title: FATAL ERROR lock file "postmaster.pid" already exists
body: <p>I have recently installed PostGIS on my Mac (El Capitan 10.11.4, Postgres is version 9.5.1) using Homebrew, and I am following these instructions - <a href="http://morphocode.com/how-to-install-postgis-on-mac-os-x/" rel="noreferrer">http://morphocode.com/how-to-install-postgis-on-mac-os-x/</a></p>
  
  <p>When I try to start Postgres using</p>
  
  <pre><code>pg_ctl -D /usr/local/var/postgres start 
  </code></pre>
  
  <p>I get the following error:</p>
  
  <pre><code>$ FATAL:  lock file "postmaster.pid" already exists
  HINT:  Is another postmaster (PID 280) running in data directory "/usr/local/var/postgres"?
  </code></pre>
  
  <p>So I spent a few hours researching how to address this, but to no avail.</p>
  
  <p>Notably, I tried to kill the PID as recommended in an answer on Superuser - <a href="https://superuser.com/questions/553045/fatal-lock-file-postmaster-pid-already-exists-">https://superuser.com/questions/553045/fatal-lock-file-postmaster-pid-already-exists-</a>  (in the case above, I ran <code>kill 208</code>), but as soon as I tried to start Postgres again, I got the same error, albeit with a different PID number.  I saw a few people recommended deleting the postmaster.pid file, but I feel like maybe I should save that as a last resort...</p>
  
  <p>Admittedly part of the reason I'm not sure how to fix this is that I'm not really clear on what the postmaster even is - I'm just starting to learn about all of this.</p>
  
  <p>Hopping into a Postgres database via the <code>psql db_name</code> command works just fine, for what it's worth.</p>