To monitor the memory usage on the server is essential in order to make your server stable. If your server goes out of memory then you may need to reboot the server to come back your server online again. There are few commands available to monitor the memory usage on the server:
This command gives you the information about processes, memory, paging, block IO, traps, and CPU activity. For all the options refer the manual of this command (man vmstats).
It displays the amount of free and used memory in the system. For all the options, refer the manual of this command (man free).
This file contains the information of current memory status (Free memory, used memory etc.).