Shell Programming and Scripting

Can someone explain to me please why the scale=3 fails to give me an answer to 3 decimal places in the first example of calling bc below but succeeds in the second?

I know I can use a printf modifier to get the answer to 3 decimal places but I'd like to understand what's going on with bc.

Thanks.

Code:

start=$(date +%s.%N)
sleep 1
stop=$(date +%s.%N)

timeTaken=$(echo "scale=3; $stop - $start" | bc)
echo "timeTaken = $timeTaken"
# timeTaken = 1.004179356

num1="100"
num2="3"
result=$(echo "scale=3; $num1 / $num2" | bc)
echo "result = $result"
# result = 33.333

scale is implementation dependent, meaning: it affects "some" operations. Algorithms also look at length - the number of significant digits in the operands.

That is why 10 /3 got scaled and the other one did not.

bc was/is a front end for another program, dc.
This was true in earlier versions of HPUX and BSD. The source (dc and bc @Bell Labs) worked that way.

Although bc is "POSIX" there are NO standards about how to implement scale or ibase, or obase; just slightly what bc -l entails, and not even what functions or values are declared. The only positive standard is bc -l sets scale to default 20.

Not much help, huh.

Sometimes you have to do a pointless calculation to get "bc" to round.
In this example I am dividing by one:

Code:

timeTaken=$(echo "scale=3; ($stop - $start)/1" | bc)

Try it. It works on my version of "bc".

Thanks for the information guys.

Methyl - dividing by 1 also results in the answer being given to 3 decimal places with my version of bc.

Am I correct in thinking that for portability people just have to assume that bc's scale (to reduce precision) does not actually work, so if a result needs to be given at a certain (reduced) precision they have to use a printf modifier or such like?

Jim - when you say bc is "POSIX" does that mean there is a list of programs that are guaranteed to be on POSIX compliant OSs? If so where would I find the list? I just did a search and couldn't find anything. Or are the "" around POSIX meant to say - strictly unofficial? Is there an unofficial list?

Cheers.

Yes: bc

Thanks Radoulov.

Would you guys say that all OS's which fall in the mostly POSIX-compliant category (Eg. Linux, OpenBSD, etc.) have all of those utilities installed? So it is not necessary when writing a bash script to have code that checks if bc, sed, tr, grep, and so on are installed on the system?

Cheers.

PS. Link to all the POSIX-compliant utilities, scroll down to 'Utilities': Shell & Utilities: Table of Contents

Yes, unless the system has been modified ...
Be aware though that most operating systems usually provide more than one implementation of those utilities
(for example POSIX grep vs. standard Solaris grep).
So in order to access the POSIX versions of the above mentioned programs
on some systems (Solaris comes to mind), you'll need to set your PATH:

Code:

PATH=`getconf PATH`:$PATH export PATH

or (if you're already in a POSIX compliant shell):

Code:

PATH=$(getconf PATH):$PATH export PATH