In Linux


1) Unmounting the unresponsive DVD drive

The newbie states that when he pushes the Eject button on the DVD drive of a server running a certain Redmond-based operating system, it will eject immediately. He then complains that, in most enterprise Linux servers, if a process is running in that directory, then the ejection won’t happen. For too long as a Linux administrator, I would reboot the machine and get my disk on the bounce if I couldn’t figure out what was running and why it wouldn’t release the DVD drive. But this is ineffective.
Here’s how you find the process that holds your DVD drive and eject it to your heart’s content: First, simulate it. Stick a disk in your DVD drive, open up a terminal, and mount the DVD drive:
# mount /media/cdrom
# cd /media/cdrom
# while [ 1 ]; do echo “All your drives are belong to us!”; sleep 30; done
Now open up a second terminal and try to eject the DVD drive:
# eject
You’ll get a message like:
umount: /media/cdrom: device is busy
Before you free it, let’s find out who is using it.
# fuser /media/cdrom
You see the process was running and, indeed, it is our fault we can not eject the disk.
Now, if you are root, you can exercise your godlike powers and kill processes:
# fuser -k /media/cdrom
Boom! Just like that, freedom. Now solemnly unmount the drive:
# eject
fuser is good.

2) Getting your screen back when it’s hosed

Try this:
# cat /bin/cat
Behold! Your terminal looks like garbage. Everything you type looks like you’re looking into the Matrix. What do you do?
You type reset. But wait you say, typing reset is too close to typing reboot or shutdown. Your palms start to sweat—especially if you are doing this on a production machine.
Rest assured: You can do it with the confidence that no machine 2ill be rebooted. Go ahead, do it:
# reset

3)Format A USB Pen Drive
First of all get the USB drive name by issuing fdisk-l command. It will list all the partitions currently available. Suppose it is /dev/sdb1
Once you get got the name then issue mkfs.vfat /dev/sdb1

4)Finding MAC addresses of all devices in LAN
Recently I had to find mac addresses of all servers in the local area network (LAN) for preseeding Debian installations using PXE (I will soon write about it). Finding them is easy with nmap

I used the following command and I had the mac addresses along with their associated IPs of all devices in the LAN. To find mac addresses, nmap must be run as root

nmap -sP 192.168.2.*
Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2010-03-31 12:39 EDT
Host 192.168.2.1 appears to be up.
MAC Address: 00:02:B3:40:E0:AA (Intel)
Host 192.168.2.2 appears to be up.
MAC Address: 00:02:B3:40:E0:A5 (Intel)
Host 192.168.2.3 appears to be up.
MAC Address: 00:02:B3:40:E0:A5 (Intel)
Host 192.168.2.11 appears to be up.
MAC Address: 00:1B:2F:6B:B7:AC (Unknown)
Host 192.168.2.34 appears to be up.
MAC Address: 00:1F:C6:C9:A7:54 (Unknown)
Host 192.168.2.39 appears to be up.
MAC Address: 00:17:A4:93:59:EF (Global Data Services)
Host 192.168.2.50 appears to be up.
MAC Address: 00:1E:8C:04:A5:1F (Unknown)
Host 192.168.2.57 appears to be up.
MAC Address: 00:05:5D:E0:32:DF (D-Link Systems)
Host 192.168.2.71 appears to be up.
MAC Address: 00:03:47:A9:F3:D1 (Intel)
Host 192.168.2.79 appears to be up.
MAC Address: 00:1C:C0:9D:7F:9D (Unknown)
Host 192.168.2.80 appears to be up.
MAC Address: 00:1C:C0:9D:7D:51 (Unknown)
Host 192.168.2.82 appears to be up.
MAC Address: 00:15:58:32:5C:F4 (Foxconn)
…..
…..

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