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

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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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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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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:

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
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
5c:9b:16:56:a6:cd:11:10:3a:cd:1b:a2:91:cd:e5:1c.
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 OS 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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