Backup an entire hard disk using "dd" command

The "dd" command is one of the original Unix utilities and should be in everyone's tool box. It can strip headers, extract parts of binary files and write into the middle of floppy disks; it is used by the Linux kernel Makefiles to make boot images. It can be used to copy and convert magnetic tape formats, convert between ASCII and EBCDIC, swap bytes, and force to upper and lower case.

For blocked I/O, the dd command has no competition in the standard tool set. One could write a custom utility to do specific I/O or formatting but, as dd is already available almost everywhere, it makes sense to use it.

Like most well-behaved commands, dd reads from its standard input and writes to its standard output, unless a command line specification has been given. This allows dd to be used in pipes, and remotely with the rsh remote shell command.

Unlike most commands, dd uses a keyword=value format for its parameters. This was reputedly modeled after IBM System/360 JCL, which had an elaborate DD 'Dataset Definition' specification for I/O devices.

Using "dd" you can create backups of an entire harddisk or just a parts of it. This is also useful to quickly copy installations to similar machines. It will only work on disks that are exactly the same in disk geometry, meaning they have to the same model from the same brand.

Full Hard Disk copy
dd if=/dev/hdx of=/dev/hdy
dd if=/dev/hdx of=/path/to/image
dd if=/dev/hdx | gzip > /path/to/image.gz

Hdx could be hda, hdb etc. In the second example gzip is used to compress the image if it is really just a backup.

Restore Backup of hard disk copy
dd if=/path/to/image of=/dev/hdx

gzip -dc /path/to/image.gz | dd of=/dev/hdx

MBR backup

In order to backup only the first few bytes containing the MBR and the partition table you can use dd as well.

dd if=/dev/hdx of=/path/to/image count=1 bs=512

MBR restore

dd if=/path/to/image of=/dev/hdx

Add "count=1 bs=446" to exclude the partition table from being written to disk. You can manually restore the table.

Another popular tools are: "Clonezilla, Mondo Rescue"

Regards,

Nishith N.Vyas

Zombie Process Understanding.

A zombie process is a process that has completed execution but still has an entry in the process table, allowing the process that started it to read its exit status.

When a process ends, all of the memory and resources associated with it are deallocated so they can be used by other processes.

However, the process's entry in the process table remains. The parent is sent a SIGCHLD signal indicating that a child has died; the handler for this signal will typically execute the wait system call, which reads the exit status and removes the zombie.

The zombie's process ID and entry in the process table can then be reused. However, if a parent ignores the SIGCHLD, the zombie will be left in the process table.

In some situations this may be desirable, for example if the parent creates another child process it ensures that it will not be allocated the same process ID.

If you have zombie processes it means those zombies have not been waited for by their parent.

To remove zombies from a system, the SIGCHLD signal can be sent to the parent manually, using the kill command. If the parent process still refuses to reap the zombie, the next step would be to remove the parent process. When a process loses its parent, init becomes its new parent. Init periodically executes the wait system call to reap any zombies with init as parent.

How to find "zombie" process in Linux ?

Execute "top" command & read top left corner to check zombie process. If you are unable to identify the process, press "z" & you will get "red" colored identification for easy understanding.

How to kill "zombie" process in Linux ?

Run this command.
ps aux | awk '{ print $8 " " $2 }' | grep -w Z

Output would be,

Z 3456
Z 2107
Z 1708

Use "Kill command" for all three processes as given below.

kill -9 3456
kill -9 2107
kill -9 1708

That's it. Enjoy Linux.

