how to show the free memory on a Linux system

[root@HCMRTG_BY_YaoBin ~]# free

             total       used       free     shared    buffers     cached

Mem:       3500784    3345300     155484          0      66100    3044924

-/+ buffers/cache:     234276    3266508

Swap:       779112         80     779032

If you prefer to see information in MB

[root@HCMRTG_BY_YaoBin ~]# free -m

             total       used       free     shared    buffers     cached

Mem:          3418       3278        139          0         64       2973

-/+ buffers/cache:        240       3178

Swap:          760          0        760

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