How to generate random numbers under OpenWRT?

Question

With a "normal" (i mean "full") linux distro, it works just fine:

sleep $(echo "$[ ($RANDOM % 9 ) ]")

ok, it waits for about 0-9 sec

but under OpenWRT [not using bash, rather "ash"]:

$ sleep $(echo "$[ ($RANDOM % 9 ) ]") sleep: invalid number '$[' $

and why:

$ echo "$[ ($RANDOM % 9 ) ]" $[ ( % 9 ) ] $

So does anyone has a way to generate random numbers under OpenWRT, so i can put it in the "sleep"?

Thank you

Answer 1

You might try something like this:

sleep `head /dev/urandom | tr -dc "0123456789" | head -c1`

Which works on my WhiteRussian OpenWRT router.

I actually don't know if this will always return a number, but when it does, it will always return 0-9, and only 1 digit (you could make it go up to 99 if you made the second head -c2).

Good luck!

Answer 2

you could also use awk

sleep $(awk 'BEGIN{srand();print int(rand()*9)}')

Answer 3

For some scenarios, this might not yield a sufficient diversity of answers. Another approach is to use /dev/urandom directly (eg https://www.2uo.de/myths-about-urandom/):

echo $(hexdump -n 4 -e '"%u"' </dev/urandom)

When using awk, note that awk uses the time of day as the seed (https://linux.die.net/man/1/awk). This might be relevant for scenarios where the time of day is reset (eg no battery backed time of day clock), or synchronised across a fleet (eg group restart).

srand([expr])

Uses expr as a new seed for the random number generator. If no expr is provided, the time of day is used. The return value is the previous seed for the random number generator.

This is confirmed by looking at the source in busybox (https://github.com/mirror/busybox/blob/master/editors/awk.c):

seed = op1 ? (unsigned)L_d : (unsigned)time(NULL);

At least for some versions of Openwrt, it seems an explicit call to srand() is required to avoid obtaining the same answers repeatedly:

# awk 'BEGIN{print rand(), rand()}'
0 0.345001

# awk 'BEGIN{print rand(), rand()}'
0 0.345001