Arduino & Raspberry Pi

I would like to know for a project I am doing if it is possible for my Arduino board to communicate with a Raspberry Pi board so that the arduino can communicate in a way that tells the raspberry pi to open a certain application.

Any help would be great.

You will need to have a program running on the RPI that is listening for input from the Arduino and which can open the appropriate RPi application depending on the message received.

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You can hook the RPi to the Arduino using I2C, Serial, SPI, or most simply a single I/O pin, so if it is HIGH, for example, the RPi will execute the command you want, the set the line LOW again.

Most of your coding will be on the RPi.

Ok because for the project I will be creating a robot I can communicate with using a GSM shield. So for example if I send an SMS to the robot saying maybe to open a certain application on the RPi the Arduino would recieve, with the help of the GSM shield of course, the SMS and use that command so it can send it over to the RPi and it could use that as an input to open that certain app...

So my question is, what would be best to do that pekasus, I2C, Serial, SPI, or a I/O pin?

I'm unclear about your setup. Do you want to send an SMS to Arduino, which will then send a command to RPi?

If that's the case, then it depends on the type of command to send from the Arduino to the RPi. If it is a simple command, then you can use the GPIO. The RPi watches for the pin to go HIGH or LOW, then executes a pre-programmed command.

If you need to send data with the command, say coordinates, then you should use Serial as that is the simplest, and it could pass text from the SMS to the RPi.