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