title: grep of 50000 strings in a big file performance improvement
body: <p>I have a file, which is about 200 MB of size, with about 1.2 M lines in it. Let's say it as reading.txt. I have another file, input.txt,
  in which there are about 50000 lines. I want to take a string in each line from input.txt file and grep in reading.txt. For a matched line,
  in reading.txt get that complete line and write into other file, output.txt.
  As of now, I am looping through every string of input.txt file, grep in reading.txt file. This approach is consuming more than 1 hour time.
  Is there any option to increase performance so that time consumption reduces for this process.</p>
  <pre><code>    while read line
      do
              LC_ALL=C grep ${line} reading.txt 2&gt;/dev/null
      done&lt;input.txt &gt;&gt; output.txt
  </code></pre>