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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LEARN ABOUT NETBSD

shuffle

SHUFFLE(1)						    BSD General Commands Manual 						SHUFFLE(1)

NAME

     shuffle -- print a random permutation of the command line arguments

SYNOPSIS

     shuffle [-0] [-f filename ...] [-n number] [-p number] [arg] [...]

DESCRIPTION

     The shuffle program prints a random permutation (or ``shuffle'') of its command line arguments.  This can be useful in shell scripts for
     selecting a random order in which to do a set of tasks, view a set of files, etc.

     If the -f option is given, the data is taken from that files' contents or if the filename is - ``stdin''.

     If the -n option is given, its argument is treated as a number, and the program prints a random permutation of the numbers greater than or
     equal to 0 and less than the argument.

     If the -p option is given, its argument is treated as a number, and the program prints that number of randomly selected lines or arguments in
     a random order.

     The -0 option changes the field separator character from 
 to , so that the output is suitable to be sent to xargs(1) (to handle filenames
     with whitespace in them).

EXAMPLES

	   $ shuffle a b c d
	   c
	   b
	   d
	   a
	   $ shuffle -p 1 a b c d
	   d
	   $ shuffle -n 4 -p 2
	   0
	   3

SEE ALSO

     jot(1), random(6)

HISTORY

     The shuffle program first appeared in NetBSD 1.4.

AUTHORS

     Written by Perry E. Metzger <perry@piermont.com>.

BSD
								 February 18, 2009							       BSD