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We use a linux (centOS I believe) cluster for our research. My professor somehow forgot the new password he set for root, and now can't login as 'su'. What is the way (or best way) to reset the root password without damaging something. I don't know whether it's even setup for 'sudo'. What will the procedure with and without 'sudo'?
The typical method is to boot into single-user mode. I'm not 100% sure that Redhat/Centos lets you do this without a password, but its worth a try.
Start up the computer. When you see the grub menu, then hit any key to stop the count, then:
With the desired entry higlighted, hit "e" for edit.
Select the kernel line, and "e" again
Add the word "single" (no quotes) at the end
hit return (enter), then "b" to boot
The machine should come up in a terminal mode, root account, with no password needed.
Assuming all this works, enter "passwd root", the new password, and then "init 5" to go back to normal. (Or maybe "init 3" with a server?)
If this does not work, the only options I know is to boot the system from live CD, mount the partition containing /etc, and edit the "passwd" file to remove the first "x" in the root entry. This then allows root to login with no password.
Distribution: Gentoo, RHEL (Fedora, CentOS, OEL), Ubuntu, FreeBSD, Solaris 10
You have to reboot your box:
Select the kernel
Press the e key to edit the entry
Select second line (the line starting with the word kernel)
Press the e key to edit kernel entry so that you can append single user mode
Append the word Single to the end of the (kernel) line
Press ENTER key
Now press the b key to boot the Linux kernel into single user mode