free - Display amount of free and used memory in the system.
Display the amount of memory in kilobytes (default):
free -k
free
Display the amount of memory in megabytes:
free -m
Display the amount of memory in gigabytes:
free -g
Display memory status for regular intervals (5 seconds in this case):
free -s 5
Show low and high memory statistics:
free -l
Display free version:
free -V