Generating a POSIX random number? Post: 302977390

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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

07-17-2016

Registered User

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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SHTOOL-TABLE.TMP(1)					      GNU Portable Shell Tool					       SHTOOL-TABLE.TMP(1)

NAME

       shtool-table - GNU shtool pretty-print a field-separated list

SYNOPSIS

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DESCRIPTION

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OPTIONS

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EXAMPLE

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HISTORY

       The GNU shtool table command was originally written by Ralf S.  Engelschall <rse@engelschall.com> in 1999 for GNU shtool.

SEE ALSO

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18-Jul-2008							   shtool 2.0.8 					       SHTOOL-TABLE.TMP(1)

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