If one needs a Raspberry Pi that shuts down save by just unplugging and without having the risk of corrupting the SD card one has to change the SD card to a read-only mode.
In the fstab:
$ sudo nano /etc/fstab
one has to change the SD card to read only (ro) and one has to make a temporary virtual drive (30MB), too:
/dev/mmcblk0p2 / ext4 defaults,noatime,ro 0 1 tmpfs /tmp tmpfs defaults,size=30M 0 0
One has to disable some programs, rsyslog and dphys-swapfile:
$ sudo systemctl disable rsyslog $ sudo systemctl disable dphys-swapfile
If one needs a programs that requires writing one has to make sure that will be in the temporary drive. That could be done for example with a symbolic link:
$ ln -s /home/pi/tmp /tmp
For changes in the system one has to remount the partition writable:
$ sudo mount / -o remount,rw