Ubuntu Manpage: free - Display amount of free and used memory in the system

free displays the total amount of free and used physical and swap memory in the system, as well as the buffers and caches used by the kernel. The information is gathered by parsing /proc/meminfo. The displayed columns are:

total: Total installed memory (MemTotal and SwapTotal in /proc/meminfo)
used: Used memory (calculated as total - free - buffers - cache)
free: Unused memory (MemFree and SwapFree in /proc/meminfo)
shared: Memory used (mostly) by tmpfs (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
available: Estimation of how much memory is available for starting new applications, without swapping. Unlike the data provided by the cache or free fields, this field takes into account page cache and also that not all reclaimable memory slabs will be reclaimed due to items being in use (MemAvailable in /proc/meminfo, available on kernels 3.14, emulated on kernels 2.6.27+, otherwise the same as free)

OPTIONS

-b, --bytes

Display the amount of memory in bytes.

-k, --kilo

Display the amount of memory in kilobytes. This is the default.

-m, --mega

Display the amount of memory in megabytes.

-g, --giga

Display the amount of memory in gigabytes.

--tera

Display the amount of memory in terabytes.

-h, --human

Show all output fields automatically scaled to shortest three digit unit and display the units of print out. Following units are used.

  B = bytes
  K = kilos
  M = megas
  G = gigas
  T = teras

If unit is missing, and you have petabyte of RAM or swap, the number is in terabytes and columns might not be aligned with header.

-w, --wide

Switch to the wide mode. The wide mode produces lines longer than 80 characters. In this mode buffers and cache are reported in two separate columns.

-c, --count count

Display the result count times. Requires the -s option.

-l, --lohi

Show detailed low and high memory statistics.

-s, --seconds seconds

Continuously display the result delay seconds apart. You may actually specify any floating point number for delay, usleep(3) is used for microsecond resolution delay times.

--si