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I'm learning to script in bash, trying to solve a little exercise. The exercise is this:

If the variable named "basenew" contains the contents of the variable named "valuebase". "basenew" must contain more than 113,469 characters. If both conditions are met, the script must then print the last 20 characters of the variable "basenew".

My code is

#!/bin/bash

basenew="8dm7KsjU28B7v621Jls"
valuebase="ERmFRMVZ0U2paTlJYTkxDZz09Cg"


for i in {1..40}
do
    basenew=$(echo $basenew | base64)


if [[ $basenew =~ $valuebase && ${#basenew} -ge 113469 ]] ; then
    echo $i
    echo $basenew | wc -c
    StrLen=`echo ${basenew} | wc -c`
    From=`expr $StrLen - 20`
    echo $basenew | cut -c ${From}-${StrLen}
else
    echo "error"

fi



done

But I'm stuck, because it prints in the 28th iteration, and it is the 20 last, but it isn't the correct answer.

Any advice to print the last 20 characters using tail -c 20?

Thanks

terdon
  • 242,166
joop
  • 11
  • 1
  • fix your indentation so we (and you) can actually see what the script does. 2) you're already using ${#basenew}, so why echo $basenew | wc -c too? 3) look into the substring expansion ${var:pos:count}, so you don't need the cut. Also arithmetic expansion $(( ... )), no need for expr. 4) quote the variable expansions, see When is double-quoting necessary? and http://mywiki.wooledge.org/WordSplitting
    • – ilkkachu Aug 17 '21 at 08:36
  • The script you show will always print error since you are just repeating the exact same comparison 48 times: if [[ $basenew =~ $valuebase && ${#basenew} -ge 113469 ]]. Your variables $basenew and $valuebase never change value, so this is probably not the actual script you are asking about. Please [edit] your question and show us the actual script you are using that "prints in the loop 28". – terdon Aug 17 '21 at 12:22
  • @terdon i fix it, the basenew is in a for loop, that encode to base64 40 times – joop Aug 17 '21 at 14:25
  • OK, that's better, but you are telling us "it is the wrong output" and you don't show us what the right output would be and you don't show us the wrong output you get. What is wrong with the current version of your script? Why is U2paTlJYTkxDZz09Cg== the wrong output? – terdon Aug 17 '21 at 14:34
  • @terdon because in the exercise i have a form to put the correct answer but it fails when i put that output. Is rare because it match both if.

    For the use of "tail -c 20" i need to save the output inside a .txt?

     – joop Aug 17 '21 at 14:40
  • To get the base64 encoding of the contents of a variable, that should rather be b64=$(printf %s "$var" | base64 -w0). echo adds a newline character, mangles backslashes and/or some values starting with - (though the last two won't occur here). – Stéphane Chazelas Aug 17 '21 at 14:46
  • Another thing here is that your script has the loop over those 40 iterations, but the problem description you presented doesn't contain any mention of that. The script adds another level of Base64 encoding on the value of basenew on each iteration, but we don't see the description of how that should be done and can't check if the script is doing it correctly. Remember: it's your exercise, pretty much no-one else can know what exactly should be done, so you might want to take care to tell the details correctly and in full. – ilkkachu Aug 17 '21 at 15:21