Floating point results in Bash integer division

Question

I have a backup script on my server which does cron jobs of backups, and sends me a summary of files backed up, including the size of the new backup file. As part of the script, I'd like to divide the final size of the file by (1024^3) to get the file size in GB, from the file size in bytes.

Since bash does not have floating point calculation, I am trying to use pipes to bc to get the result, however I'm getting stumped on basic examples.

I tried to get the value of Pi to a scale, however,

even though the following works:

~ #bc -l bc 1.06.95 Copyright 1991-1994, 1997, 1998, 2000, 2004, 2006 Free Software Foundation, Inc. This is free software with ABSOLUTELY NO WARRANTY. For details type `warranty'. 4/3 1.33333333333333333333 22/7 3.14285714285714285714 q 0 quit 

A non interactive version does not work:

#echo $(( 22/7 )) | bc 3 

This works:

#echo '22/7' | bc -l 3.14285714285714285714 

But I need to use variables. So it doesnt help that the following does not work:

#a=22 ; b=7 #echo $(( a/b )) | bc -l 3 

I'm obviously missing something in the syntax for using variables in Bash, and could use with some 'pointers' on what I've misunderstood.

As DigitalRoss said, I can use the following:

#echo $a / $b | bc -l 3.14285714285714285714 

However I cant use complex expressions like:

#echo $a / (( $b-34 )) | bc -l -bash: syntax error near unexpected token `(' #echo $a / (( b-34 )) | bc -l -bash: syntax error near unexpected token `(' #echo $a / (( b-34 )) | bc -l -bash: syntax error near unexpected token `(' 

Can someone give me a working correct syntax for getting floating point results with complicated arithmetic expresssions?

Answer 1

Just double-quote (") the expression:

echo "$a / ( $b - 34 )" | bc -l 

Then bash will expand the $ variables and ignore everything else and bc will see an expression with parentheses:

$ a=22 $ b=7 $ echo "$a / ( $b - 34 )"  22 / ( 7 - 34 )  $ echo "$a / ( $b - 34 )" | bc -l -.81481481481481481481