Putting the contents of /dev/urandom into a variable?

Question

This is a follow on from my previous thread :- Passing date command into a variable?

Ok so effectively I would like /dev/urandom to generate 4 digits for me. And place them into a variable.

Here is an example of my bash script :-

#!/bin/bash
for (( c=0; c<=10; c++))
do
        a="$(tr -dc '[:digit:]' < /dev/urandom | fold -w 4)"

        echo $a
done

The only reason I wish to "Echo" the results is so I can actually see that /dev/urandom has generated 4 digits for me correctly.

For my bash script I wish to make, I need to do this programmatically as I wish to do further operation's on the generated results.

So I need not "Echo" the results. so is a="$(tr -dc '[:digit:]' < /dev/urandom | fold -w 4)"

Actually placing 4 random digits into the variable "a"? when I Echo "a" from my script I just get a hanging console and have to use "CTRL+C" to bring my console back.

All I am trying do is place 4 random digits from /dev/urandom into a variable.

I m a new to bash scripting so all advice welcome and thank you in advanced for any help or suggestions.

Answer 1

If you don't need security, you can use bash's built-in RANDOM variable. To get a random number between 0 and 9999 inclusive, $((RANDOM % 10000)) gives a small bias towards values less than 767, because the values of $RANDOM are uniform in the range [0, 32767]. To get a uniform distribution, and put leading zeros:

while a=$((RANDOM + 10000)); ((a <= 39999)); do :; done
a=${a#?}

Note again that this is not applicable anywhere security is a concern. The output from $RANDOM is not unpredictable.

If you do need security, then /dev/urandom is the right place to get random data. Calling fold is unnecessary on most current unices: you can grab 4 characters with head -c.

a=$(</dev/urandom tr -dc 0-9 | head -c 4)

head -c is not POSIX and not present on OpenBSD, but it's present in other BSD and on Linux (both with GNU coreutils and BusyBox).

If you need portability to systems that don't have head -c, then fold -w 4 | head -n 1 is a good replacement that's POSIX standard. Just note that you do need the head call, otherwise fold will keep outputting lines of 4 digits forever.

Answer 2

fold will just fold the infinite stream of digits and thus a is being defined indefinitely. You will need to add a terminating command, simplest head:

 a="$(tr -dc '[:digit:]' < /dev/urandom | fold -w 4 | head -n1)"

Just compare the bahaviour of these two commands

tr -dc '[:digit:]' < /dev/urandom | fold -w 4  
tr -dc '[:digit:]' < /dev/urandom | fold -w 4 | head -n1

The first one will show the infinite stream of 4-digit random numbers, while the second will abort after one line.