Can we allocate 10 gb RAM to postgres out of 30gb Team, 
Can we allocate 10 gb RAM to postgres out of 30gb on Linux machine. 
If yes can someone share me the steps
Yes one can allocate the Shared Memory with a Kernel Flag this is documented in Postgresql Documentation

https://www.postgresql.org/docs/current/kernel-resources.html 

Also may want to look into Huge Page same link further down

note this is Shared Memory other settings eat up memory independently,  see the postgres.conf  for more details