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Top Forums Shell Programming and Scripting Generating a POSIX random number? Post 302977390 by wisecracker on Sunday 17th of July 2016 08:40:24 AM
Old 07-17-2016
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:
 

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RAND(3) 						   BSD Library Functions Manual 						   RAND(3)

NAME
rand, rand_r, srand, sranddev -- bad random number generator LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <stdlib.h> int rand(void); int rand_r(unsigned *seed); void srand(unsigned seed); void sranddev(void); DESCRIPTION
These interfaces are obsoleted by random(3). The rand() function computes a sequence of pseudo-random integers in the range of 0 to RAND_MAX (as defined by the header file <stdlib.h>). The srand() function sets its argument seed as the seed for a new sequence of pseudo-random numbers to be returned by rand(). These sequences are repeatable by calling srand() with the same seed value. If no seed value is provided, the functions are automatically seeded with a value of 1. The sranddev() function initializes a seed, using the random(4) random number device which returns good random numbers, suitable for crypto- graphic use. The rand_r() function provides the same functionality as rand(). A pointer to the context value seed must be supplied by the caller. SEE ALSO
random(3), random(4) STANDARDS
The rand() and srand() functions conform to ISO/IEC 9899:1990 (``ISO C90''). The rand_r() function is as proposed in the POSIX.4a Draft #6 document. BSD
May 25, 1999 BSD
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