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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
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UNIX / Linux: Explain The Nine Permissions Bits On Files

Can you completely explain the nine permissions bits on UNIX or Linux files?
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psql: FATAL: Ident authentication failed for user "username" Error and Solution

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:

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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