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Brew Services Postgres Suddenly won’t Start

Aaron SnyderAaron Snyder

I was working on one of our Rails apps when it suddenly couldn’t connect to Postgres. I checked Brew services and saw the word “started” colored yellow. This yellow “started” indicates that Brew tried starting Postgres but had issues.

The next step is to open the file ~/Library/LaunchAgents/homebrew.mxcl.postgresql.plist mentioned in the brew services list. Near the bottom of the file, there is a section that points to the Postgres error log file.


The contents of /usr/local/var/log/postgres.log then pointed to our issue:

FATAL:  lock file "postmaster.pid" already exists
HINT:  Is another postmaster (PID 532) running in data directory "/usr/local/var/postgres"? 

Usually, this points to postgres not being shut down gracefully. A quick ps aux | grep /usr/local/var/postgresl shows that there aren’t any postgres processes running, confirming a bad shutdown. For me, it was probably the laptop’s battery dying.

The fix is to remove the postmaster.pid file and start postgres. This is acomplished by rm usr/local/var/postgres/postmaster.pid followed by brew services stop postgresql && brew services start postgresql



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