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I am writing a bash script for a c program, where the program asks for a 4 numerical pin inputs. However, when I wrote the script, the output seems to run in a loop, but it doesn't break where it gets identified as the correct number the program will accept.

#!/bin/bash RANGE=9000 count=${RANDOM:0:4} while [[ "$count" -le $RANGE ]] do number=$RANDOM (( "number %= $RANGE" )) echo $number if [[ "$count" == "$RANGE" ]]; then break fi done

When I run it, I can see some numbers in the output that returned as 2 or 3 digits, instead of 4. So in theory, what I want to do is find a random number that is 4 digits that the program will take, but I don't know what is the random number, so essentially it is a brute force, or just me manually guessing the pin number.

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  • Why are you putting your arithmetic expression in quotes? It's just (( number %= RANGE )); no need for $s or quotes. Commented Sep 6, 2020 at 0:40
  • Anyhow, if you get a 3 digit number that just means the first digit was 0; if you get a 2-digit number, the first two digits are 0. Commented Sep 6, 2020 at 0:41
  • ...btw, it's not clear what you mean by "for a C program" in this context. Commented Sep 6, 2020 at 0:42
  • This is a c program that I am given that asks for a numerical input. I did not write the c program myself. Also, when I typed the arithmetic expression in quotes, I'm still a newbie at bash, but I will take that in consideration for future scripts. Commented Sep 6, 2020 at 0:50

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If all you need is a random 4-digit number, you can do that with:

printf -v number "%04d" $((RANDOM % 10000))

The $RANDOM gives you a random number 0..32767, the % 10000 translates that to the range 0..9999 (not perfectly distributed, but should be good enough for most purposes), and the printf ensures leading zeros are attached to it (so you'll see 0042 rather than 42, for example).

You can test it with the following script:

(( total = 0 )) (( bad = 0 )) for i in {1..10000} ; do printf -v x "%04d" $((RANDOM % 10000)) (( total += 1 )) [[ "${x}" =~ ^[0-9]{4}$ ]] || { echo Bad ${x}; (( bad += 1 )); } done (( good = total - bad )) echo "Tested: ${total}, bad ${bad}, good ${good}"

which should give you:

Tested: 10000, bad 0, good 10000
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4 Comments

Insofar as the OP is trying to make guesses at a number, it probably wouldn't do to not be able to guess combinations where the first digit is 0.
BTW, consider printf -v x '%04d' "$x" instead of the command substitution; makes the code bash-specific, but much faster (and it's already bash-specific anyhow).
Good points, @Charles, I've bought them on-board with another slight improvement that combines the random generation with the printf. I don't think being bash-specific is an issue since the question is tagged bash.
Thank you for help, that cleared up some confusion about the scripting. I should've known that was the case with the numbers showing as 2 digits, when I was pretty sure I only asked for a certain range of numbers.

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