title: use commandline arguments in a sed command
body: <p>I want to use commandline arguments in a sed command, where I am currently wrapping the arguments in ''. My command is:</p>
  
  <pre><code>mycomputer$ sed 's/^\(.\{'$1'\}\)./\1'$2'/' out2.fa 2 A
  </code></pre>
  
  <p>which essentially looks at a line in a file, and changes the 2nd position to an A. This works when I hardcode it in, but the above results in:</p>
  
  <pre><code>sed: -e expression #1, char 21: Invalid content of \{\}
  </code></pre>
  
  <p>also should I be using $2 and $3 as the commandline arguments given that the filename is technically the first argument?</p>