free command displays the number of free and used memory in the system.
-b: displays the amount of memory in bytes.
-k: displays the memory size in kilobytes (default).
-m: displays the amount of memory in megabytes.
-sn: command repeats every n seconds.
-t: displays the string with the total number of free and used memory.
The free command displays information about the physical memory (RAM) and swap space (on disk). In the output information shows the total amount of memory, as well as the number used and free memory.