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I am trying to understand the output of the free -hm command. I know about the columns other than "shared". Shared is taking up 50% of my RAM and I couldnt find much relevant info on it what it is and what it does. I am running ubuntu 16.10.

It will be helpful if anyone can explain it :)

              total        used        free      shared  buff/cache   available
Mem:           7,7G        2,2G        498M        3,7G        5,0G        1,5G
Swap:          3,9G        352M        3,6G
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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

Source: see man free

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