psql: could not connect to server: no such file or directory is the server running locally and accepting connections on unix domain socket "/var/run/postgresql/.s.pgsql.5432"?

                                    The error states that the psql utility can't find the socket to connect to your database server. Either you don't have the database service running in the background, or the socket is located elsewhere, or perhaps the pg_hba.conf needs to be fixed.

Step 1: Verify that the database is running
The command may vary depending on your operating system. But on most *ix systems the following would work, it will search for postgres among all running processes

ps -ef | grep postgres
On my system, mac osx, this spits out

501   408     1   0  2Jul15 ??         0:21.63 /usr/local/opt/postgresql/bin/postgres -D /usr/local/var/postgres -r /usr/local/var/postgres/server.log
The last column shows the command used to start the server, and the options.

You can look at all the options available to start the postgres server using the following.

man postgres
From there, you'd see that the options -D and -r are respectively the datadir & the logfilename.

Step 2: If the postgres service is running
Use find to search for the location of the socket, which should be somewhere in the /tmp

sudo find /tmp/ -name .s.PGSQL.5432
If postgres is running and accepting socket connections, the above should tell you the location of the socket. On my machine, it turned out to be:

/tmp/.s.PGSQL.5432
Then, try connecting via psql using this file's location explicitly, eg.

psql -h /tmp/ dbname
Step 3: If the service is running but you don't see a socket
If you can't find the socket, but see that the service is running, Verify that the pg_hba.conf file allows local sockets.

Browse to the datadir and you should find the pg_hba.conf file.

By default, near the bottom of the file you should see the following lines:

# "local" is for Unix domain socket connections only
local       all       all       trust
If you don't see it, you can modify the file, and restart the postgres service.

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