free -k 输出 (RHEL 5 and RHEL 6):

             total       used       free     shared    buffers     cached
Mem:       7778104    2971960    4806144          0     211756    1071092
-/+ buffers/cache:    1689112    6088992
Swap:      4194296          0    4194296

free -k 输出 (RHEL 7/8):

              total        used        free      shared  buff/cache   available
Mem:        1012952      252740      158732       11108      601480      543584
Swap:       1048572        5380     1043192

/proc/meminfo相关值

MemTotal:        7778104 kB
MemFree:         4806144 kB
Buffers:          211756 kB
Cached:          1071092 kB
SwapTotal:       4194296 kB
SwapFree:        4194296 kB
MemAvailable:     543584 kB   --RHEL 7/8 中新增的一个值，该值用来替换-/+ buffers/cache

free 输出	相应的/proc/meminfo值
Mem: total	MemTotal
Mem: used	MemTotal - MemFree
Mem: free	MemFree
Mem: shared (can be ignored nowadays. It has no meaning.)	N/A
Mem: buffers	Buffers
Mem: cached	Cached
-/+ buffers/cache: used	MemTotal - (MemFree + Buffers + Cached)
-/+ buffers/cache: free	MemFree + Buffers + Cached
Swap: total	SwapTotal
Swap: used	SwapTotal - SwapFree
Swap: free	SwapFree

7.1以后可参照下表：

free输出	相应的/proc/meminfo值
Mem: total	MemTotal
Mem: used	MemTotal - MemFree - Buffers - Cached - Slab
Mem: free	MemFree
Mem: shared	Shmem
Mem: buff/cache	Buffers + Cached + Slab
Mem:available	MemAvailable
Swap: total	SwapTotal
Swap: used	SwapTotal - SwapFree
Swap: free	SwapFree

参考：
https://access.redhat.com/solutions/406773