check /etc/shadow and say if you found correct password or not

check-login.sh

#!/bin/bash
# usefull if you don't remember which password you setted
# you can try misc password and the script will warn you 
# when you found the correct one
# Requirement:
# you need to install "whois" package

if [ "$EUID" -ne 0 ]
  then echo "Please run as root"
  exit
fi

if [ -z $1 ];then
    read -p 'Enter username: ' USERNAME
else
    USERNAME=$1
fi
read -s -p 'Enter Password: ' PASSWORD

line=$(grep "$USERNAME" /etc/shadow)
if [ -z $line ];then
    echo "User $USERNAME does not exist"
    exit 1
fi
format=$(echo $line |  sed "s/^$USERNAME:\$\(.*\)\$\(.*\)\$\([^:]*\).*/\1/")
salt=$(echo $line |  sed "s/^$USERNAME:\$\(.*\)\$\(.*\)\$\([^:]*\).*/\2/")
truepassword=$(echo $line |  sed "s/^$USERNAME:\$\(.*\)\$\(.*\)\$\([^:]*\).*/\3/")

case $format in
    1)  method="md5";;
    2a) method="Blowfish";;
    2y) method="Blowfish";;
    5)  method="sha-256";;
    6)  method="sha-512";;
esac

if [ "$method" = "Blowfish" ];then
  echo "method not supported by this tool"
  exit 1
fi

# depending of system the following command syntax can change
# here its tested for raspbian, try a "mkpasswd --help" to check syntax
encrypted=$(mkpasswd -m$method $PASSWORD $salt | sed "s/.*\$\(.*\)$/\1/")


if [ "$encrypted" = "$truepassword" ];then
    echo -e "\r\nyou found it!"
else
    echo -e "\r\nmissed! try again"
fi