As you know I am getting to grips with POSIX and hit this stumbling block.
Generating two random numbers 0 to 255 POSIXly. Speed in not important hence the 'sleep 1' command.
I have done a demo that works, but it sure is ugly! Is there a better way?
Code:
#!/bin/sh
# Random numbers POSIX style.
byteone=0
bytetwo=0
while true
do
dd if=/dev/urandom of=/tmp/rand bs=1 count=1 > /dev/null 2>&1
byteone=$( od -tu1 -An < /tmp/rand )
byteone=$( echo $byteone ) # Ignore this line as it simply strips leading whitespaces.
dd if=/dev/urandom of=/tmp/rand bs=1 count=1 > /dev/null 2>&1
bytetwo=$( od -tu1 -An < /tmp/rand )
bytetwo=$( echo $bytetwo ) # Ignore this line as it simply strips leading whitespaces.
echo "$byteone $bytetwo"
sleep 1
done
/dev/urandom is not specified by POSIX. So, besides being ugly, it doesn't really conform to POSIX.
The shell variable RANDOM is not specified by POSIX either, but if you're using ksh or bash, the following code is a LOT faster and simpler:
Code:
#!/bin/sh
while true
do
byteone=$((RANDOM % 255))
bytetwo=$((RANDOM % 255))
echo "$byteone $bytetwo"
sleep 1
done
Note that on many systems, /bin/sh is not a POSIX-conforming shell. And, as you have seen in many of my earlier posts, /bin/awk or /usr/bin/awk might not be a POSIX-conforming awk utility. But as long as /bin/sh is a shell that recognizes Bourne shell syntax, the following usually works on any POSIX-conforming system:
Code:
#!/bin/sh
PATH=`getconf PATH`
export PATH
sh <<-"EOF"
awk '
BEGIN { srand()
while(1)
printf("%d %d\n", 256 * rand(), 256 * rand())
}' | while read byteone bytetwo
do echo "$byteone $bytetwo"
sleep 1
done
EOF
There is other implementation-defined initialization code that is needed on some systems to really set up a POSIX-conforming environment, but the above should work for the minimal features used by this script.
This User Gave Thanks to Don Cragun For This Post:
/dev/urandom is not specified by POSIX. So, besides being ugly, it doesn't really conform to POSIX.
Hmmm, I didn't know that and it looks like there are very many on the WWW that don't either as that was where I got my main info from; that means that /dev/random is not POSIX compliant either. Ouch!
Quote:
The shell variable RANDOM is not specified by POSIX either, but if you're using ksh or bash, the following code is a LOT faster and simpler:
Code:
#!/bin/sh
while true
do
byteone=$((RANDOM % 255))
bytetwo=$((RANDOM % 255))
echo "$byteone $bytetwo"
sleep 1
done
I already have this and Shell Check pointed out that RANDOM is undefined that is why I went the direction that I did.
Quote:
Note that on many systems, /bin/sh is not a POSIX-conforming shell. And, as you have seen in many of my earlier posts, /bin/awk or /usr/bin/awk might not be a POSIX-conforming awk utility. But as long as /bin/sh is a shell that recognizes Bourne shell syntax, the following usually works on any POSIX-conforming system:
Code:
#!/bin/sh
PATH=`getconf PATH`
export PATH
sh <<-"EOF"
awk '
BEGIN { srand()
while(1)
printf("%d %d\n", 256 * rand(), 256 * rand())
}' | while read byteone bytetwo
do echo "$byteone $bytetwo"
sleep 1
done
EOF
There is other implementation-defined initialization code that is needed on some systems to really set up a POSIX-conforming environment, but the above should work for the minimal features used by this script.
Hmm, awk again, why did I not think of that after all I use it a lot for floating point stuff for CygWin...
You are a star Don thanks, consider my ugly non-compliant code scrubbed I am homing in on your 'awk' example...
This POSIX lark is much more difficult than I expected.
As for your final part about 'awk', if one does not use extensions that are not POSIX compliant then surely that particular 'awk' variant IS still technically conforming to the POSIX _environment_?
---------- Post updated at 11:57 AM ---------- Previous update was at 10:02 AM ----------
As an addendum here I decided to make it look a little clearer for me so I put this into Shell Check...
Code:
#!/bin/sh
awk ' BEGIN \
{ srand()
while(1)
printf( "%d %d\n", 16 * rand(), 16 * rand() )
}' | while read -r byteone bytetwo
do
echo "$byteone $bytetwo"
done
And got this _error_:-
Code:
$ shellcheck myscript
Line 2:
awk ' BEGIN \
^-- SC1004: You don't break lines with \ in single quotes, it results in literal backslash-linefeed.
$
Do I take it that Shell Check is correct and the backslash is a no-no even inside a completely different language; the above works perfectly for my needs.
the while(1) inside the awk script will make it generate random numbers until hell freezes, is that needed at all to fill byteone and bytetwo?
Piping into a while loop executing in a subshell will not make the variables available in the parent shell. If you want to use those, a different mechanism is needed.
In some awks, calling srand() is pointless as it preseeds the random generator from the system clock which is done anyway when that awk starts.
So, although some people in these fora rant about one liners - just kidding - you might want to consider this:
the while(1) inside the awk script will make it generate random numbers until hell freezes, is that needed at all to fill byteone and bytetwo?
Piping into a while loop executing in a subshell will not make the variables available in the parent shell. If you want to use those, a different mechanism is needed.
In some awks, calling srand() is pointless as it preseeds the random generator from the system clock which is done when awk starts anyway.
So, although some people in these fora rant about one liners - just kidding - you might want to consider this:
Except that I tried removing while(1) , (I assumed while(true) ) and it disabled the Ctrl-C on this MBP. I could not exit the program.
I will stick up for Don here it worked perfectly for my needs but I will try your method and see how it works, watch this space...
I've got some difficulty to understand why Ctrl-C should be disabled. Anyway, above (post#4) would supply exactly two random bytes, no need to interrupt nor exit.
EDIT:while(true) in awk shouldn't do anything as true is not defined, evaluates to 0 and thus FALSE.
Addendum to Rudis' post:-
MacBook Pro 13 inch, circa August 2012, OSX 10.7.5, default terminal calling OSX's 'sh'.
Test code, not conforming entirely to POSIX!
Code:
#!/bin/sh
for n in {1..4}
do
TMP=$(awk ' BEGIN {printf( "%d %d\n", 16 * rand(), 16 * rand() )}')
byteone=${TMP% *}; bytetwo=${TMP#* }
echo "$byteone $bytetwo"
done
So I assume 'srand()' is needed if only to change the seed per call...
EDIT:
Using 'srand()' and without 'sleep 1' then with 'sleep 1'...
I do test on various *nix style platforms including CygWin.
Code:
#!/bin/sh
for n in {1..4}
do
TMP=$(awk ' BEGIN { srand(); printf( "%d %d\n", 16 * rand(), 16 * rand() ); }')
byteone=${TMP% *}; bytetwo=${TMP#* }
echo "$byteone $bytetwo"
#sleep 1
done
So to conclude the 'srand()' function is only accurate to the _timer's_ second value and not 'milli' or 'micro' second values. Hence the 'sleep 1' to advance the _timer_ by 1 second.
Don's code works with or without 'sleep', hence I can now understand why 'while(1)' is being used.
Last edited by wisecracker; 07-17-2016 at 10:01 AM..
Reason: See EDIT:
Hi all...
Apologies for any typos, etc...
This took a while but it didn't beat me...
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07-16-2016
