[Ilugc] doubt in dd
- From: girishvenkatachalam@xxxxxxxxx (Girish Venkatachalam)
- Date: Wed Oct 11 19:50:37 2006
On Wed, Oct 11, 2006 at 01:57:41PM +0100, pnv kannan wrote:
Hi,
can dd command be used to check the contents of a file?
please clarify
thanks
kannan v
dd stands for "disk dump" just like grep stands for "graphic regular expression
print" and ping stands for "packet internet groper"...
It is mostly used for copying from device files though it is sometimes used to
copy back to them as well. It gives you raw access. For instance, this is
really cool.
dd if=/dev/cdrom of=cd.iso
This will get you the cd ISO. In my linux box, cdrom device is /dev/hdc
Note that since it is raw access there is no need to mount.
It also takes offsets using skip and seek. And you customize how much you want
to read or write with count= and bs= parameters.
I use dd for backing up or restoring my MBR since it is guaranteed to be 512
bytes.
dd if=/dev/hda of=mbr bs=512 count=1
Another thing I do is create memory filesystems known in linux as loopback
device files.
/dev/zero is a special file like /dev/null which gives an inexhaustible string
of zeroes.
So you can create a file of arbitrary size quickly with
#dd if=/dev/zero of=1mfile count=1024 bs=1024\*1024
And later you can mount it using the loopback mechanism and then create a
filesystem on it.,
I have already mailed this before, check the archives.
If you want to create a CD/USB/DVD image file you can use dd.
For instance, my USB flash stick has only 256 MB space. So I do a
# dd if=/dev/zero of=usb-flash bs=1024 count=256\*1024\*1024
Then I
#losetup -f usb-flash
#mkfs.vfat /dev/loop/0(this cud be different in your case)
After that, I can mount the loopback device just like a USB flash disk. Copy
all the files, run grub or syslinux on it and then.
You plug in the USB flash drive and do this.
#dd if=usb-flash of=/dev/sda
You are done. :-)
Or still better send this file to someone else and ask him to make the flash
stick for you.
regards,
Girish
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