Can not view man page on CentOS

I have been having issues with viewing man page when I logged in as a regular user. Everytime, I have to add sudo in front of man command. After troubleshooting the problem for a while, I found a solution:

Add execute permission for the following folder and its contents

sudo chmod o+x -R /usr/share/man/

Others also suggest that the $MANPATH is incorrect. However, this is not the case for me.

IPtables: open port for specific IP

The following are examples of allow a host or a range of hosts visit port 8080

sudo vim /etc/sysconfig/iptables

# the following rule specifies only Ip 101.2.3.44 can visit port 8080

-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp -s 101.2.3.44 --dport 8080 -j ACCEPT

# a range of IPs are allowed to visit port 8080

-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp -src-range 101.2.3.44-1.2.3.54 --dport 8080 -j ACCEPT

# restart iptables service to make the change take effect

sudo /etc/init.d/iptables restart

Use sudo

1. when you forget to run a command with sudo, instead of retyping the command you could use

# to run the last command in history with sudo
sudo !!
# or you could use if you prefer
sudo !-1
# run the second to the last command in the history with sudo
sudo !-2 

2. To have an interactive shell as root:

sudo -i
# or
sudo su -

3. when you use vim and forget to type sudo

:w !sudo tee %

A command line tool to record your terminal activity

Script is a simple command line tool to record your terminal activity, including your command history and outputs of the commands, basically everything showed on the terminal screen.
To use it

script [file_name]
#or
script

By type this command, script will create a typecript file at your current directory if you do not provide a file name in the command.
After finished, execute exit to save your command history and outputs.

Obtain IP address of Linux

Two commands used to obtain ip address of linux machine:

#set 1
#displays all cuurentlly active network interfaces
/sbin/ifconfig
# IP address in the output of this command is you IP
/sbin/ifconfig eth0 | grep 'inet addr:'
#set 2
/sbin/ip addr list
/sbin/ip addr list eth0

Restore CentOS top panel

Have you ever messed up top panel of your CentOS?
Here is a way to restore it to it's default.

sudo mv ~/.gconf/apps/panel ~/.gconf/apps/panel.bak

sudo reboot

Disable system beep for CentOS

Here is way to disable the system beep for CentOS

# unloading the pcspkr module: 
rmmod -v pcspkr

In addition, add the following line in /etc/rc.d/rc.local file to automatic disable it during boot.

/sbin/rmmod pcspkr

if you ever want to re-enable the speaker, run the following command:

/sbin/modprobe pcspkr