Q:

How do I access Postgres when I get an error about "/var/run/postgresql/.s.PGSQL.5432"?

I am running Ubuntu 16.  I have installed Postgresql.  Postgresql used to work, but then I rebooted.  
nmap commands show port 5432 is open.  Postgres seems to be working correctly:
service postgresql status

postgresql.service - PostgreSQL RDBMS    Loaded: loaded
  (/lib/systemd/system/postgresql.service; enabled; vendor preset:
  enabled)    Active: active (exited) since Sat 2017-07-29 18:42:59 EDT;
  1min 4s ago   Process: 201 ExecStart=/bin/true (code=exited,
  status=0/SUCCESS)  Main PID: 201 (code=exited, status=0/SUCCESS)
  Memory: 0B    CGroup: /system.slice/postgresql.service

I ran this: psql
But I got this:

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 file listed above does not seem to exist.
How do I get into Postgresql?  Normally I'd run psql or sudo -i -u postgres then psql.  But these commands are not working.  I keep getting an error about "could not connect to server."  Several reboots have not helped.
Update:
I ran this command: dpkg -l | grep postgres

rc postgresql-9.5                9.5.6-0ubuntu0.16.04                   amd64        object-relational SQL database, version 9.5 server
ii postgresql-client             9.5+173                                all          front-end programs for PostgreSQL (supported version)
ii postgresql-client-9.5         9.5.7-0ubuntu0.16.04                   amd64        front-end programs for PostgreSQL 9.5
ii postgresql-client-common      173                                    all          manager for multiple PostgreSQL client versions
ii postgresql-common             173                                    all          PostgreSQL database-cluster manager

A:

You probably have multiple PostgreSQL versions installed. If so, the other version probably defaults to unix_socket_directories = '/tmp/' but the libpq your psql is linked to probably defaults to /var/run/postgresql/.
Try
psql -h /tmp

If that works, the above is the problem. You can add export PGHOST=/tmp to your .bashrc to change the default locally for your user.
If that doesn't work, make sure PostgreSQL is actually running
ps aux |grep postgres

and if not, start it. How depends on how you installed it, but it'll be via the service or systemctl command(s) if you installed using packages.

A:

If your Postgres service is up and running without any error or there is no error in starting the Postgres service and still you are getting the mentioned error, follow these steps
Step1: Running pg_lsclusters will list all the postgres clusters running on your device
eg: 
Ver Cluster Port Status Owner    Data directory               Log file
9.6 main    5432 online postgres /var/lib/postgresql/9.6/main /var/log/postgresql/postgresql-9.6-main.log

most probably the status will be down in your case . Try restarting Postgres clusters and service
Step 2: Restart the pg_ctlcluster
#format is pg_ctlcluster <version> <cluster> <action>
sudo pg_ctlcluster 9.6 main start

#restart postgresql service
sudo service postgresql restart

Step 3: Step 2 failed and threw an error
If this process is not successfull it will throw the error.
My error was(You can see the error log on /var/log/postgresql/postgresql-9.6-main.log) 
FATAL: could not access private key file "/etc/ssl/private/ssl-cert-snakeoil.key": Permission denied
Try adding `postgres` user to the group `ssl-cert`

Step 4: check ownership of postgres
Make sure that postgres is the owner of /var/lib/postgresql/version_no/main
eg: sudo chown postgres -R /var/lib/postgresql/9.6/main/
Step 5: Check Postgres user belongs to ssl-cert user group
It happened to me and it turned out that I removed erroneously the Postgres user from "ssl-cert" group. Run the below code to fix the user group issue and fixing the permissions
#set user to group back with
sudo gpasswd -a postgres ssl-cert

# Fixed ownership and mode
sudo chown root:ssl-cert  /etc/ssl/private/ssl-cert-snakeoil.key
sudo chmod 740 /etc/ssl/private/ssl-cert-snakeoil.key

sudo service postgresql restart

A:

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"?

This error generally means that the server is not running. Based on dpkg -l output and the thread of comments, it was due to the postgresql-9.5 main package being somehow uninstalled. Since the uninstall hasn't been called with the --purge option to dpkg, the data and configuration files are still there, so apt-get install postgresql-9.5 can fix the problem.

