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Users & Groups Management HowTos

Learn all about users and groups on Linux and Unix like operating systems. Topic include user & group creations, permissions & ownership, access control, ACL, and more ( rss feed ).

Debian / Ubuntu Linux Public key Blacklisted (see ssh-vulnkey(1)) Error and Solution

Q. When ever I try to login to my remote Debian Linux server called in013.example.com, I get the following error message in /var/log/auth.log file:

Jul 1 17:04:36 in013 sshd[14447]: Public key 48:de:55:22:xx:yy:zz:yy:xx:yy:zz:yy::88:e8:87:47 blacklisted (see ssh-vulnkey(1))
Jul 1 17:04:36 in013 sshd[14447]: Public key 48:de:55:22:xx:yy:zz:yy:xx:yy:zz:yy::88:e8:87:47 blacklisted (see ssh-vulnkey(1))

I'm using Ubuntu Linux as desktop operating system. How do I fix this error?
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UNIX / Linux Command To Check Existing Groups and Users

How do I check the existing Linux / UNIX users and groups under Linux operating system?
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Linux Change Default User Home Directory While Adding A New User

By default base directory for the system user is set to /home directory. I'd like to add user to /iscsi/home/${user} directory instead of the default /home. How do I force useradd command to add user to /iscsi/home under CentOS / RHEL / Fedora Linux server?
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Linux sudo Configuration

Q. Can you tell me how to configure sudo to provide access to end users? How do I allow to run programs with the security privileges of another user using sudo?
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Remove Linux User From a Secondary Group ( Supplementary Groups )

Q. User tom is a member of a group called sales and printer. I'd like to remove tom from a group called printer without editing any user configuration text files stored at /etc/ directory?
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FreeBSD Install BASH Shell

I'd like to Bash shell under FreeBSD for bash programming. How do I install bash shell under FreeBSD UNIX operating system?
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Stop Ubuntu / Debian Linux From Deleting /tmp Files on Boot

Q. I know /tmp as it named is a temporary dircory, Debian policy is to clean /tmp at boot. However, I'd like to configure my Ubuntu Server to stop deleting files from /tmp on boot due to custom configuration issue. How do I configure behavior of boot scripts to stop deleting files on boot?
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Linux 64 bit Server

Q. Can you tell me more about Linux 64-bit server distribution for production webhosting environment?
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Linux logged out a user automatically

Q. We have a few users who frequently forget to log out and I'd like to force them or automatically log out a user in case of inactivity detected. How do I force user to be logged out automatically?

A. BASH provides TMOUT variable. It is necessary for security reasons to have an inactive user logged out after a period of inactivity from Linux / UNIX ssh / telnet session. This can be accomplished by setting an environment variable TMOUT.

Automatically log out a user

Add the TMOUT variable to your /etc/bashrc file:
# vi /etc/bashrc
Set TMOUT to 300 seconds (5 minuets):
Save an close the file. Above config would automatically logout users after 300 seconds of inactivity. Please note that this hack only works with run level 2, 3 i.e. it will not work with GUI sessions.

Change MySQL root password

Q. How do I change MySQL server root password under Linux / UNIX operating system at shell prompt?

A. You need to mysqladmin command which is a client for performing administrative operations. You can use it to check the server’s configuration and current status, to create and drop databases, change mysql root password and more.

How do I change mysql root password?

If you are setting a root password first time, use any one of the following command syntax:
mysqladmin password "NEW-PASSWORD"
mysqladmin -u root password "NEW-PASSWORD"
mysqladmin -u root -h localhost password "NEW-PASSWORD"

To set password to d3s9cXq, enter:
# mysqladmin -u root password "d3s9cXq"

Please note that if the NEW-PASSWORD value contains spaces or other characters that are special to your command interpreter, you need to enclose it within quotes. Thus, the next time you invoke mysqladmin (or any other client program such as mysql) using the same account, you will need to specify the new password. So to login as root, enter the command:
# mysql -u root -p
# mysql -h localhost -u root -p


  • -u root: Specify username
  • -p : Prompt for password

If you would like to change current password d3s9cXq to OlzzYs$qz, enter the following command:
# mysqladmin -u root -p password "OlzzYs$qz"

Enter password: <enter old password>