simple ruby example to read and write bytes to Arduino serial port.

arduino-serial.ino

uint8_t c;

void setup()  
{
  // Open serial communications and wait for port to open:
  Serial.begin(4800); //  8E2 : 8bits, 2 stops bits, even parity.
  // Serial.begin(4800, SERIAL_8E2); //  8E2 : 8bits, 2 stops bits, even parity.
  while (!Serial) {
    ; // wait for serial port to connect. Needed for Leonardo only
  }
  
  c='a';
}


void loop() {
  
  loop2();

}

void loop1() {
  Serial.write(c);
  c++;
  delay(1000);  
}


void loop2() {
    if (Serial.available() > 0) {
      // read the incoming byte:
      c = Serial.read();
      // c++;
      // Serial.println(c);  
      Serial.write(c);   
    }
}

void loop3() {
  Serial.write(c);
  c++;
  delay(1000);  
}

arduino-serial.rb

# link : http://playground.arduino.cc/interfacing/ruby

#simplest ruby program to read from arduino serial, 
#using the SerialPort gem
#(http://rubygems.org/gems/serialport)

require "pp"
require "serialport"

class TTy

	def initialize

		# defaults params for arduino serial
		baud_rate = 4800
		data_bits = 8
		stop_bits = 1
		parity = SerialPort::NONE

		# serial port
		@sp=nil
		@port=nil
	end
	
	def open port
		@sp = SerialPort.new(port, @baud_rate, @data_bits, @stop_bits, @parity)
	end
	
	
	def shutdown reason

		return if @sp==nil
		return if reason==:int

		printf("\nshutting down serial (%s)\n", reason)

		# you may write something before closing tty
		@sp.write(0x00)
		@sp.flush()
		printf("done\n")
	end
	
	def read
		@sp.flush()
		printf("# R : reading ...\n")
		c=nil
		while c==nil
			c=@sp.read(1)
			break if c != nil
		end
		printf("# R : 0x%02x\n", c.ord)
		return c
		# @sp.readByte()
	end
	
	def write c
		@sp.putc(c)
		@sp.flush()
		printf("# W : 0x%02x\n", c.ord)
	end
	
	def flush
		@sp.flush
	end
end


# serial port should be connected to /dev/ttyUSB*
ports=Dir.glob("/dev/ttyUSB*") 
if ports.size != 1 
	printf("did not found right /dev/ttyUSB* serial")
	exit(1)
end

tty=TTy.new()
tty.open(ports[0])

at_exit     { tty.shutdown :exit }
trap("INT") { tty.shutdown :int  ; exit}

# reading thread
Thread.new do
	d=0x16
	pp d
	while true do

		sleep(0.01)
		tty.write(d)
		tty.flush()
		
		# build a loop with a value (d) cycling from 0 to 255
		d=d+1
		d=d%255
	end
end

#just read forever
while true do
   while (true) do 
	  c=tty.read()
      sleep(0.01)
    end
end

sleep 500
tty.shutdown