On the kernel level, I2C communication is handled by the i2c-dev kernel driver. On the lowest level, the bus is seen as a file and using a file descriptor one can read from it or write to it. The shell interface is a set of compiled programs that wrap this functionality for specific uses at a higher level. The intended use is interactive debugging to see if something works or not. For programmatic use it's better to take a look at the C API for C/C++ and the smbus2 library for Python.

These programs are part of the i2c-tools package that usually comes preinstalled in most Linux distributions.

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