How to generate random numbers under OpenWRT?

Question

With a "normal" (I mean full) Linux distro, it works just fine:

sleep $(echo "$[ ($RANDOM % 10 ) ]")

OK, it waits for about 0-9 seconds.

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

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

and why:

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

So does anyone have a way to generate random numbers under OpenWRT, so I can put it into the "sleep"?

Answer 1

Ash doesn't understand $[] and it's deprecated in Bash. Also, the echo is unnecessary. Use this instead for both Bash and Ash:

sleep $(( $RANDOM % 10 ))

Answer 2

Also consider that Busybox included in OpenWrt does not usually have the $RANDOM compiled in.

You can have it if at compilation time you select the BUSYBOX_CUSTOM and BUSYBOX_CONFIG_ASH_RANDOM_SUPPORT configuration options in the OpenWrt buildroot.

Otherwise, you can use a command like this:

hexdump -n 2 -e \"%u\" /dev/urandom

That generates a number between 0 and 65535. So, if you want to run sleep for 0-9 seconds you can do something like:

sleep $(($(hexdump -n 2 -e \"%u\" /dev/urandom) % 10))

where the $((... % 10)) part is needed for reducing the 0-65535 space to 0-9.

Answer 3

Here's an (unreliable) way to produce an integer between 0 and 9 (inclusive):

sleep $(head -30 /dev/urandom | tr -dc "0123456789" | head -c1)

This reads 30 lines from /dev/urandom, uses tr to discard all characters that aren't digits, and then grabs only the first character of the remaining digits.  This is unreliable insofar as there is a possibility (with very low probability) that /dev/urandom will yield 30 lines that happen to contain no digits.

This can be extended to generate a three-digit number (in the range 0-999) as follows:

$(head -30 /dev/urandom | tr -dc "0123456789" | head -c3)