title: What does if [ "x" != x ] do in bash?
body: <p>I'm looking at the source code of virtualenv, and the activate script contains this code:  </p>
  
  <pre><code>if [ -z "$VIRTUAL_ENV_DISABLE_PROMPT" ] ; then  
      _OLD_VIRTUAL_PS1="$PS1"    
      if [ "x" != x ] ; then
          PS1="$PS1"
      else 
      if [ "`basename \"$VIRTUAL_ENV\"`" = "__" ] ; then
          # special case for Aspen magic directories
          # see http://www.zetadev.com/software/aspen/
          PS1="[`basename \`dirname \"$VIRTUAL_ENV\"\``] $PS1"
      else
          PS1="(`basename \"$VIRTUAL_ENV\"`)$PS1"
      fi
      fi
      export PS1
  fi
  </code></pre>
  
  <p>What does the line <code>if [ "x" != x ]</code> do?  x is not defined anywhere else in the script. </p>