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Bash Shell: Check File Exists or Not

How do I test existence of a text file in bash running under Unix like operating systems?
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HowTo: Use cat Command In Linux / UNIX

How do I use cat command in Linux? How do I use cat command in UNIX? How can I use cat command in UNIX or Linux shell scripts?
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Samba Restrict IPC$ ( list of shared resources )

The IPC$ share allows users to anonymously fetch a list of shared resources from a server. It can be used as a point of attack into a system. How do I disable or limit IPC$ under Samba to certain subnet such as 10.0.0.0/8?
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UNIX / Linux: Explain The Nine Permissions Bits On Files

Q. I’ve installed Postgresql under Red Hat Enterprise Linux 5.x server. I’ve created username / password and database. But when I try to connect it via PHP or psql using following syntax:

psql -d myDb -U username -W

It gives me an error that read as follows:

psql: FATAL: Ident authentication failed for user “username”

How do I fix this error?
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Display Apache Server Status with mod_status

Q. How do I display Apache server status such as current server activity and performance under Red Hat Enterprise Linux / UNIX / BSD?
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What Would Cause a Lot of ARP Broadcasts From One System / Machine

Q. I’m experiencing decreased network performance due to ARP broadcasts coming from one system. What would cause a lot of ARP broadcasts from one server / computer?
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Windows Displaying the Contents of DNS

Q. How do I display the contents of my DNS client resolver cache under Windows server 2000 / 2003 / 2008 / XP / Vista operating systems?
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Linux Copy Command Examples

How do I copy files under Linux operating systems?
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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:

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@ 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 OX X?
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