Generating a POSIX random number? Post: 302977366

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Top Forums Shell Programming and Scripting Generating a POSIX random number? Post 302977366 by Don Cragun on Saturday 16th of July 2016 09:20:44 PM

07-16-2016

Registered User

/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.

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rand

rand(3C)																  rand(3C)

NAME

       rand(), rand_r(), srand() - simple random-number generator

SYNOPSIS

DESCRIPTION

       uses  a	multiplicative,  congruential, random-number generator with period 2^32 that returns successive pseudo-random numbers in the range
       from 0 to 2^15-1.

       can be called at any time to reset the random-number generator to a random starting point.  The generator is initially seeded with a  value
       of 1.

       returns a random number at the address pointed to by the randval parameter.  The seed parameter can be set at any time to start the random-
       number generator at an arbitrary point.

   Note
       The spectral properties of leave a great deal to be desired.  provides a much better, though more elaborate, random-number  generator  (see
       drand48(3C)).

RETURN VALUE

       If seed or randval is NULL, returns 0.  Otherwise, returns a psuedo-random integer.

EXAMPLES

       The following:

	      int x, y;
	      srand(10);
	      x = rand();
	      y = rand();

       would produce the same results as:

	      int x, y, s = 10;
	      x=rand_r(&s);
	      y=rand_r(&s);

WARNINGS

       Users  of  should  note	that  rand_r()	now conforms with POSIX.1c.  The old prototype of is supported for compatibility with existing DCE
       applications only.

SEE ALSO

       drand48(3C), random(3M), thread_safety(5), random(7).

STANDARDS CONFORMANCE

																	  rand(3C)