title: Colon at the beginning of line in docker entrypoint bash script
body: <p>I was looking through <a href="https://github.com/docker-library/postgres/blob/cd294bf8dfdf4a74b2077aa6413fa579f9bf07de/9.4/docker-entrypoint.sh" rel="noreferrer">an entrypoint script for a docker image</a> and they had the following lines (53-54)</p>
  
  <pre><code>: ${POSTGRES_USER:=postgres}
  : ${POSTGRES_DB:=$POSTGRES_USER}
  </code></pre>
  
  <p>I saw this thread <a href="https://stackoverflow.com/questions/3224878/what-is-the-purpose-of-the-colon-gnu-bash-builtin">What is the purpose of the : (colon) GNU Bash builtin?</a></p>
  
  <p>and was able to figure out <code>:</code> meant true and that the <code>:=</code> was used to assign default values, but it doesn't really touch upon what does <code>:</code> at the beginning of the line at least for this specific case.</p>