/bin/sh

a=10

until [ $a -lt 10 ]

do

echo $a
 a=`expr $a + 1`

done

This loop continues forever because a is always greater than or equal to 10and it is never less than 10.

The break Statement

The break statement is 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 steps down to the code following the end of the loop.

Syntax
The following break statement is used to come out of a loop −
break

The break command can also be used to exit from a nested loop using this format −
break n

Here n specifies the nth enclosing loop to the exit from.

Example
Here is a simple example which shows that loop terminates as soon as abecomes 5 −

#!/bin/sh

a=0

while [ $a -lt 10 ]
do

echo $a

if [ $a -eq 5 ]

then

break

fi

a=`expr $a + 1`

done

Upon execution, you will receive the following result −

0
1
2
3
4
5

Here is a simple example of nested for loop. This script breaks out of both loops if var1 equals 2 and var2 equals
0−

#!/bin/sh

for var1 in 1 2 3

do

for var2 in 0 5
 do

if [ $var1 -eq 2 -a $var2 -eq 0 ]

then

break 2

else

echo "$var1 $var2"

fi

done

done

Upon execution, you will receive the following result. In the inner loop, you have a break command with the
argument 2. This indicates that if a condition is met you should break out of outer loop and ultimately from the
inner loop as well.
1 0
1 5

The continue statement

The continue statement is similar to the break command, except that it causes the current iteration of the loop
to exit, rather than the entire loop.

This statement is useful when an error has occurred but you want to try to execute the next iteration of the loop.

Syntax
continue
Like with the break statement, an integer argument can be given to the continue command to skip commands from
nested loops.
continue n

Here n specifies the nth enclosing loop to continue from.

Example
The following loop makes use of the continue statement which returns from the continue statement and starts
processing the next 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

Upon execution, you will receive the following result −

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

he shell performs substitution when it encounters an expression that contains one or more special characters.

Example
Here, the printing value of the variable is substituted by its value. Same time, "\n" is substituted by a new line −
#!/bin/sh

a=10

echo -e "Value of a is $a \n"

You will receive the following result. Here the -e option enables the interpretation of backslash escapes.
Value of a is 10
Following is the result without -e option −
Value of a is 10\n

The following escape sequences which can be used in echo command −

Sr.No. Escape & Description

1
\\

backslash

2 \a

alert (BEL)

3 \b

backspace

4 \c

suppress trailing newline

5 \f

form feed
 6 \n

new line

7 \r

carriage return

8 \t

horizontal tab

9 \v

vertical tab

You can use the -E option to disable the interpretation of the backslash escapes (default).
You can use the -n option to disable the insertion of a new line.