Final answer:
Use the 'free' command to check the memory usage on a Linux system, providing details on free and used memory, including buffers. The 'top' and 'htop' commands can also give dynamic, real-time information in a detailed and user-friendly interface.
Step-by-step explanation:
To check the quantity of memory a Linux system is using, you can use the free command. This command provides information about the total amount of free and used physical and swap memory in the system, as well as the buffers used by the kernel. Another useful command is top, which gives a dynamic real-time view of a running system. It displays a detailed overview, including the usage of memory. Additionally, the htop command can be used for an interactive view that is more user-friendly than top.