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How do I log into my Linux laptop if I have forgotten both the username and password?

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What Linux flavor is it? Do you know what the boot-loader (Grub or Lilo) is? –  Caleb Aug 1 '11 at 20:43
    
It is Ubuntu 9.04 server, 2.6.28.18. Has GRUB v 0.97 –  abc Aug 1 '11 at 20:44
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2 Answers

up vote 13 down vote accepted

You can drop into single mode from Grub. During boot press Esc on the Grub boot screen when it prompts you to. It may just show you Grub with listings of each kernel - if that's the case don't press Esc.

From here select the first entry and press e to edit that entry. Page down to the line that starts with kernel and press e again.

This will allow you to edit the entire line. Scroll to the right until you reach the end and remove splash quiet from the line, replacing it with single. Press Enter to accept the changes and press b to boot into the modified kernel line. This will boot you into single user mode and should drop you into a root shell once the boot has completed.

From here you can add users to the system, change user passwords, etc.

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+1 A much better answer than what I was going to suggest. –  Nathan Osman Aug 1 '11 at 21:24
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If the above fails -- unlikely if GRUB is password -- boot from an Ubuntu livecd (preferrably 9.04 server), mount your Ubuntu partitions, then chroot into it. You can then issue the passwd command to reset root, your user, etc. –  laebshade Aug 13 '11 at 15:40
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You might do as Marco suggested, but his will not work on all distros. More precisely, it will not work without the need of giving the root password on systems using openrc.

The more generic way, without using any other bootable media, is to append init=/bin/bash or init=/bin/sh to boot options (that's where you'd put single following Marco's answer).

Another option, maybe a last resort, is to boot from a removable media (like livecd), mount the root partition of your system and either chroot to it or do a dirty job on /etc/shadow. By this I mean editing it and removing the password hash field. But it is a dirty (read: dangerous) method: Firstly, /etc/shadow is a vital security file and an error in its structure can create a security breach or just break a lot of things. To prevent errors, it should only be edited using vipw -s command - but this is only possible when you chroot into the system. Other problems could be caused by a conflict/error detected by advanced security mechanisms working on the system. But I haven't used such so far, so cannot tell what those problems might be.

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some dirty job like what?? –  amyassin Aug 12 '11 at 13:21
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I didn't want to write about it in the answer, because of the dirtyness: I meant to remove the password hash, then login with no password and set a new one. –  rozcietrzewiacz Aug 12 '11 at 14:04
    
I think it is the best solution... what's dirty about it?? –  amyassin Aug 12 '11 at 14:09
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Editing /etc/shadow by hand is dangerous: one can easily mess up the system or maybe even create a conflict in some advanced security mechanism. Ok, I'll update the answer. –  rozcietrzewiacz Aug 13 '11 at 10:43
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