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HowTO: Linux Disable a User Account Command

How do I disable a user account under Linux operating systems? How can I completely disable a user account Linux server?
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Change the Password in UNIX

How do I change the password in UNIX?
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Linux deny or block user login

Q. How do I deny or block user login for user id tom? I’m using CentOS 5 Linux server OS.

A.. You need to use passwd command.

Deny user login by locking out account

Pass -l option to passwd command. It is used to lock the specified account and it is available to root only. The locking is performed by rendering the encrypted password into an invalid string and by prefixing the encrypted string with an !.


passwd -l {username}

Unlock account or allow login

To allow login use passwd command as follows:
passwd -u {username}

This is the reverse of the -l option – it will unlock the account password by removing the ! prefix.

/sbin/nologin shell

/sbin/nologin displays a message that an account is not available and exits non-zero. It is intended as a replacement shell field for accounts that have been disabled or login is blocked.

Example: Deny login for tom user

Type the command as follows (login as root user):
# passwd -l tom
You can also change shell to /sbin/nologin:
# usermod -s /sbin/nologin tom

Example: Allog login for tom user

Type the command as follows (login as root user):
# passwd -u tom
You can also need change back shell from /sbin/nologin to /bin/bash:
# usermod -s /bin/bash tom

For more information and other options read passwd command man page.

Move or migrate user accounts from old Linux server to a new Linux server

Q. How do I Move or migrate user accounts to from old Linux server a new Cent OS Linux server including mails? This new system a fresh installation.

A. You can migrate users from old Linux server to new Linux sever with standard commands such as tar, awk, scp and others. This is also useful if you are using old Linux distribution such as Redhat 9 or Debian 2.x.

Following files/dirs are required for traditional Linux user management:
* /etc/passwd – contains various pieces of information for each user account

* /etc/shadow – contains the encrypted password information for user’s accounts and optional the password aging information.
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Understanding /etc/shadow file

Can you explain /etc/shadow file format used under Linux or UNIX-like system?
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