cluster already running - error while startup postgres postmaster

cluster already running / removed stale pid file, error you may get while start up the postmaster.

postmaster daemons create pid files when start up, remove them when the process exits. some time pid file exist even after postmaster daemons exit because the process crashed before it could remove the pid file or the system crashes and processes do not get a chance to remove their pidfiles even if they want to.

--> in this case just find out & remove postmaster pid file ( you could find out pid file in data directory of postgresql installed path)

or
--> find out the the running process id (PID) using port no & kill PID

lsof | grep 5432 --(lsof | grep PortNo) and kill the pid
kill -9 PID

or
--> find out the the running process id (PID) using process name & kill PID

ps -ef | grep postmaster --(lsof | grep PortNo) and kill the pid
kill -9 PID

Once stop the running postmaster using pid, start the postgres database server.