FATAL ERROR lock file "postmaster.pid" already exists

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 - http://morphocode.com/how-to-install-postgis-on-mac-os-x/

When I try to start Postgres using

pg_ctl -D /usr/local/var/postgres start

I get the following error:

$ FATAL:  lock file "postmaster.pid" already exists
HINT:  Is another postmaster (PID 280) running in data directory "/usr/local/var/postgres"?

So I spent a few hours researching how to address this, but to no avail.

Notably, I tried to kill the PID as recommended in an answer on Superuser - https://superuser.com/questions/553045/fatal-lock-file-postmaster-pid-already-exists- (in the case above, I ran kill 208), 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...

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.

Hopping into a Postgres database via the psql db_name command works just fine, for what it's worth.

Posting this in case it helps someone else:

I was having this same problem as the OP after a hard reboot when my laptop crashed. What helped me was running the following command to see what PID was associated with postmaster.pid:

cat /usr/local/var/postgres/postmaster.pid

The first number that appears will be the PID. Looking in Activity Monitor, I was able to see that Postgres was running, but without a PID number that matched the one shown.

Instead of the steps outlined in the answer referenced on Superuser, I restarted my laptop properly and then opened up Terminal and ran

brew services restart postgresql

This worked without having to remove postmaster.pid, which I saw a few other posts recommend. Sometimes it's the simple solutions that work.

It often happens to me in OSx, when my system shutdown unexpectedly.

You can just remove the file postmaster.pid.

cd Library/Application Support/Postgres/var-{postgres-version}

and remove the postmaster.pid file

in case you use brew then your path should be something like:

/usr/local/var/postgres/postmaster.pid

restart the Postgres by using this command

pg_ctl -D /usr/local/var/postgres restart

Since you can connect to the database, you don't need to start the server again - it's already running.

pg_ctl is used to control the PostgreSQL server. Since your server is already started, your command:

pg_ctl -D /usr/local/var/postgres start

Returns an error, saying that there is a lock on postmaster.pid - which is true since there is already a server running under that PID.

There are two ways:

The most basic way - skip that step, your server is already running!
Executing a needless operation - stopping the server, and then starting it again.

You could stop your server doing :