linux141:~ # free --help
free: invalid option -- '-'
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
可以用该指令进行动态监控：
free -m -s 3 -c 2 表示没3s打印一次，一共打印两次
