These tools show the memory usage of all running processes (for example when you are running Ubuntu with Unity, but no particular application program running).
I am also using
free -m # Display the amount of memory in megabytes.
and I think the column 'used' is useful (no pun intended).
The output of
free is explained in the manual
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/mem‐
info. The displayed columns are:
Total installed memory (MemTotal and SwapTotal in /proc/meminfo)
Used memory (calculated as total - free - buffers - cache)
Unused memory (MemFree and SwapFree in /proc/meminfo)
Memory used (mostly) by tmpfs (Shmem in /proc/meminfo, available on kernels 2.6.32, displayed as zero if not available)
Memory used by kernel buffers (Buffers in /proc/meminfo)
Memory used by the page cache and slabs (Cached and Slab in /proc/meminfo)
Sum of buffers and cache
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)
top will show the same (or almost the same) result (tested in 16.04 LTS), (but in older versions the output of top was more difficult to match to the output of
htop, more user friendly
You will get a similar result as 'used' with
htop installed with
sudo apt install htop