Edit /etc/passwd file from grub

Question

I tried installing zsh on ubuntu ( 14.04). When I switched to zsh, I was unable to go into super user mode.

If I do a su it asks for passwd and on entering the correct password, it says:

Cannot execute which zsh: No such file or directory

But when I enter an incorrect password and press enter 4-5 times, it shows nothing.

On checking my /etc/passwd file, it showed:

root:x:0:0:root:/root:which zsh

I tried to edit the file and save but it shows

Unable to save /etc/passwd

I have tried to change the runlevel to 1 via grub, but it results in an error:

APCI PCC probe failed

I am running Ubuntu on VirtualBox.

Solution 1

You cannot edit files from the Grub prompt (Grub can only read files, it doesn't have any write support). You need to boot Linux¹.

If you can't log in normally, you can completely bypass the normal boot process and start just a kernel and a shell. At the Grub prompt, press e to edit the boot sequence, add init=/bin/bash to the end of the linux=… line, and press Ctrl+X to boot. You'll get a root shell and nothing else. Run

mount -t proc proc /proc
mount -o remount,rw /

Now run chsh -s /bin/zsh root to repair your user database.

Next, reboot with Alt+SysRq+U followed by Alt+SysRq+B, or boot normally with

mount -o remount,ro /
umount /proc
exec /sbin/init

Tip: run chsh without a user argument. That way, it validates the shell.

Tip: when you're changing some configuration, always keep a shell open and test the configuration. If you made a mistake, you'll still have a command line to repair it.

Tip: you can create a toor account with UID 0 (so it's still root), with the same password as root, and where the shell remains /bin/sh, so that you keep another way to log in if things break. Better yet, install a statically linked shell (busybox-static or zsh-static) and use that as toor's login shell.

¹ _{Or you can use guestfs if the host is Linux.}

Solution 2

You should remove from the password file the which word... It should be something like :

root:x:0:0:root:/root:/bin/zsh

Where /bin/zsh is the path to the Z shell binary.
In the current config, the login process is trying to execute which zsh and that command is failing.

Also, edit the file as root, with sudo vim /etc/passwd for example because it is a file which is writable only by root.

If you can't sudo or su, then boot into recovery mode (choose Advanced option for Ubuntu in GRUB boot loader, then choose Ubuntu...(recovery mode), then choose root/Drop to root shell prompt). After this, remount the / filesystem as read-write (mount -o remount,rw /) and then you can edit the content of the /etc/passwd file. Tested this on Ubuntu 16.04.

Comments

[Sergiy Kolodyazhnyy]

Ubuntu grub has option to go into recovery. Also, you can change an option to boot into bash directly. Can't recall which, though

[iyerrama29]

even during recovery; setting root: Drop out to root shell asks for passwd, which when entered correct gives: which zsh: No such file or directory

[iyerrama29]

Hi there, I tried editing the file, but as I mentioned above, it shows I cannot edit the file(unless I am the root - and also I am unable to become the root because the su command doesn't work): Unable to save /etc/passwd

[Sergiy Kolodyazhnyy]

Filesystem is mounted read only. Try remount as read write

[iyerrama29]

I tried going to /etc and doing mount -o remount rw and got mount: only root can do that

[iyerrama29]

As I mentioned, I cannot do a sudo or a su, when I do sudo it shows: sudo: /usr/bin/sudo must be owned by uid 0 and have setuid bit set and on su it asks for passwd but when entered correct password it gives ** Cannot execute which zsh: No such file or directory**

[terdon]

-1, I'm afraid everything you suggest is wrong: 1) The OP can't use sudo as explained in their question; 2) You mean /etc/shells not /etc/shell; 3) $SHELL doesn't show the currently running shell, it shows the default for the user. In any case, how is that relevant?

[magor]

boot into recovery mode (choose Advanced option for Ubuntu in GRUB boot loader, then choose Ubuntu...(recovery mode), then choose root/Drop to root shell prompt). After this, remount the / filesystem as read-write (mount -o remount,rw /) and then you can edit the content of the /etc/passwd. Tested this on Ubuntu 16.04.

[iyerrama29]

I booted into the recovery mode and chose: root: Drop out to root shell prompt, it again gives: Give the root password for maintainence or press Ctrl+D to continue, on giving the right password, it shows which zsh: No such file or Directory

[magor]

in this case you should try booting an Ubuntu install disk and drop to shell (or use the Try Ubuntu option) and then try to mount the drive and change the file

[AReddy]

/etc/shell will not give you the required output.

[mstr]

Thanks a lot for the great trick with changing the init! It helped me in a situation where I had to edit fstab to allow normal booting :-)