title: How to get names of variables are available/set in shell scripts
body: <p>I want all variables names are set in shell script. I have a file which contains key value pairs and I was read content from that file and store/set into variables. I want to do some processes if a variable is available/set otherwise I don't need to do those processes. How to achieve this. </p>
  
  <p>For example I run loop in shell scripts in each iteration it gives one of the variables is set before that command.</p>
  
  <p>If code like this</p>
  
  <pre><code>a=test1
  b=test2
  c=test3
  
  for i in ???
  do
      echo $i
  done
  </code></pre>
  
  <p>then I want output like this </p>
  
  <p>a
  b
  c</p>
  
  <p>What command is used o achieve this.</p>