Re: [Help-bash] $RANDOM excludes value zero when called in a subscript

[Roger Price]

On Tue, 6 Aug 2019, Greg Wooledge wrote:

> First mistake is making us go to a web page to retrive the script.

This is contrary to the advice given in other mailing lists, and is not 
mentioned in the page https://lists.gnu.org/mailman/listinfo/help-bash

> Second mistake:
> declare a HIST3      # Histogram of $RANDOM in sub-script
>
> Pretty sure you meant to use "-a" there, not "a".

Yes, clearly a mistake.

> Third mistake: ... one, using a static name in a publicly writable directory, 
> and two, expecting to be able to execute a program in /tmp, which may or may 
> not be mounted with noexec permissions.

Agreed for production code, but for a quick demonstration, I take the risk.

> OK, now let's take a look at the actual meat of the issue.
>
>      R3=$( $SS )       # $RANDOM in sub-script
>
> So, you're executing this script repeatedly in a tight loop.  You didn't
> seed the random number generator, so it's going to use whatever its
> fallback seeding strategy is, which is not documented, ...
>
> The real stumper is that you didn't find *lots* more issues with the
> randomness, using the default seeding of $RANDOM in a tight loop.  I'm
> amazed you're not just getting the same value every single time.

I'm surprized - I was assuming that $RANDOM is based on an algorithm, not on 
external entropy.  If indeed $RANDOM uses, and can run out of external entropy, 
then why doesn't it stop and wait, or issue an error message?  Is $RANDOM 
purely algorithmic, or not?

> ...then you really need to understand the care and feeding of a linear 
> congruential pseudorandom number generator.

Your terminology makes me suspect that $RANDOM is of the form

 X_{n+1} = (a X_n + b) mod m

with X_0 based on (tv.tv_sec ^ tv.tv_usec ^ getpid ()) or whatever value is 
assigned to RANDOM.

> Whatever you're really doing, you need to design it so that it uses a
> single instance of a PRNG,

I reduced my test script to one single use of $RANDOM and still saw the same 
anomaly, so it's not caused by multiple instances.

> Knowing more about your actual project and why you're trying to farm
> out the $RANDOM stuff would help us help you design it properly.

My use case was originally very simple - pseudo random integers in the interval 
10..20 to drive a sleep command which slows down network intensive activity and 
plays nice with someone else's server. While debugging this I came across the 
anomaly, and having done a lot of simulations with algorithmic RNG's I became 
curious.

I read your message carefully, several times, but I see no explanation for why 
$RANDOM does not issue zero values from a sub-script.

Roger