Ah! I missed that $$ part of the command. You are right, that would
have solved it too. Anyway %N is even better so I'll stick to the
uploaded variant.
// Ola
On Fri, May 24, 2013 at 08:48:03PM -0600, Bob Proulx wrote:
> Hi Ola,
>
> Ola Lundqvist wrote:
> > > RANDOM=$(dd if=/dev/urandom count=1 2> /dev/null | cksum | awk
> > > '{print$1}')
> > >
> > > Splitting on whitespace is more resilient to input differences than
> > > splitting on each space character. IMNHO using awk for field
> > > selection is almost always better than using cut. The variations in
> > > 'wc' output is a good example of where awk works as desired but cut
> > > would not.
> >
> > Ah, I see. So if we happend to get a tab here cut would fail. In that case
> > Checked it and you are right.
> >
> > ola@quartz:~/svn/fsp/cron-apt$ echo "1204231524"$'\t'"512"
> > 1204231524 512
> > ola@quartz:~/svn/fsp/cron-apt$ echo "1204231524"$'\t'"512" | awk '{print$1}'
> > 1204231524
> > ola@quartz:~/svn/fsp/cron-apt$ echo "1204231524"$'\t'"512" | cut -d' ' -f1
> > 1204231524 512
> >
> > I think we should use awk as well. Good then I'll do so.
>
> Yes. This is also just a general style of defensive shell scripting.
> For example 'wc' notoriously has different output on different
> systems. On some systems it has leading spaces and on others it does
> not. And having experienced problems with differing whitespace from
> various utilities I like to avoid being sensitive to them.
>
> > > Since this has come up for discussion here it gives me an opportunity
> > > to cringe once again at using /dev/*random and the unportability of
> > > it and perhaps suggest using awk instead. In my own scripts I have
> > > been using portable awk for generating random numbers. Therefore I
> > > would suggest this instead and eliminate the use of /dev/*random.
> > > Noting that in Debian this works with either mawk or gawk.
> > >
> > > RANDOM=$(awk -v rs=$RUNSLEEP -v s=$$$(date +%M%S)
> > > 'BEGIN{srand(s);print(int(rs*rand()));}')
> >
> > I do not really think this is a good approach. The reason is that you
> > initialize the random vector with the same minute and second every day.
> > Which means that you will end up in no randomness at all.
>
> Your point about "$(date +%M%S)" being the same because this runs from
> cron at the same time every day is a good one and one that I had
> missed. In my application that isn't true but I failed to see that
> point when suggesting it here. My bad.
>
> However that is only half of the seed. The "$$" is the other part of
> the seed value. That will be different every run. Each time the
> script is run it will get a different process id.
>
> That is why I had originally had both $$ and $(date ...) so that
> either one or the other would be different each time.
>
> > > Command line test case:
> > >
> > > $ awk -v rs=3600 -v s=$$$(date +%M%S)
> > > 'BEGIN{srand(s);print(int(rs*rand()));}'
> > > 1194
> >
> > This is easy to see if you run the above command fast like this:
> > ola@quartz:~/svn/fsp/cron-apt$ awk -v rs=3600 -v s=$$$(date +%M%S)
> > 'BEGIN{srand(s);print(int(rs*rand()));}'
> > 82
> > ola@quartz:~/svn/fsp/cron-apt$ awk -v rs=3600 -v s=$$$(date +%M%S)
> > 'BEGIN{srand(s);print(int(rs*rand()));}'
> > 82
> > ola@quartz:~/svn/fsp/cron-apt$ awk -v rs=3600 -v s=$$$(date +%M%S)
> > 'BEGIN{srand(s);print(int(rs*rand()));}'
> > 555
> > ola@quartz:~/svn/fsp/cron-apt$ awk -v rs=3600 -v s=$$$(date +%M%S)
> > 'BEGIN{srand(s);print(int(rs*rand()));}'
> > 555
>
> In the command line testing run the $$ is always the same process id.
> It is the command shell's process id. But in a script that part would
> always be different. In normal use in a script the seed would always
> be different.
>
> > So thanks for the suggestion, but I'll stick to /dev/*random where
> > we know that we will get real random data.
>
> That is fine of course. :-)
>
> > I can use this variant in case /dev/urandom does not exist...
> > Actually I will do that. However I will change +%M%S to +%N to have
> > better randomness in it.
>
> Good plan. GNU date used in Debian includes %N. But in my application
> I need it to run on HP-UX and IBM AIX and neither of those provide %N.
>
> > Changes are uploaded now.
>
> I am sure that whatever you ended up using is fine. :-)
>
> Bob
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