bash "let" barfs on addition

[Matthew Flaschen]

Evuraan::ഏവൂരാന്‍ wrote:

> I know, I can use bc,dc,awk, python and even a desk calculator  wide
> enough :) - but, I am curious as to why is "let" letting us down?

First of all, it is nothing specific to let, as you can see from:

echo $((7540113804746346429+4660046610375530309))

Second, if you use the source (apt-get source, or just look at
http://www.opensource.apple.com/darwinsource/Current/bash-76.2/bash/expr.c)
you can see:

"All arithmetic is done as intmax_t integers with no checking for
overflow (though division by 0 is caught and flagged as an error)."

In this case, intmax_t is obviously a 64-bit signed two's complement
integer.  Your some exceeds that bound, hence the overflow into a
negative.  Now, if you're wondering "How come that's the limit?  Where
is it specified?" it /isn't/.  Math is not in the POSIX shell
specification
(http://www.unixinfo.org/books/ProUnixShellProg/ProUnixShellProg-Posix.html),
so bash considers itself free to provide as much or as little as it
likes.

You are right to use bc if you crave more.  It is /documented/
(http://www.opengroup.org/onlinepubs/000095399/utilities/bc.html) to
support arbitrary precision.

Matt Flaschen