Commands to check "Disk Usage" in Linux

'du' = Finding the "disk usage"

du
Typing the above at the prompt gives you a list of directories that exist in the current directory along with their sizes. The last line of the output gives you the total size of the current directory including its subdirectories. The size given includes the sizes of the files and the directories that exist in the current directory as well as all of its subdirectories. Note that by default the sizes given are in kilobytes.

du /home/nishith
The above command would give you the directory size of the directory /home/nishith

du -h
This command gives you a better output than the default one. The option '-h' stands for human readable format. So the sizes of the files / directories are this time suffixed with a 'k' if its kilobytes and 'M' if its Megabytes and 'G' if its Gigabytes.

du -ah
This command would display in its output, not only the directories but also all the files that are present in the current directory. Note that 'du' always counts all files and directories while giving the final size in the last line. But the '-a' displays the filenames along with the directory names in the output. '-h' is once again human readable format.

du -c
This gives you a grand total as the last line of the output. So if your directory occupies 100MB the last 2 lines of the output would be

100M .
100M total

The first line would be the default last line of the 'du' output indicating the total size of the directory and another line displaying the same size, followed by the string 'total'. This is helpful in case you this command along with the grep command to only display the final total size of a directory as shown below.

du -ch | grep total
This would have only one line in its output that displays the total size of the current directory including all the subdirectories.

du -s
This displays a summary of the directory size. It is the simplest way to know the total size of the current directory.

du -S
This would display the size of the current directory excluding the size of the subdirectories that exist within that directory. So it basically shows you the total size of all the files that exist in the current directory.

du --exclude=mp3
The above command would display the size of the current directory along with all its subdirectories, but it would exclude all the files having the given pattern present in their file names. Thus in the above case if there happens to be any mp3 files within the current directory or any of its subdirectories, their size would not be included while calculating the total directory size.

'df' = Finding the "disk free" space

df
Typing the above, outputs a table consisting of 6 columns. All the columns are very easy to understand. Remember that the 'Size', 'Used' and 'Avail' columns use kilobytes as the unit. The 'Use%' column shows the usage as a percentage which is also very useful.

df -h
Displays the same output as the previous command but the '-h' indicates human readable format. Hence instead of kilobytes as the unit the output would have 'M' for Megabytes and 'G' for Gigabytes.
Example :

I have my Linux installed on /dev/hda1 and I have mounted my Windows partitions as well (by default every time Linux boots). So 'df' by default shows me the disk usage of my Linux as well as Windows partitions. And I am only interested in the disk usage of the Linux partitions. This is what I use :

$ df -h | grep /dev/sda1 | cut -c 41-43

This command displays the following on my machine

78%

Please Note: You can find your drive letter by typing "fdisk -l / df -kh" command line.



Thanks,
Nishith N.Vyas

Kickstart installation guide for CentOS 5.5

This guide explain how to install and configure kickstart server for network based deployments of CentOS, from an NFS share.

The instructions should work the same on RedHat and Fedora.

Requirement:

* CentOS 5.5 DVD
* Static IP address for the Kickstart/DHCP server
* /data partition or any other

Installation Steps:

1. Login to the CentOS server using Root account.

2. Mount the CentOS DVD. Command would be : mount /dev/cdrom /media

3. Move to the CentOS RPM folder inside the DVD: cd /media/CentOS

4. Run the command bellow to install the TFTP-Server:
rpm -ivh xinetd-2.3.14-10.el5.i386.rpm
rpm -ivh tftp-server-0.49-2.el5.centos.i386.rpm

(If you get dependency error, download all necessary packages using "yum")

5. Run the command bellow to install the DHCP server:
rpm -ivh dhcp-3.0.5-23.el5.i386.rpm

6. Create new folder for the Kickstart server:
mkdir -p /data/kickstart

7. Edit using "vi", the file /etc/xinetd.d/tftp and change the following settings:
From:
disable = yes To: disable = no
From:
server_args = -s /tftpboot To: server_args = -s /data/kickstart

8. Run the command bellow to start the TFTP server:
/sbin/service xinetd start

9. Run the command bellow to start the TFTP server run at startup:
chkconfig xinetd on

10. Edit using "vi", the file /etc/dhcpd.conf and add the following lines:
ddns-update-style none;
allow bootp;
allow booting;
subnet 10.1.1.0 netmask 255.255.255.0 {
option routers 10.1.1.254;
option domain-name-servers 10.1.1.2;
next-server 10.1.1.1;
filename "pxelinux.0";
range dynamic-bootp 10.1.1.200 10.1.1.210;
}Note 1: Replace 10.1.1.0 with the correct network ID.
Note 2: Replace 255.255.255.0 with the correct subnet mask.
Note 3: Replace 10.1.1.254 with the correct default gateway.
Note 4: Replace 10.1.1.1 with the Kickstart server IP address.
Note 5: Replace 10.1.1.200 with the first IP of the DHCP pool.
Note 6: Replace 10.1.1.210 with the last IP of the DHCP pool.
Note 7: Replace 10.1.1.2 with the correct DNS server.

