title: "exec not found" when using variable in exec
body: <p>I met a strange problem.</p>
  
  <pre><code>mkfifo "spch2008"
  exec 100&lt;&gt;"spch2008"
  </code></pre>
  
  <p>It's OK. But, when i use variable to replace "100", error occurs.</p>
  
  <p>exec: 100: not found</p>
  
  <pre><code>PIPE_ID=100
  mkfifo "spch2008"
  exec ${PIPE_ID}&lt;&gt;"spch2008"
  </code></pre>
  
  <p>I don't know the reason.  please hlep me,thanks.</p>