title: Prevent execution of command shell in a Makefile
body: <p>The example below define a variable called <code>GUNICORN_PID_FILE</code>, this variable run the command <code>cat</code> to obtain the PID inside the file.</p>
  
  <pre><code>SHELL := /bin/sh
  
  GUNICORN_PID_FILE := gunicorn.pid
  GUNICORN_PID = "$(shell cat ${GUNICORN_PID_FILE})"
  </code></pre>
  
  <p>The issue is when the file <code>gunicorn.pid</code> not exists.</p>
  
  <pre><code>cat: gunicorn.pid: No such file or directory
  </code></pre>
  
  <p>Have a way to prevent this issue?</p>