How to get memory used(RAM used) using Linux command?

Here is sample output from free:

% free
             total       used       free     shared    buffers     cached
Mem:      24683904   20746840    3937064     254920    1072508   13894892
-/+ buffers/cache:    5779440   18904464
Swap:      4194236        136    4194100

The first line of numbers (Mem:) lists

• total memory
• used memory
• free memory
• usage of shared
• usage of buffers
• usage filesystem caches (cached)

In this line used includes the buffers and cache and this impacts free. This is not your "true" free memory because the system will dump cache if needed to satisfy allocation requests.

The next line (-/+ buffers/cache:) gives us the actual used and free memory as if there were no buffers or cache.

The final line (Swap) gives the usage of swap memory. There is no buffer or cache for swap as it would not make sense to put these things on a physical disk.

To output used memory (minus buffers and cache) you can use a command like:

% free | awk 'FNR == 3 {print $3/($3+$4)*100}'
23.8521

This grabs the third line and divides used/total * 100.

And for free memory:

% free | awk 'FNR == 3 {print $4/($3+$4)*100}' 
76.0657

I am going to mention on how to parse the information on free command. To find the percentage, you can use as suggested in the other 2 answers.

This is clearly explained here. I would try to explain from what I have in my system.

free -m
             total       used       free     shared    buffers     cached
Mem:          7869       4402       3466          0        208       3497
-/+ buffers/cache:        696       7173
Swap:         3999        216       3783

Now, let us see what the various numbers actually represent.

Line1

• 7869 Indicates memory/physical RAM available for my machine.
• 4402 Indicates memory/RAM used by my system.
• 3466 Indicates Total RAM free and available for new process to run.
• 0 Indicates shared memory. This column is obsolete and may be removed in future releases of free.
• 208 Indicates total RAM buffered by different applications in Linux.
• 3497 Indicates total RAM used for Caching of data for future purpose.

Line2

-/+ buffers/cache:        696       7173

How to calculate the values 696 and 7173 obtained in Line2?

Total used (4402) - Total buffer RAM (208) - Total RAM for caching(3497) should constitute the Actual used RAM in the system. It returns me 697 which is the actual used RAM output in the second line.

Now, Total Available (7869) - Actual Used (696) should give you the free memory which is 7173 in this case which is also got as output in the second line.

Though a duplicate as pointed by @szboardstretcher , my preference from the solutions (in the original question) is the one below, specially since you want to parse to a webpage.

$ free | awk '/Mem/{printf("used: %.2f%"), $3/$2*100} /buffers\/cache/{printf(", buffers: %.2f%"), $4/($3+$4)*100} /Swap/{printf(", swap: %.2f%"), $3/$2*100}'

Output:

used: 82.68%, buffers: 42.81%, swap: 1.27%