When I modified the /etc/profile file today, I finally forgot to add: $PATH, and it logged out of the system. Finally, I kept looping through the login interface and could not enter the system.
Finally, the problem was solved by modifying the /etc/profile file in Recovery Mode.
I am using the Ubuntu 14 version. Select the option starting with Advances in Ubuntu when the system starts. In the interface that enters, select the option followed by (Recovery Mode). After entering the recovery mode
Select the option starting with root - then enter the command line interface. At this time, the file system is Read Only and the /etc/profile file cannot be modified.
At this time, you need to execute the following command mount -o remount rw /
After executing the above command, the file system is Writable. You can then modify the /etc/profile file.