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system administrator

I have recently started to managing FreeBSD server along with Ubuntu Linux. However, sudo command, always shows me the following error:

zsh: command not found: sudo

How do I solve this problem and use sudo under FreeBSD 9.x system?
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Howto: Shutdown Linux System From Terminal

How do I shutdown Linux system from terminal or command line options?
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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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Linux Change Password

How do I change my own password as well as the passwords of other users as required under Linux operating system without using GUI systems? How can I change or set user password on Linux operating systems?
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How do I regenerate OpenSSH sshd server host keys stored in /etc/ssh/ssh_host_* files? Can I safely regenerate ssh host keys using remote ssh session as my existing ssh connections shouldn't be interrupted on Debian or Ubuntu Linux?
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Mac OS X Remove SSH Known Host

I've reinstalled Linux on remote server and now trying to connect it over ssh using Mac OS X:
ssh user@server1.example.com

But I'm getting an error which read as follows:

Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that the RSA host key has just been changed.
The fingerprint for the RSA key sent by the remote host is
Please contact your system administrator.
Add correct host key in /Users/user/.ssh/known_hosts to get rid of this message.
Offending key in /Users/user/.ssh/known_hosts:1
RSA host key for server1.example.com has changed and you have requested strict checking.
Host key verification failed.

How do I fix this problem under Mac OX X?
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I am trying to copy a file called act.dat. I do have enough disk space to copy this file. I am getting an error "file size limit exceeded" under Linux. How do I get rid of this error?
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