I did a clean Debian Jessie installation and was trying to set the fish as default shell, I've utilized the command
usermod -s /usr/bin/fish username (from here), restarted the session and now I got an error:
There was an error creating the child process for this terminal
Failed to execute child process "/usr/bin/fish" (Input/output error)
And the terminal is unaccessible. How can I undo that command?