If you just did
sudo chmod 666 /usr then things are no too bad - since that will have changed the permissions only on the parent
/usr directory, not any of the files of subdirectories within it.
If however you added the recursive flag
-R then you are almost certainly better off re-installing the system. Even so, there should be no reason to lose data: you can boot from a live USB or DVD and backup your important files from there first.
If you only did a non-recursive
sudo chmod 666 of the
/usr directory, and you have physical access to the machine, then the simplest fix should be
- Boot into recovery mode via the grub advanced menu and select 'Drop to root shell'
Remount the filesystem in read-write mode
mount -o remount,rw /
Execute the command
chmod 755 /usr
Don't add any other command line options and don't use any shell wildcards.
exit to continue booting normally
chmod lives in
/bin rather than
/usr/bin so shouldn't be affected by the permissions on
/usr. If (for example) you'd change the permission bits on the
/bin directory, or on the
/ directory itself, then this method would not be appropriate since you wouldn't be able to execute
chomd from recovery mode - the best option in that case would probably be to boot a live CD/DVD/USB of any available Linux distribution (it doesn't need to be Ubuntu); identify and mount the broken system's root device (at
/mnt in the live system, for example); and then run
chmod 755 /mnt/usr from the live system.