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How to setup and use I2C and SPI on RPi

Guides

Sparkfun

Adafruit

Enable Interfaces

Make sure I2C and/or SPI are turned on 

sudo raspi-config

Install Software

WiringPi is used to program (download) (docs)

I2C tools 

sudo apt-get install -y i2c-tools

i2cdetect program will probe all the addresses on a bus 

i2cdetect -Y 1