title: What means the -z value in an if expression on a Linux script?
body: <p>In this script I found this <code>if</code> expression:</p>
  
  <pre><code>if [ -z $1 ]; then
      echo "Usage: createpkg.sh &lt;rev package&gt;"
      exit
  else
      CURRENT_VERSION=$1
  fi
  </code></pre>
  
  <p>My problem is that I can't find what exactly means this <code>-z</code> value.</p>
  
  <p>From the content of the echo I can deduct that (maybe) <code>$1</code> variable represents the sotware version. and that (maybe) <code>-z</code> is a void value. So if I execute the script without passing to it the version of the software that I would packing it print me the correct procedure to execute the script.</p>
  
  <p>But I am not sure about the real meaning of the <code>-z</code> value.</p>