free — Display Information about Memory utilization
Summary:-
Displays the total amount of free and used physical memory & swap space in the system, as well as the buffers & cache consumed by the kernel.
Examples :-
$ free — Display the mem info in KB
$ free -m — Display the mem info in MB
$ free -m -t — Display the total mem info
$ free -s 2 — Display the mem info for every 2 sec
$ free -s 2 -c 5 — Display the mem info for every 2 sec, but only 5 times
$ cat /proc/meminfo — This is the original file which is used by free command for the report
Read: man free