Bug#695285: /etc/cron.daily/apt backup RNG is very wasteful

[Michael Vogt <mvo@debian.org>]

On Thu, Jan 10, 2013 at 02:44:42AM -0500, sacrificial-spam-address@horizon.com wrote:
> > Thanks, I fixed this now in my bzr tree and it will be part of the
> > next upload.
> 
> Er, assuming that's revision 2269:
> http://anonscm.debian.org/loggerhead/apt/debian-sid/revision/2269/debian/apt.cron.daily

Great, thanks for double checking.
 
> You don't need the final "cut -c1-5".  It is true that cksum returns an
> unsigned 32-bit number, and some shells only do 32-bit signed math, but
> they simply convert the number to negative.
> 
> You either mask off the sign bit with
> RANDOM=$(($(dd if=/dev/urandom bs=2 count=1 2> /dev/null | cksum | cut -d' ' -f1) % 0x7fffffff))
> 
> Or, of you want to emulate the range of Bash $RANDOM exactly:
> RANDOM=$(($(dd if=/dev/urandom bs=2 count=1 2> /dev/null | cksum | cut -d' ' -f1) % 0x7fff))

Thanks, indeed, simulating this more closely is probably a good idea,
I added that in my bzr branch now.

> The "od -d -N2 -An" solution works with a recent enough od.
> I was looking to see how old that can be.  The uppercase option flags
> are not in 7th edition, but are in POSIX.1 as of 2001:
[..]

Thanks for checking this. I like this, its more compact than the
current code. However the reason why I did not use this, was that the
following fragment:

$ while true; do res=$(od -N2 -d /dev/urandom | cut -s -d' ' -f2); echo
$res; if [ -z "$res" ]; then break;  fi ; done
61581
42056
38021

$ 

give me a empty string in $res sometimes. I haven't really put any
though into why this is, maybe just a side effect of running it in th
while loop?

Cheers,
 Michael