psql: could not connect to server: No such file or directory (Mac OS X)

None of the above worked for me. I had to reinstall Postgres the following way :

• Uninstall postgresql with brew : brew uninstall postgresql

• brew doctor (fix whatever is here)

• brew cleanup

• Remove all Postgres folders :

• rm -r /usr/local/var/postgres

• rm -r ~/Library/Application\ Support/Postgres

• Reinstall postgresql with brew : brew install postgresql

• Start server : brew services start postgresql

• You should now have to create your databases... (createdb)

WARNING: If you delete postmaster.pid without making sure there are really no postgres processes running you, could permanently corrupt your database. (PostgreSQL should delete it automatically if the postmaster has exited.).

SOLUTION: This fixed the issue--I deleted this file, and then everything worked!

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

--

and here is how I figured out why this needed to be deleted.

1. I used the following command to see if there were any PG processes running. for me there were none, I couldn't even start the PG server:

   ps auxw | grep post
   

2. I searched for the file .s.PGSQL.5432 that was in the error message above. i used the following command:

   sudo find / -name .s.PGSQL.5432 -ls
   

   this didn't show anything after searching my whole computer so the file didn't exist, but obviously psql "wanted it to" or "thought it was there".

3. I took a look at my server logs and saw the following error:

   cat /usr/local/var/postgres/server.log
   

   at the end of the server log I see the following error:

   FATAL:  pre-existing shared memory block (key 5432001, ID 65538) is still in use
   HINT:  If you're sure there are no old server processes still running, remove the shared memory block or just delete the file "postmaster.pid".
   

4. Following the advice in the error message, I deleted the postmaster.pid file in the same directory as server.log. This resolved the issue and I was able to restart.

So, it seems that my macbook freezing and being hard-rebooted caused Postgres to think that it's processes were still running even after reboot. Deleting this file resolved. Hope this helps others! Lots of people have similar issues but most the answers had to do with file permissions, whereas in my case things were different.