Generating a POSIX random number?


Login or Register for Dates, Times and to Reply

 
Thread Tools Search this Thread
Top Forums Shell Programming and Scripting Generating a POSIX random number?
# 1  
Generating a POSIX random number?

Hi Guys and gals...

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

# 2  
/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:
# 3  
Quote:
Originally Posted by Don Cragun
/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.
# 4  
Twothree comments:
  • 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:

Code:
TMP=$(awk ' BEGIN {printf( "%d %d\n", 16 * rand(), 16 * rand() )}')
byteone=${TMP% *}; bytetwo=${TMP#* }


Last edited by RudiC; 07-17-2016 at 09:35 AM..
# 5  
Quote:
Originally Posted by RudiC
Twothree comments:
  • 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:

Code:
TMP=$(awk ' BEGIN {printf( "%d %d\n", 16 * rand(), 16 * rand() )}')
byteone=${TMP% *}; bytetwo=${TMP#* }

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...
# 6  
I assume MBP stands for MacBook Pro?

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.

Last edited by RudiC; 07-17-2016 at 09:46 AM..
# 7  
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

Results:-
Code:
Last login: Sun Jul 17 13:31:28 on ttys000
AMIGA:barrywalker~> cd Desktop/Code/Shell
AMIGA:barrywalker~/Desktop/Code/Shell> ./RAND2.sh
13 6
13 6
13 6
13 6
AMIGA:barrywalker~/Desktop/Code/Shell> ./RAND2.sh
13 6
13 6
13 6
13 6
AMIGA:barrywalker~/Desktop/Code/Shell> ./RAND2.sh
13 6
13 6
13 6
13 6
AMIGA:barrywalker~/Desktop/Code/Shell> _

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

Results:-
Code:
AMIGA:barrywalker~/Desktop/Code/Shell> ./RAND2.sh
12 13
12 13
12 13
12 13
AMIGA:barrywalker~/Desktop/Code/Shell> ./RAND2.sh
3 9
1 15
7 6
13 12
AMIGA:barrywalker~/Desktop/Code/Shell> ./RAND2.sh
11 14
11 14
11 14
11 14
AMIGA:barrywalker~/Desktop/Code/Shell> ./RAND2.sh
3 2
1 9
14 7
12 6
AMIGA:barrywalker~/Desktop/Code/Shell> _

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:
Login or Register for Dates, Times and to Reply

Previous Thread | Next Thread
Thread Tools Search this Thread
Search this Thread:
Advanced Search

Test Your Knowledge in Computers #733
Difficulty: Medium
AmigaBASIC is a complied BASIC programming language implementation for the Amiga, designed and written by Microsoft.
True or False?

9 More Discussions You Might Find Interesting

1. OS X (Apple)

Generate a random number in a fully POSIX compliant shell, 'dash'...

Hi all... Apologies for any typos, etc... This took a while but it didn't beat me... Although there are many methods of generating random numbers in a POSIX shell this uses integer maths and a simple C source to create an executable to get epoch to microseconds accuracy if it is needed. I take... (8 Replies)
Discussion started by: wisecracker
8 Replies

2. Shell Programming and Scripting

Generating a Random String of 'n' length

Hi, How can I generate a string of random characters (alpha+numeric) of a particular length ? For e.g. for n=5, output = 'kasjf' n=10, output = 'hedbcd902k' Also, please let me know if random (valid) dates could also be generated. Thanks (7 Replies)
Discussion started by: rishigc
7 Replies

3. Shell Programming and Scripting

Random number generating script?

Having a hard time with this. Very new to scripting and linux. Spent all sunday trying to do this. Appreciate some help and maybe help breaking down what the syntax does. Create a Bash program. It should have the following properties • Creates a secret number between 1 and 100 i. The... (3 Replies)
Discussion started by: LINUXnoob15
3 Replies

4. Shell Programming and Scripting

Generating Random Number in certain range

Hi there I am trying to generate a random number between 40 and 70 using the shell here is my code so far and it keeps going above 70. all help much appreciated! comp=$(( RANDOM%70+40 )) echo $comp (4 Replies)
Discussion started by: faintingquiche
4 Replies

5. Programming

Generating Random Number in Child Process using Fork

Hello All, I am stuck up in a program where the rand functions ends up giving all the same integers. Tried sleep, but the numbers turned out to be same... Can anyone help me out how to fix this issue ? I have called the srand once in the program, but I feel like when I call fork the child process... (5 Replies)
Discussion started by: manisum
5 Replies

6. Programming

C Help; generating a random number.

Im new to C, and Im having a hard time getting a random number. In bash, I would do something similar to the following to get a random number; #!/bin/bash seed1=$RANDOM seed2=$RANDOM seed3=$RANDOM SEED=`expr $seed1 * $seed2 / $seed3` echo ${SEED%.*} Now, in online examples... (4 Replies)
Discussion started by: trey85stang
4 Replies

7. Programming

generating 16 digit random number in C

Hi, How can we generate 16 digit random nos in C. (10 Replies)
Discussion started by: ajaysahoo
10 Replies

8. Shell Programming and Scripting

Generating random numbers

Hi, I am having trouble with generating random numbers. can this be done with awk? So I have a file that looks like this: 23 30 24 40 26 34 So column1 is start and column2 is end. I want to generate 3 random #'s between start and stop: So the output will look like this: ... (9 Replies)
Discussion started by: phil_heath
9 Replies

9. Shell Programming and Scripting

Generating random number within a specific range (0.5-1.5)

Hello, need a way to generate numbers within 0.5-1.5 range Has to be totally random: 0.6 1.1 0.8 1.5 0.6 and so on.... How to? (10 Replies)
Discussion started by: TehOne
10 Replies

Featured Tech Videos