I used the "chmod 700 /usr" command, and now what?

Question

I no longer have access to my documents and I can’t enter as root to undo this nonsense. I use Ubuntu 18.04.

Answer 1

If you have taken safety precautions when installing your operating system, you should have changed or created a password for the user root. Whereas it has done so, it should still be possible to issue the command su root that will log you in as root unused /usr/bin/sudo, what should work.

However, if this is not the case, you still have two other options. You can try logging in as Single User Mode (which will log you into a shell directly like root) and from there you can undo this "nonsense".

If none of this works, another computer or other disk/operating system (in this case Linux) will be needed to help you. You will need to add the affected disk to the target system, bootar and log into the healthy and functional operating system, and from there will utilize the tool chroot to change the base path of your shell into the partition where your installation of Ubuntu with problems is.

This will allow you to correct the permissions (once the sudo will use the permissions of the shell in which it was launched, so you do not need to be root of your Ubuntu). Also allows you to change your password root (as a precaution) in case something else goes wrong. You can learn more about chroot clicking here. This is the installation documentation for Gentoo, but it is in Portuguese and can provide you with some instructions on how to do it.

Remembering that if you opt for the last method once you access your partition from the other computer, I strongly recommend that BACK UP YOUR PERSONAL DATA, because making mistakes at this stage can cause irreparable damage to your data.