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Linux / UNIX: See If Particular Users Are Logged Into Server

How do I see if my friends or coworkers are logged into the same Linux or BSD or UNIX (AIX, HP-UX, Apple OS X) server / workstation as I am from a command line?
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Linux /bin/false VS /sbin/nologin: Politely Refuse a Login

How do I deny access to user account? Do I need to use /bin/false or /sbin/nologin to refuse a login?
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Linux Display Date And Time Of Login

How do I display user last login date and time under Linux operating systems?
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SSH System Bootup In Progress Please Wait Error and Solution

Q. I've Debian Linux server and whenever someone trying to ssh into this box, they get an error as follows and server closes the connection:

System bootup in progress -please wait

How do I fix this problem and allow ssh login?
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Reboot Linux System Command

How do I reboot my Linux based system? How do I reboot a remote Linux system or server using command line?
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Linux: Openssh (ssh server) deny root user access

Q. How do I block access to root user over ssh session?

A.. sshd (OpenSSH Daemon) is the daemon program for ssh. Server side ssh configuration is defined in /etc/ssh/sshd_config file.

You need to use DenyUsers option to block access to root user.

This option can be followed by a list of user name patterns, separated by spaces. Login is disallowed for user names that match one of the patterns. Only user names are valid; a numerical user ID is not recognized. By default, login is allowed for all users. If the pattern takes the form USER@HOST then USER and HOST are separately checked, restricting logins to particular users from particular hosts.

Open /etc/ssh/sshd_config file

Use vi command:
# vi /etc/ssh/sshd_config

Deny root user access

Append or modify as follows to block root user:
DenyUsers root

If you want to block additional user just append names to DenyUsers
DenyUsers root, user2, user3

Save and close the file. Restart sshd service:
#/etc/init.d/sshd restart