title: grep based on first column
body: <p>I have a big data file called fileA having the following format</p>
  
  <pre><code>col1    0.1111,0.2222,0.33333,0.4444
  col5    0.1111,0.2222,0.33333,0.4444
  col3    0.1111,0.2222,0.33333,0.4444
  col4    0.1111,0.2222,0.33333,0.4444
  </code></pre>
  
  <p>The separator between 1st and 2nd columns is \t. Other separators are comma.
  I have another file containing the name of rows I am interested in, called fileB, which looks like:</p>
  
  <pre><code>col3
  col1
  ...
  </code></pre>
  
  <p>Both files are not sorted. I want to retrieve all the rows from fileA with names appearing in fileB. The code <code>grep -f fileB fileA</code> does this job, but I think it will search all fileds in fileA, which takes long time. How can I specify only to search the 1st column in fileA?</p>