title: How can I avoid an error: No such environment variable?
body: <p>In my code I am using environment variables, but if it (env.var) doesn't exist, I get the error message NAME_ENV_VAR: no such variable, and my script stops executing.
  For example, in the line</p>
  
  <pre><code> myeval $env($File)
  </code></pre>
  
  <p>I receive an error:</p>
  
  <pre><code> can't read "env(NIKE_TECH_DIR)": no such variable
      while executing
  "myeval $env($File)"
      (procedure "chooseRelevantFiles" line 39)
      invoked from within
  "chooseRelevantFiles $::GlobalVars::reqStage"
  (file "/vobs/tavor/src/Scripts/ReproduceBug.tcl" line 575)
  </code></pre>
  
  <p>How can I avoid this error and go on to execute my script?</p>