FATAL ERROR lock file "postmaster.pid" already exists

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.

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:

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

You could stop your server doing :

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

So that you won't have the lock on postmaster anymore and you could use your command to start it again.

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

restart the Postgres by using this command

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