Hi all,

I'm trying to implement a linear congruential pseudorandom number generator (<http://en.wikipedia.org/wiki/Linear_congruential_generator>), since $RANDOM and /dev/random aren't standardized. I'm referring to the Shell & Utilities volume of POSIX.1-2008, but I'm running into some odd issues. This works fine:

Code:

#! /bin/sh

rand_x=1

rand()
{
    rand_x=$(((($rand_x * 1103515245) + 12345) % 4294967296))
    echo $rand_x
}

while true; do
    rand
    sleep 2
done

But I want to use the generated integer in an arithmetic expansion, for modular arithmetic (I know it's a sloppy way of limiting the range of values, but it's good enough for my purpose).

I've tried the following, but it simply repeats "1103527590 0" over and over:

Code:

#! /bin/sh

rand_x=1

rand()
{
    rand_x=$(((($rand_x * 1103515245) + 12345) % 4294967296))
    echo $rand_x
}

while true; do
    r=`rand`
    printf '%i %i\n' $r $(($r % 3))
    sleep 2
done

Similarly, this repeats "1103527590":

Code:

#! /bin/sh

rand_x=1

rand()
{
    rand_x=$(((($rand_x * 1103515245) + 12345) % 4294967296))
    echo $rand_x
}

while true; do
    printf '%i\n' `rand`
    sleep 2
done

It looks like, if I try to store or use the output of rand(), either $rand_x is being reset to 1 each time or the arithmetic substitution in rand() isn't being used.

But if I access $rand_x directly, as in the following script, I get the correct sequence:

Code:

#! /bin/sh

rand_x=1

rand()
{
    rand_x=$(((($rand_x * 1103515245) + 12345) % 4294967296))
}

while true; do
    rand
    printf '%i %i\n' $rand_x $(($rand_x % 3))
    sleep 2
done

However I'd like to avoid exposing $rand_x like that (rand() will eventually become a library function). So does anyone have any idea why using echo and capturing/using the output of rand() isn't working?

I'm testing this in GNU Bash and dash on GNU/Linux. When I get this working, I'll test it in more shells.

---------- Post updated at 23:58 ---------- Previous update was at 23:26 ----------

The former seems to be the case. If I echo $rand_x from rand() and just store the output of rand(), $rand_x is reset to 1 for some reason.

The following script illustrates this:

Code:

#! /bin/sh

rand_x=1

rand()
{
    rand_x=$(((($rand_x * 1103515245) + 12345) % 4294967296))
    echo $rand_x >&2
    echo $rand_x
}

while true; do
    r=`rand`
    printf '%i %i\n' $rand_x $(($rand_x % 3))
    sleep 2
done

Note that in rand(), $rand_x is now also printed to standard error (file descriptor 2) so it appears on the terminal even though standard output is stored in $r.

Output (file descriptors 1 and 2):
Anyone have any idea why this is happening?