How to manage different versions of PostgreSQL servers on the same host?

I have PostgreSQL 9.4 and 9.5 on the same host with Ubuntu 14.04. My problems are:

1. When I run service postgresql start|stop, both versions start/stop. But I want to control which one to do.
2. When I use psql to connect to PostgreSQL, it always connects to 9.4. But I want to control which server to do. I know I can connect by different port, 5432 for 9.4 and 5433 for 9.5. But I want to connect by peer auth, that is via Unix socket.

I managed to find these clues for above two questions separately but haven't resolved my problems:

1. I find a command pg_ctlcluster may do the work but I don't know how to specify correct parameters, say, cluster-name. How could I figure it out? Or you have some other way?
2. Unix socket can be configured by unix_socket_directories = '/var/run/postgresql'. But both versions have the same config dir, within which there're subdirs 9.4-main.pg_stat_tmp 9.4-main.pid 9.5-main.pg_stat_tmp 9.5-main.pid for different versions. When I do psql -h /var/run/postgresql, it just connects to the old 9.4 version.

Could anyone help? Thanks!

Well, after some more effort, I get the answers myself:

1. Ubuntu and Debian offer a pg_ctlcluster as well as a serial pg_xxxcluster commands to manage multiple versions/instances of PostgreSQL on the same host. To find out the version and cluster name, just do pg_lscluster, which outputs like 9.4 main 5432 online postgres /var/lib/postgresql/9.4/main /var/log/postgresql/postgresql-9.4-main.log 9.5 main 5433 online postgres /var/lib/postgresql/9.5/main /var/log/postgresql/postgresql-9.5-main.log For example, to stop 9.4, you use pg_ctlcluster stop 9.4 main. BTW, to disable autostart 9.4, edit /etc/postgresql/9.4/main/start.conf
2. psql need a --port, -p option to know which instance to connect, even for peer auth by Unix Socket, since all versions have the same unix_socket_directories. For example, psql -p 5433 dbname can connect by Unix Socket to version 9.5 running with port 5433, while default psql dbname connects to default port 5432, which belongs to my old 9.4 version.

That's it!

On Debian-based distributions like Ubuntu, psql (and a few other PostgreSQL commands) is a symlink to pg_wrapper, which offers the --cluster option. See man pg_wrapper for details.

For local connections, you can use

psql --cluster 9.4/main
psql --cluster 9.5/main

To see your versions, clusters and ports, use pg_lsclusters. Sample output:

# pg_lsclusters
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
11  main    5433 online postgres /var/lib/postgresql/11/main  /var/log/postgresql/postgresql-11-main.log

And to set the default, you can set your preferred version to use port 5432 in /etc/postgresql/*/main/postgresql.conf, and to some other port for the other version.

(This was also answered here: Running multiple versions of PostgreSQL on the same Ubuntu server )