Question about free -m

(answer by jfeinbaum)

The free command is used to measure the total amount of free and in use physical and swap memory in your system, and adding the -m option will modify it to show all outputs in MegaBytes (MB). As for the different categories it shows, lets look at an example output to get a better understanding:

free -m
              total        used        free      shared  buff/cache   available
Mem:           3946         167        3115           7         663        3543
Swap:           511           0         511

As you can see, the output gives us a handful of categories. Going through them by column from left to right, the definition of each is below:

total:

• The Total installed memory (MemTotal and SwapTotal in /proc/meminfo)
  used:
• The total Used memory (calculated as total - free - buffers - cache)
  free:
• The total Unused memory (MemFree and SwapFree in /proc/meminfo)
  shared Memory:
• The memory that is used (for the most part) by tmpfs (the temporary file storage system) (Shmem in /proc/meminfo, available on kernels 2.6.32, displayed as zero if not available)
  buffers:
• Memory used by kernel buffers (Buffers in /proc/meminfo)
  cache:
• Memory used by the page cache and slabs (Cached and Slab in /proc/meminfo)
  buff/cache: Sum of buffers and cache

These descriptions were taken, (and slightly modified) from the Manual page for the free command. Try this command to check it out yourself! man free