Matthew Gregan writes: > On Thu, Aug 08, 2002 at 08:54:44PM -0400, Andrew Tarr wrote: > > > > > su -; dd if=/dev/zero of=/dev/hda > > > > > > So what does the above command do, should I try it out and see ;-) > > > 'su' gives you 'superuser' or root priviledges. > > In the case above, what ``su -'' will do is spawn the shell specified > for uid 0 (root) in /etc/passwd as a login shell owned by uid 0. > > Since ``su -'' spawns a shell, ``dd if=/dev/zero of=/dev/hda'' will not > execute until the new shell exits, and will execute with the uid of the > original user *not* uid 0; this means you would not have permission to > overwrite the device unless the original user has write access to the > device, see below for the user=root, group=disk stuff.
Yes, I read your previous message to that effect, and I can see that now. > > > The '-' tells the 'su' program that it's a 'login' shell, not really > > sure what that means in this case. > > See the ``INVOCATIONS'' section of the bash(1) manpage. > > > So 'dd if=/dev/zero of=/dev/hda' writes zeros to the first hard-drive > > of the system. Thankfully, you can only do this as root, hence the su. > > It depends on the permissions of /dev/hda, for example: > > $ ls -la /dev/hda > brw-rw---- 1 root disk 3, 0 Mar 15 10:51 /dev/hda > > If a normal user was in the group ``disk'', in this case, they could > also overwrite the entire device with that ``dd'' command. I was aware of this, just for the record. I just thought it would be clearer if I said 'only root can do this' without going into the details... the exposition of how 'dd' works, what with involving device files and so forth. Andrew.
