Bash script question

HarryBoy · 08-31-2010, 11:17 AM

Hi,

I'm new to bash scripting,I am doing the following:

echo 'hi'
my_new_no=($((my_old_no))*2.2)
echo $my_new_no

but the output is:
hi
1600028*2.2

I want this to be a 'value' (the result of my calculation)and not a string. How do I do this?

Thanks

alunduil · 08-31-2010, 11:26 AM

I don't remember which utility it is (bc maybe?) but bash by itself is unable to do floating point operations. When I try to run the simple math command: `echo $(( 160028*2.2 ))` I get the following output:

Code:

bash: 160028*2.2 : syntax error: invalid arithmetic operator (error token is ".2 ")

Perhaps perl or awk would be easier to do math with?

Regards,

Alunduil

custangro · 08-31-2010, 11:28 AM

Quote:

Originally Posted by HarryBoy

Hi,

I'm new to bash scripting,I am doing the following:

echo 'hi'
my_new_no=($((my_old_no))*2.2)
echo $my_new_no

but the output is:
hi
1600028*2.2

I want this to be a 'value' (the result of my calculation)and not a string. How do I do this?

Thanks

I THINK that you can only do whole numbers...1, 2, 3 (someone correct me if I'm wrong)

You can use "bc" like so...

Code:

#!/bin/bash
echo 'hi'
my_old_no=3
my_new_no=$( echo "${my_old_no}*2.2" | bc -l)
echo $my_new_no

HarryBoy · 08-31-2010, 11:44 AM

Code:

newno=$( echo "${oldno}*2.2" | bc -1)

gives me:

-bash: bc: command not found

???

Thanks

PTrenholme · 08-31-2010, 11:51 AM

Try replacing the my_new_no=($((my_old_no))*2.2) by my_new_no=$((${my_old_no}*2.2))

custangro · 08-31-2010, 12:02 PM

Quote:

Originally Posted by HarryBoy

Code:

newno=$( echo "${oldno}*2.2" | bc -1)

gives me:

-bash: bc: command not found

???

Thanks

Well it depends on what version on Linux you are using...bc should be standard...try /usr/bin/bc

valen_tino · 08-31-2010, 12:36 PM

@HarryBoy

You should be typing bc -l and not bc -1

jay73 · 08-31-2010, 01:45 PM

declare -i makes a variable an integer.

declare -i my_old_no=...
declare -i my_new_no=$((my_old_no*2.2))

Note: without "declare -i", the calculation will still produce the proper result. That result will be a string but it will look as if it were numerical.

colucix · 08-31-2010, 02:20 PM

If bc is not available, an alternative option to do floating point arithmetic is awk:

Code:

my_new_no=$(awk -v i=$my_old_no 'BEGIN{print i*2.2}')

schneidz · 08-31-2010, 10:02 PM

here's another one:

Code:

[schneidz@hyper ~]$ a=5
[schneidz@hyper ~]$ b=7.5
[schneidz@hyper ~]$ echo $a $b | awk '{c = $1 * $2} END {print " c = " c}'
 c = 37.5

John VV · 08-31-2010, 10:07 PM