title: change all word occurrences under each directory recursively
body: <p>Haw can I change a word after searching under some directory :
  for example change the word <strong><em>abc</em></strong> to <strong><em>ddd</em></strong> located in all .c files.</p>
  
  <pre><code>grep --include=\*.c -rnw '/path/to/somewhere/' -e "abc"
  </code></pre>
  
  <p>Then how can I modify these occurrences to be <strong><em>ddd</em></strong>? 
  What is the command line?</p>