Help in Shell Scripting

[Mohan1289]

Hi guys i just started Shell scripting today and doing a very basic Shell script but i am not getting the correct output i don't now why please help me.

I am using Ubuntu 12.10

The code i written is:

#!/bin/bash
echo Enter Sides
read l
read b
area='expr $l \* $b'
echo The area of rectangle with sides $l and $b is $area
echo Enter radius
read r
area='expr $r \* $r \22'
area='expr $area / 7'
echo The area of circle with radius $r is $area

The Output i'm getting is:

Enter sides
9
8
The area of rectangle with sides 9 and 8 is expr $l\ *$b
Enter Radius
15
The area of the circle with radius 15 is expr $area / 7

What's wrong in my shell script??

[kjohri]

For substituting the command output in `command`, single back quotes need to be used.


#!/bin/bash
echo Enter Sides
read l
read b
area=`expr $l \* $b`
echo The area of rectangle with sides $l and $b is $area
echo Enter radius
read r
area=`expr $r \* $r`
area=`expr $area \* 22`
area=`expr $area / 7`
echo The area of circle with radius $r is $area

[Lars Noodén]

For substituting the command output in `command`, single back quotes need to be used.


It's even more clear to use $()


area=$(expr $l \* $b)

[Mohan1289]

For substituting the command output in `command`, single back quotes need to be used.


#!/bin/bash
echo Enter Sides
read l
read b
area=`expr $l \* $b`
echo The area of rectangle with sides $l and $b is $area
echo Enter radius
read r
area=`expr $r \* $r`
area=`expr $area \* 22`
area=`expr $area / 7`
echo The area of circle with radius $r is $area


Thanks for the help kiohri

[Mohan1289]

It's even more clear to use $()


area=$(expr $l \* $b)


Can i use like that too instead of back quotes`
Lars Noodén (http://ubuntuforums.org/member.php?u=168643)

[Lars Noodén]

Yes, they're used in about the same way. $() can even be nested if the occasion needs it.

It might be worth skimming through these three guides to see if anything stands out as useful:


http://mywiki.wooledge.org/BashGuide
http://mywiki.wooledge.org/BashFAQ
http://mywiki.wooledge.org/BashPitfalls

[Mohan1289]

Hi guys i am newly joined in Ubuntu forums and i am very beginner of programming. I wish to have strong programming skills in shell scripting and to gain profound knowledge on Ubuntu Operating system and languages.
Please suggest me some programs/projects which may help to improve my knowledge.
I am practical person i learn things by work rather than reading, So can you guys please suggest me what to do....

[Lars Noodén]

The only two additional resources that I can recall just now are these:


http://tldp.org/LDP/abs/html/
http://tldp.org/LDP/Bash-Beginners-Guide/html/


Those with the Wooledge pages linked in the post above ought to be a good start.

One of the places that Ubuntu uses shell scripts is in the System V init scripts. Those are slowly going away to be replaced by Upstart, but those that have not been moved to Upstart can be found in /etc/init.d/

[Mohan1289]

Thanks Lars

[Vaphell]

when using bash expr is not required
you can easily perform integer math inside (())


$ b=10; c=12
$ ((a=b*b/7))
$ echo $a
14
$ a=$(( c*c/7 ))
$ echo $a
20

note that integer division doesn't deal with fractions at all and you need to use bc command in order not to lose precision.


$ a=$( echo "$b*$b/7" | bc -l )
$ echo $a
14.28571428571428571428
$ a=$( echo "$c*$c/7" | bc -l )
$ echo $a
20.57142857142857142857

[Mohan1289]

Thank you Vaphell. I am very New to both Linux and Shell Scripting i wish to learn about Both Scripting and Linux.

Thank you

[Habitual]

These references may come in handy:

http://www.tldp.org/HOWTO/Bash-Prog-Intro-HOWTO.html
http://www.tldp.org/LDP/Bash-Beginners-Guide/html/index.html
http://www.gnu.org/software/bash/manual/html_node/index.html
http://www.grymoire.com/Unix/Sh.html
http://www.tldp.org/LDP/abs/html/
http://mywiki.wooledge.org/BashFAQ
http://mywiki.wooledge.org/BashPitfalls
and
https://help.ubuntu.com/community/CommandLineResources

[Mohan1289]

Thankyou Habitual

[Some Penguin]

Hi guys i am newly joined in Ubuntu forums and i am very beginner of programming. I wish to have strong programming skills in shell scripting and to gain profound knowledge on Ubuntu Operating system and languages.
Please suggest me some programs/projects which may help to improve my knowledge.
I am practical person i learn things by work rather than reading, So can you guys please suggest me what to do....

One very traditional sort of thing to try is building a postfix calculator -- e.g. parsing "5 6 10 * +" in such a way that its execution could be modeled as



+
/ \
5 *
/ \
6 10


and evaluating accordingly. It's commonly used as a very basic introduction to stacks.

[Mohan1289]

er... Sorry guys due to some technical problems i am unable to connect to the internet :D.

[Mohan1289]

One very traditional sort of thing to try is building a postfix calculator -- e.g. parsing "5 6 10 * +" in such a way that its execution could be modeled as



+
/ \
5 *
/ \
6 10


and evaluating accordingly. It's commonly used as a very basic introduction to stacks.

I am sorry i don't understand the problem..
i mean how can display a tree like that??
what's a POST FIX calculator? what does it do?

[Vaphell]

postfix calculator means that the operation to perform is listed AFTER the numbers.
3+4 would be written as 3 4 + (you can read it as 'there are 2 numbers, now add them)

That graph shows the order of operations
5 (6 10 *) +
to calculate the sum you need to multiply first and the graph shows it - * is one of the arguments required to produce the addition.

[Mohan1289]

postfix calculator means that the operation to perform is listed AFTER the numbers.
3+4 would be written as 3 4 + (you can read it as 'there are 2 numbers, now add them)

That graph shows the order of operations
5 (6 10 *) +
to calculate the sum you need to multiply first and the graph shows it - * is one of the arguments required to produce the addition.

So The Answer for the above example is 65?