title: What is the purpose of setting a variable default to empty in bash?
body: <p>In general, this syntax is used to guarantee a value, potentially a default argument.</p>
  
  <blockquote>
    <p>(from the Bash reference manual)</p>
    
    <p>${parameter:-word}</p>
    
    <p>If parameter is unset <strong>or null</strong>, the expansion of word is substituted. 
        Otherwise, the value of parameter is substituted.</p>
  </blockquote>
  
  <p>What would be the purpose of defaulting a variable to empty if the substitution is only chosen when the variable is empty anyway?</p>
  
  <p>For reference, I'm looking at /lib/lsb/init-functions.</p>