The free command is used to view how much memory (RAM) is available. The free command without any options will display the available memory in bytes.
~]# free
total used free shared buffers cached
Mem: 20453604 19756444 697160 16 189332 3692964
-/+ buffers/cache: 15874148 4579456
Swap: 2097148 389432 1707716
Typically, the -h (human readable) option is used to view the available memory as M (megabytes) and G (gigabytes). When contemplating "do I have enough memory", if you are on version 6 of Red Had, look at the "free" column and add up mem and buffers/cache. I ususally do not include swap in this calculation, since software may not perform as well on swap memory.
~]# free -h
total used free shared buffers cached
Mem: 19G 19G 510M 16K 185M 3.6G
-/+ buffers/cache: 15G 4.3G
Swap: 2.0G 380M 1.6G
When contemplating "do I have enough memory", if you are on version 7 of Red Had, look at "available".
~]# free -h
total used free shared buff/cache available
Mem: 19G 19G 510M 16K 3.6G 8.0G
Swap: 2.0G 380M 1.6G