I received this error when I modified the /etc/environment
file and changed the PATH variable.
I went to this to solve the problem.
On the login screen Press CTRL+ALT+F1, then type in your regular name and password.
If you get something like "/bin
is unavailable" because it's not included the PATH environment then you need to go to you /etc/environment
file and see if it looks like this
PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games"
I had extra lines because I was trying to add java to the path
If you type in
export PATH=/usr/bin:/bin
you will be able to us the ls command but that will only be for this login session
I edited that file with the pico editor
sudo pico /etc/environment
saved it, logged out and the pressed CTRL+ALT+F7 to get back to the graphic log in and was able to log in.