RAM usage of PostgreSql

I have installed PostgreSQL 9.4 (open source) version on my CentOS Linux Red Hat 7 production server and kept default parameters which are in postgresql.conf file.So my basic question is, once I start using postgres how much RAM the postgres processes  consumes (postgres related processes only). 

There are lot of allocations in postgresql.conf file, for example shared_buffers, work_mem...etc.

As per my knowledge, all postgres processes should not consume the RAM more than the value assigned in shared_buffers.Please clarify and let me know if I misunderstand the concept..
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