This condition is more common due to modify variables, a single change variables, even press CTRL + Alt + f2 (ubuntu18, before the old version of the watch online say + f1), due to the change of the path, sudo, ls, vim, etc. Some of the most basic command cannot be used, can't change back to the modified profile file, because I was using a virtual machine, even with et2fsd software cannot see any file Linux system,
So how to solve, relative path cannot be used directly to use absolute paths, in the login screen press CTRL + Alt + f2, a command line window, enter a login name and password, after the success of the login, enter/usr/bin/sudo/usr/bin/vim/etc/profile, then open the profile file, according to the modified place change after a come back, or line # beginning to join the line commented out, according to the colon: after press esc to exit the input wq exit can be saved, the f1 cut back to the login screen, select the user to enter the password, you can enter the desktop,