11. Start the DHCP server
service dhcpd start or /etc/init.d/dhcpd start

12. Run the command bellow to start the DHCP server run at startup:
chkconfig dhcpd on

13. Copy Boot Files
cp /usr/lib/syslinux/{pxelinux.0,menu.c32,memdisk,mboot.c32,chain.c32} /data/kickstart

14. Create a folder for the PXE menu files:
mkdir -p /data/kickstart/pxelinux.cfg

15. Move to the CentOS DVD root folder:
cd /media

16. Copy vmlinuz and initrd.img from the DVD to the images directory:
cp /media/images/pxeboot/{vmlinuz,initrd.img} /data/kickstart/images

17. Create the CentOS DVD structure:
cp -r CentOS /data/kickstart/
cp -r isolinux /data/kickstart/
cp -r repodata /data/kickstart/
cp -r images /data/kickstart/

18. Create using "vi", the file /data/kickstart/pxelinux.cfg/default with the following content:
default menu.c32
prompt 0
MENU TITLE PXE Menu
LABEL CentOS
MENU LABEL CentOS
KERNEL images/vmlinuz
append initrd=images/initrd.img vga=normal network ks=nfs:10.1.1.1:/data/kickstart/ks.cfg textNote: Replace 10.1.1.1 with the Kickstart server IP address.
19. Create an unattended installation script /data/kickstart/ks.cfg

Note: Make sure the file starts with the following lines:
install
nfs --server=10.1.1.1 --dir=/data/kickstartNote 1: Replace 10.1.1.1 with the Kickstart server IP address.

Note: Make sure the lines beginning with “cdrom” and “url” does not exist on the file.

Note: To review ks.cfg file options, see the link:
http://www.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/5.4/html/Installation_Guide/s1-kickstart2-options.html

20. Edit using "vi", the file /etc/exports and add the following line:
/data/kickstart *(ro,no_root_squash)

21. Start the NFS service:
service portmap start
service nfs start
chkconfig nfs on

That' it.

Alert "Disk Usage" on your email id

Hello,

Below shell script shows "Hard Disk Usage" on your email address on regular basis.

Step:1

Make a file & named "disk-alert" & copy below contents in it.

######################################################################

# set admin email so that you can get email

ADMIN="abc@xyz.com" (Mention Your Mail Id Here)

# set alert level 90% is default (Set Usage Level as per the need)

ALERT=90

df -H | grep -vE '^Filesystem|tmpfs|cdrom' | awk '{ print $5 " " $1 }' | while read output;

do

#echo $output

usep=$(echo $output | awk '{ print $1}' | cut -d'%' -f1 )

partition=$(echo $output | awk '{ print $2 }' )

if [ $usep -ge $ALERT ]; then

echo "Running out of space \"$partition ($usep%)\" on $(hostname) as on $(date)" |

mail -s "Alert: Almost out of disk space $usep" $ADMIN

fi

done

######################################################################

Step:2

Edit "crontab" & make below setting.

crontab -e

59 23 * * * /root/script/diskalert

Save & Exit (:wq)

Step:3

service crond start

service sendmail start

chkconfig crond on

chkconfig sendmail on

Please Note : You can use any other mail service in your network as per the availability.

Conclusion:

This shell script will give you "Disk Usage" output on the mentioned email id on daily basis @ 23 Hrs:59 Minutes

Note: Thanks to "cyberciti.biz" to make such a nice effort to make my post more powerful.