Have terminal bash script act as virtual "button" for Leonardo?

I've searched quite a lot for this and tried to piece bits of code together but with no luck so far.

All i'm looking to do is have a command I can run in terminal/bash script on the Mac trigger some code to run on the Arduino (Leonardo)

It doesn't matter what data the Mac sends, I only have one command to run so it doesn't need to look like separate buttons or anything.

So far i've tried

int incomingByte = 0;   // for incoming serial data
void setup() {
Serial.begin(9600);
}

void loop() {

    // send data only when you receive data:
    if (Serial.available() > 0) {
            // read the incoming byte:
            incomingByte = Serial.read();
//run commands
  }
    }

and then from terminal/script both

echo "1" > /dev/tty.usbmodemHIDPC1 9600

and

screen > /dev/tty.usbmodemHIDPC1 9600

I see the Arduino light flash when I run those two commands in terminal which would seem to suggest that its receiving the data, but nothing runs.

I have stripped out my code that is due to be run by the way for simplicity. I've got working keyboard presses ready to go to enter in the "run commands".