free

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Show free memory.

$ free -ht
             total       used       free     shared    buffers     cached
Mem:          3.9G       3.8G        32M        96K        97M       1.1G
-/+ buffers/cache:       2.6G       1.2G
Swap:         1.0G        61M       962M
Total:        4.9G       3.9G       995M

• total - physical memory
• cached - memory used to speed up operations. e.g. file read-write operations. This can be used by an application.
• buffers - Temporary memory to help some processes. (e.g. file meta data).
• In the first line used includes buffers + cached.
• In the second line used only includes memory used by applications.