How to know if a user is existing or not on Linux.
$ cat /etc/passwd | grep user_name
To identify the password we don't have a direct method but following process can help.
Example: Trying to find password for "root" user
$ cat /etc/shadow
Get the hash of the corresponding user, in our case "root" user.
Then, use the programs like from the website "hashcat.net(On Kali Linux its available default)" and convert the hash.
Logging: /var/log/auth.log
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