How to show memory information in Linux

Memory information checks help confirm whether a Linux host has the expected RAM capacity before sizing workloads, opening a support case, or comparing a server to an inventory record. The first reading should separate usable memory from cache and swap so a capacity check does not get mistaken for a pressure incident.

The free command and /proc/meminfo read kernel memory counters and show what the running system can allocate. These values are the right source for total memory, available memory, cache, swap, huge pages, and other runtime counters that change as workloads run.

Hardware-level commands use a different source. lsmem reads memory block information from sysfs, while dmidecode reads DMI and SMBIOS firmware tables for memory device size, type, speed, and slot details. Containers and some virtual machines hide those interfaces, so run hardware-detail commands on the host or full guest that owns the memory.

1. Display total, used, free, cache, available, and swap memory.

   $ free --human
                  total        used        free      shared  buff/cache   available
   Mem:           3.3Gi       1.9Gi       366Mi       109Mi       1.3Gi       1.4Gi
   Swap:          3.8Gi          0B       3.8Gi

   available estimates memory that applications can allocate without immediate swapping. buff/cache is mostly reclaimable file cache, not permanently occupied application memory.
   Related: How to check memory usage in Linux

2. Read the raw kernel memory counters from /proc/meminfo.

   $ cat /proc/meminfo
   MemTotal:        3461112 kB
   MemFree:          375492 kB
   MemAvailable:    1448432 kB
   Buffers:           38452 kB
   Cached:          1242596 kB
   SwapCached:            0 kB
   Active:          2135860 kB
   Inactive:         546660 kB
   ##### snipped #####
   SwapTotal:       3984380 kB
   SwapFree:        3984380 kB
   ##### snipped #####

   MemTotal is the physical memory visible to the kernel. MemAvailable is the safer capacity signal for new allocations because it accounts for reclaimable cache.

3. Summarize virtual memory counters with vmstat.

   $ vmstat --stats
         3461112 K total memory
         2010704 K used memory
         2134860 K active memory
          546660 K inactive memory
          377468 K free memory
           38452 K buffer memory
         1305588 K swap cache
         3984380 K total swap
               0 K used swap
         3984380 K free swap
   ##### snipped #####

   vmstat –stats is useful when a report needs one snapshot that includes memory, swap, paging, and CPU counter totals.

4. List memory blocks on the full host or virtual machine.

   $ lsmem
   RANGE                                 SIZE  STATE REMOVABLE BLOCK
   0x0000000040000000-0x000000013fffffff   4G online       yes  8-39
   
   Memory block size:                128M
   Total online memory:                4G
   Total offline memory:               0B

   If lsmem is missing on a minimal Ubuntu system, install the package that provides it with sudo apt install util-linux-extra. In containers, lsmem can fail when /sys/devices/system/memory is not exposed.

5. Show firmware memory-device details when slot, speed, type, or module information is needed.

   $ sudo dmidecode --type memory
   # dmidecode 3.6
   Getting SMBIOS data from sysfs.
   SMBIOS 3.6.0 present.
   
   Handle 0x0008, DMI type 17, 92 bytes
   Memory Device
           Size: 4 GB
           Form Factor: DIMM
           Locator: Not Specified
           Type: DRAM
           Speed: Unknown
           Manufacturer: Not Specified
           Serial Number: Not shown
           Part Number: Not shown
   ##### snipped #####

   dmidecode requires root access and can print serial numbers, asset tags, and part numbers. Remove hardware identifiers before sharing command output outside the support case or asset record that needs them.

6. Match the command to the question being answered.

   Use free, /proc/meminfo, or vmstat for memory visible to the running kernel. Use lsmem or dmidecode for host hardware layout and module inventory. Firmware capacity can be slightly higher than MemTotal because firmware, devices, or the hypervisor may reserve memory before Linux reports usable pages.