title: How to check if an environment variable exists and get its value?
body: <p>I am writing a shell script. In this shell script, I am have a variable that either takes a default value, or the value of an environment variable. However, the environment variable doesn't have to be present.</p>
  
  <p>For instance, assume, before running the script, I perform the following operation:</p>
  
  <pre><code>export DEPLOY_ENV=dev
  </code></pre>
  
  <p>How do I tell the script to search for this environment variable, and store its value in a variable inside the script. Moreover, how do I tell the script that if this environment variable does not exist, store a default variable?</p>