Unix memory management

When using top/free to see memory usage in Unix, there're 'used/free/shared/buffers/cached' categories. What do those exactly mean? I've tried to look for a wiki but didn't find detail information. Please advise, thanks.

Answer

Most of that information is in the man page for top.

Shared: The amount of shared memory used by a task. It simply reflects memory that could be potentially shared with other processes.
Free: Actually unused memory.
Used: Allocated to processes.
Buffers: Memory used for various OS buffers. Such as OS file-structures.
Cache: Memory used for various OS caches, generally the block and file caches.

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