title: Write a single byte to the serial port with Bash
body: <p>I have an Arduino which I have coded to read from a USB serial port and power an LED.
  I know it is working because it works on the built serial monitor. Now I want to write a Bash script which writes to the serial port.</p>
  
  <p>Here is the command:</p>
  
  <pre><code> echo 121 &gt; /dev/cu.usbmodem411
  </code></pre>
  
  <p>It outputs the string "123". How can I instead write a single byte with a value of 121?</p>