systemd's debug-shell service is intended to
diagnose systemd related boot issues with various systemctl
commands. Once enabled and following a system reboot, the root shell
will be available on tty9 which is access by pressing
CTRL-ALT-F9. The debug-shell service should only be used
for systemd related issues and should otherwise be disabled.
By default, the debug-shell systemd service is already disabled.
Ensure the debug-shell is not enabled by the systemd.debug-shel=1
boot paramenter option.
Check that the line GRUB_CMDLINE_LINUX="..."
within /etc/default/grub
doesn't contain the argument systemd.debug-shell=1.
Run the following command to update command line for already installed kernels:
# grubby --update-kernel=ALL --remove-args="systemd.debug-shell"