Now, obviously, we don't want to have to manually start the alarm control script each time the Raspberry Pi boots up, for example, after a power failure—for a start, we may not even be there. Therefore, we need to set up our operating system so that it will automatically start up the alarm-control.sh script at boot time.

To do this, we need to edit the rc.local file using Nano:

$ sudo nano /etc/rc.local

Before the line containing exit 0, insert the following line:

sudo /etc/pi-alarm/alarm-control.sh &

Your rc.local file should now look something like this:

#!/bin/sh -e
#
# rc.local
#
# This script is executed at the end of each multiuser runlevel.
# Make sure that the script will "exit 0" on success or any other
# value on error.
#
# In order to enable or disable this script just change the execution
# bits.
#
# By default this script does nothing.

# Print the IP address
_IP=$(hostname -I) || true
if [ "$_IP" ]; then
  printf "My IP address is %s
" "$_IP"
fi

sudo /etc/pi-alarm/alarm-control.sh &
exit 0

The operating system runs the rc.local script after the system boots up, so you can put anything in there that you want to happen automatically at this time.