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I just reinstalled postgres via brew install postgres

I ran initdb /usr/local/var/postgres -E utf8 but got this:

The files belonging to this database system will be owned by user "atal421".
This user must also own the server process.

The database cluster will be initialized with locale "en_US.UTF-8".
The default text search configuration will be set to "english".

initdb: directory "/usr/local/var/postgres" exists but is not empty
If you want to create a new database system, either remove or empty
the directory "/usr/local/var/postgres" or run initdb
with an argument other than "/usr/local/var/postgres".

so, I rm -rf the postgres folder and ran it again:

 initdb /usr/local/var/postgres -E utf8

it said everything was okay:

Success. You can now start the database server using:

    postgres -D /usr/local/var/postgres

so, I ran that command and got:

postgres -D /usr/local/var/postgres

FATAL:  lock file "" already exists
HINT:  Is another postmaster (PID 13731) running in data directory "/usr/local/var/postgres"?

Now when I look at my Activity Monitor I can see 6 instances of postgress.

How do I fix this?

I received this error after, I think, my computer crashed. PostgreSQL couldn't even start because of this error so killing the process wasn't the solution. I simply backed-up and then deleted the file and then the error stopped and PG was able to start again.

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Another possibility is that you had a hard shutdown and the postgres process died without cleaning up its pid file. This happens to me when my laptop's battery dies.

This solution is not for a production system, and you should really make sure the postgres daemon is not running, but I use my laptop for coding and I'm not worried about needing to regenerate my databases.

So if another process -- or none at all -- is running on that port, just delete the pid file, e.g.

rm /usr/local/var/postgres/

and postgres will soon start up fine.

To find out if another process is running on that port, you can do

ps wax | grep `head -1 /usr/local/var/postgres/`

Then run

tail -f /usr/local/var/postgres/server.log 

to see if it worked. You should see

FATAL:  lock file "" already exists
HINT:  Is another postmaster (PID 933) running in data directory "/usr/local/var/postgres"?
FATAL:  lock file "" already exists
HINT:  Is another postmaster (PID 933) running in data directory "/usr/local/var/postgres"?
LOG:  database system was interrupted; last known up at 2014-05-25 09:41:32 PDT
LOG:  database system was not properly shut down; automatic recovery in progress

(or at least that's what I just saw after I did the above :-) )

(And really, shouldn't Postgres be smart enough to realize that there is no process with PID 933 and remove the bogus pid file on its own?)

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Public service announcement: never delete Really. Great way to get data corruption.

You already had PostgreSQL installed, and you deleted the data dir without stopping the running server. So you now have some orphan PostgreSQL server processes that are managing data files that've been deleted, so they're no longer accessible in the file system and will be fully deleted when the last open file handle to them is closed. You can't use pg_ctl to shut the server down like normal because you've deleted the cluster datadir, so you must simply kill the processes. Kill the postmaster (do notuse kill -9, just an ordinary kill will do) and the rest will shut down too.

You will then be able to start a new server in the datadir against the freshly initdb'd data.

It is highly likely that you will experience conflicts down the track unless you uninstall the other older version of PostgreSQL.

In a nutshell:

cat /usr/local/var/postgres/

Note down the number on the first line, which is the pid of the postmaster.

Verify with ps that the pid is that of a postgres postmaster.

Kill the postmaster process with the following command, replacing 'PID' with the number you have noted down. Again, do not use kill -9 or kill -KILL, just use a plain kill, i.e. a SIGTERM:

kill PID

If the pid is not that of a postgres postmaster, manually kill any postgres backends that may still be running, verify that they are no longer running, and only thenremove (You must also verify that the is not on shared storage where the server could be running on some other VM/host).

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