To execute a command or script whenever we initiated a Unix-like system we are able to call it from the /etc/rc.local file. Any command or script to place that call in the file will be executed at the end of starting, that is, when all the scripts we have in the corresponding runlevel have been executed.
The term runlevel or runlevel refers to the mode in operating systems that implement the style boot system UNIX-like initiation. When the operating system enters runlevel 0, it is off, and when it enters the runlevel 6 reboots. Intermediate runlevels (1-5) differ regarding what drives are mounted, and which network services are initiated.
It is a communication bus serial bus .It is widely used in industry, mainly to communicate microcontroller and its peripherals in embedded systems, even to communicate integrated circuits that reside on the same printed circuit board.
It is a communications standard, used primarily to transfer information between integrated circuits in electronic equipment. Basically it used for the same purpose as the I2C bus but differ in some features such as: Full-Duplex communication and faster transfer rate with SPI face fewer used pins and lack of assent with I2C.
A photoresistor (or light-dependent resistor, LDR, or photocell) is a light-controlled variable resistor. The resistance of a photoresistor decreases with increasing incident light intensity.