You can output information about the memory of a machine running on CentOS by reading the /proc/meminfo file using the cat command.

Example:

$ cat /proc/meminfo
MemTotal:         131072 kB  
MemFree:            1012 kB  
Cached:            16580 kB  
Active:            76420 kB  
Inactive:          43368 kB  
Active(anon):      65108 kB  
Inactive(anon):    38100 kB  
Active(file):      11312 kB  
Inactive(file):     5268 kB  
Unevictable:           0 kB  
Mlocked:               0 kB  
SwapTotal:        131072 kB  
SwapFree:          64152 kB  
Dirty:                 0 kB  
Writeback:             0 kB  
AnonPages:        103208 kB  
Shmem:              7684 kB  
Slab:              10256 kB  
SReclaimable:       2140 kB  
SUnreclaim:         8116 kB