1. Login to the console on the machine and type “reboot” or “shutdown -r,” etc, if you have an account with privileges to initiate such an action. If you don’t have an account with suitable privilege, try control-alt-delete (and power off) or hard power-down your machine (you may need to fsck later, but that’s a given), take a deep breath and count to 11 😉
2. Power on the server and wait for the grub boot screen to come up. You’ll may not need the GUI for this to work, but it’s the only way I’ve done it. When the grub boot menu comes up, hit the up or down arrow key at least once to stop the automatic boot countdown timer. If you have multiple boot options, choose the one you know (or believe) is the one currently in use (actually, this shouldn’t matter, but loading up an older kernel might cause issues) and press the “e” key to enter edit mode.
3. After you enter edit mode, you’ll be presented with a few lines of text (dependent on how you have your grub.conf populated). Using your arrow key again, navigate to the line that starts with “kernel.” Press the “e” key again, and your cursor should show up at the end of the “kernel” line (if it doesn’t, you can move it to where you need it by using the left and/or right arrow keys as necessary).
4. Now that you’re in edit mode, and your cursor is in the correct position, type a “space” character followed by “single.” So if your boot command line was:
kernel /boot/vmlinuz-2.6.9-34.ELsmp ro root=/dev/sda1
it would now be:
kernel /boot/vmlinuz-2.6.9-34.ELsmp ro root=/dev/sda1 single
5. Now type “b” to continue the boot process and you’ll be dumped into a limited shell, as root, passwordlessly. Sometimes this has seemed not to work for me if I changed my edit-focus to a line other than the “kernel” line before typing “b”, but that could just be superstition on my part. Thankfully, I don’t have to do this all that often 🙂
6. The rest is gravy. You’re root, so all you have to do is type “passwd”, set the root password to whatever you like and reboot using your preferred method (reboot, shutdown -r, init 6, whatever works, etc). Since you’re in a single-user shell, you can also instantiate a reboot by just typing “exit.”
And you should be all set. Now you no longer have an excuse to avoid fixing problems on that machine (the downside 😉