To see which users are currently online (i.e. logged into the system), enter the command:
# who
This command will show you all the users who are currently logged into this server.
And the log of successful and not-so-successful logins is located at:
/var/log/auth.log
where, among other things, user logins to the system are recorded, along with various information related to authentication. For example, it logs every use of the "sudo" utility.
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