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Full Discussion: Generating a POSIX random number?
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
Old 07-16-2016
/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:
wisecracker
 

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sh

sh(1)							      General Commands Manual							     sh(1)

NAME

       sh - Shell, the standard command language interpreter

DESCRIPTION

       [Tru64  UNIX]  Tru64  UNIX  provides two command interpreters with the name sh.	The XCU5.0 and POSIX.2 compliant command interpreter sh is
       available in the file /usr/bin/posix/sh and is described in the sh(1p) reference page.  The Bourne shell,  historically	known  as  sh,	is
       available in the file /usr/bin/sh and is described in the sh(1b) reference page.

       [Tru64 UNIX]  Your initial, or login, shell is determined by your entry in the file /etc/passwd. This file can be changed only by your sys-
       tem administrator. You must use whatever procedures are in place at your location to have this entry changed.

       [Tru64 UNIX]  If available on your system, you may use the passwd -s or the chsh commands to change your login shell.

									  Note

       This option is not available if your site manages passwords through the Network Information Service (NIS) facility.  Check with your system
       administrator.

       [Tru64 UNIX]  Subsequent shells spawned from the initial shell depend on the value in the environment variable BIN_SH.  If this variable is
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       able  is  unset,  the  Bourne shell is started. If this variable is set to any other value, an error is reported and the results are unpre-
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NOTES

       [Tru64 UNIX]  With Tru64 UNIX Version 4.0 the Korn shell, /usr/bin/ksh is the same as the POSIX shell /usr/bin/posix/sh.

RESTRICTIONS

       [Tru64 UNIX]  The file /etc/shells must include entries for both the POSIX shell /usr/bin/posix/sh and the Bourne  shell,  /usr/bin/sh.	If
       this file is incorrect, see your system administrator.

EXAMPLES

       Using  the  Bourne,  Korn,  or  POSIX  shell, to set the variable BIN_SH to use the POSIX/ XCU5.0compliant shell, enter: BIN_SH=xpg4 export
       BIN_SH Using the Bourne, Korn, or POSIX shell, to set the variable BIN_SH to use the SVR4 compliant shell, enter: BIN_SH=svr4 export BIN_SH
       Using the Bourne, Korn, or POSIX shell, to unset the variable BIN_SH, enter: unset BIN_SH Using the C/ shell, to set the variable BIN_SH to
       use the POSIX/XCU5.0 compliant shell, enter: setenv BIN_SH xpg4 Using the C/ shell, to set the variable BIN_SH to use  the  SVR4  compliant
       shell, enter: setenv BIN_SH svr4 Using the C/ shell, to unset the variable BIN_SH, enter: unsetenv BIN_SH

FILES

       User profile.  Contains user information, including the login shell name.  Contains the names of available and permitted shells.

SEE ALSO

       Commands:  csh(1), ksh(1), Bourne shell sh(1b), POSIX shell sh(1p), passwd(1)

       Files:  passwd(4), shells(4)

       Standards:  standards(5)

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