Linux / Unix Command: free

free - display information about free and used memory on the system

usage: free [-b|-k|-m|-g] [-l] [-o] [-t] [-s delay] [-c count] [-V]
  -b,-k,-m,-g show output in bytes, KB, MB, or GB
  -l show detailed low and high memory statistics
  -o use old format (no -/+buffers/cache line)
  -t display total for RAM + swap
  -s update every [delay] seconds
  -c update [count] times
  -V display version information and exit

DESCRIPTION

free displays the total amount of free and used physical memory and swap space in the system, as well as the buffers and cache consumed by the kernel.

[root@localhost man]# free -b

             total       used       free     shared    buffers     cached
Mem:     393904128  202096640  191807488          0   35287040  101294080
-/+ buffers/cache:   65515520  328388608
Swap:    797843456          0  797843456