Checking kernel & CentOS version

To check which kernel version you are running use uname -r

# uname -r

To find out which CentOS (or RedHat) release is installed have a look at /etc/redhat-release

# cat /etc/redhat-release

Checking Disk Space

To check disk space used / free use the df command and it will display each volume. The -h switch turns the bytes reading into GB to make it more readable.

# df -h

If you want to check the size of a particular directory then use:

# du -sh /

Configuring DNS Settings

To configure which DNS servers your Linux server will use you need to edit /etc/resolv.conf

# vi /etc/resolv.conf

Just add each name server as a new line as in the following example:

nameserver
nameserver

Changing the hostname

You’ll find the hostname of the server in two places: /etc/hosts and /etc/sysconfig/network.

# vi /etc/hosts

and

# vi /etc/sysconfig/network

Show Processor & Memory Usage

This is equivilant to the Windows Task Manager, and will show each process and the consumed CPU and RAM usage. Very useful!

# top

Show Processor Information

This command will display the processor type and speed.

# cat /proc/cpuinfo

Search command history for old commands you have forgotten!

Use the history command with grep to find previously entered commands.

# history | grep command

Compiling sendmail.mc

# m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf

Updating virtusertable.db from /etc/mail/virtusertable

# makemap hash /etc/mail/virtusertable.db < /etc/mail/virtusertable

Saving IPTABLES to /etc/sysconfig/iptables

# iptables-save > /etc/sysconfig/iptables