Sudo command not found (su login not working)

Question

I had a system crash and now my Ubuntu runs into low-graphics mode. All the fixes require running commands with sudo. When I try this I get sudo: command not found. When I try logging in with su - I get an error: Authentication failed. I also tried sudo -i but that did nothing. I am really stuck here. What can I do?

Relevant info:

$ echo $PATH 
/usr/local/sbin:/usr/local/bin:/usr/bin:/usr/sbin:/sbin:/bin
$ ls -l /usr/bin/sudo
-rwsr-xr-x 1 root root 121144 Feb 28 2013 /usr/bin/sudo

-a sudo output : -bash: -a command not found
which - a sudo output: 
/usr/bin/sudo echo foo output: Permission denied

Answer 1

I think the only possibility left it that one of the directories leading to sudo doesn't have the other executable bit set, so although the directory is in your path, bash can't find the binaries inside. When you try with full path, then of course you get the permission denied message. This problem with the directories is likely also the cause of your original problem with the low-graphics mode thing.

So please try

ls -ld /usr
ls -ld /usr/bin

both of these should report:

drwxr-xr-x

permissions. If not, then it is your problem and you should set those permissions for those folders, and likely for some other folders also. But you can't do it from your installation, because you don't have access to sudo. So either boot into recovery mode or use a live CD to correct the errors. (Which were likely caused by a wrong chmod command.)