What are the differences between the free sections?

The Unix/Linux free command displays information about system memory usage. The sections it outputs typically include:

• total: The total amount of physical memory available to the system.
• used: The amount of memory in use.
• free: The amount of unused (free) memory.
• shared: Memory used simultaneously by multiple processes (usually minimal).
• buff/cache: Memory used by the kernel for buffering and caching filesystem data. This memory can be quickly freed if needed.
• available: An estimate of the memory that can be allocated to new processes without swapping. It is the sum of free and part of buff/cache that can be reclaimed.

Here is a typical output of the free -m command (in megabytes) with explanations for the sections:

              total        used        free      shared  buff/cache   available
Mem:           7890        2567        1234         123        4089        4879
Swap:          2047           0        2047

Where:

• Mem: The line related to physical RAM.
  • total: 7890 MB.
  • used: 2567 MB (actively used by processes).
  • free: 1234 MB (completely unused).
  • shared: 123 MB (used shared).
  • buff/cache: 4089 MB (buffers and cache).
  • available: 4879 MB (the most accurate representation of available memory for starting new applications).
• Swap: The line related to swap space.
  • total: 2047 MB (total swap size).
  • used: 0 MB (swap in use).
  • free: 2047 MB (free in swap).

The difference between free and available is important: free shows absolutely unused memory, while available is the memory the system can quickly free for applications, including most of buff/cache. The buff/cache section shows memory that is occupied but can be quickly released, so it should not be viewed as "lost" memory.