Understanding free command in Linux/Unix

free is a command which can give us valuable information on available RAM in Linux machine. But many new Linux users and admins misinterpret its output. In this post we will walk through its output format and show you actual free RAM.

Free command examples

Example 1: Display RAM details in Linux machine



	total used free shared buffers cached
Mem: 8027952 4377300 3650652 0 103648 1630364
-/+ buffers/cache: 2643288 5384664
Swap: 15624188 608948 15015240

Let us see what this table for you.

Line 1: Indicates Memory details like total available RAM, used RAM, Shared RAM, RAM used for buffers, RAM used of caching content.
Line 2: Indicates total buffers/Cache used and free.
Line 3: Indicates total swap memory available, used swap and free swap memory size available.

Let us dig more in to these lines to better understand it as Linux user prospective.

Line 1:Mem: 8027952 4377300 3650652 0 103648 1630364

8027952 : Indicates memory/physical RAM available for your machine. These numbers are in KB's
4377300 : Indicates memory/RAM used by system. This include even buffers and cached data size as well.
3650652 : 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 This column is obsolete and may be removed in future releases of free.
103648 : Indicates total RAM buffered by different applications in Linux
1630364 : Indicates total RAM used for Caching of data for future purpose.

Puzzled with buffers and cache?

What is the difference between buffers and Cache?

A buffer is a temporary location to store data for a particular application and this data is not used by any other application. This is similar to bandwidth concept. When you try to send burst of data through network, if your network card is capable of sending less data, it will keep these huge amounts of data in buffer so that it can send data constantly in lesser speeds. In other hand Cache is a memory location to store frequently used data for faster access. Other difference between a buffer and a cache is that cache can be used multiple times where as buffer is used single time. And both are temporary store for your data processing.

Line 2: -/+ buffers/cache: 2643288 5384664

	2643288 : This is actual size of used RAM which we get from RAM used -(buffers + cache)
A bit of mathematical calculation
Used RAM = +4377300
Used Buffers = -103648
Used Cache = -1630364
	Actual Total used RAM is 4377300 -(103648+1630364)= 2643288

Then why my Linux machine is showing 4377300 as used RAM. This is because Linux counts cached RAM, Buffered RAM to this used RAM. But in future if any application want to use these buffers/cache, Linux will free it for you. To know more about this, visit this site.

5384664 : Indicates actual total RAM available, we get to this number by subtracting actual RAM used from total RAM available in the system.

	Total RAM = +8027952
actual used RAM = -2643288
Total actual available RAM = 5384664

So from today on words don't complain that Linux ate your RAM, its our understanding of free command output which is the culprit and the teacher who thought us Linux. If any one asks what is the free RAM available, we have to give this number(5384664) instead of first line number(4377300) for free RAM available in your machine.

Line 3: Swap: 15624188 608948 15015240

This line indicates swap details like total SWAP size, used as well as free SWAP.

Swap is a virtual memory created on HDD to increase RAM size virtually. To know more about swap click here for creating swap partition and here for swap file creation.

Example2: Display RAM in human readable formats like in KB's, MB's, GB's, TB's

	free -k
free -m
free -g
free --tera


	[email protected]:/home/surendra# free -k
total used free shared buffers cached
Mem: 8027952 5323952 2704000 0 116876 1626940
-/+ buffers/cache: 3580136 4447816
Swap: 15624188 603792 15020396
[email protected]:/home/surendra# free -m
total used free shared buffers cached
Mem: 7839 5197 2642 0 114 1588
-/+ buffers/cache: 3495 4344
Swap: 15257 589 14668
[email protected]:/home/surendra# free -g
total used free shared buffers cached
Mem: 7 5 2 0 0 1
-/+ buffers/cache: 3 4
Swap: 14 0 14
[email protected]:/home/surendra# free --tera
total used free shared buffers cached
Mem: 0 0 0 0 0 0
-/+ buffers/cache: 0 0
Swap: 0 0 0

Note: If you observe with option –tera, the RAM size shows as 0, this is because my RAM is just 8GB and the output of free command will not support floating point numbers.

Example3: My actual RAM is 8GB but it is showing 7GB when using -g option, whats wrong with my RAM. This is because free command by default counts 1024 as power instead of 1000. To see difference use –si

With out –si

	free -g


	total used free shared buffers cached
Mem: 7 5 2 0 0 1
-/+ buffers/cache: 3 4
Swap: 14 0 14

with –si

	free --si -g


	total used free shared buffers cached
Mem: 8 5 2 0 0 1
-/+ buffers/cache: 3 4
Swap: 15 0 15

Example 4: Want to see combined size of both swap as well as RAM use -t option

	free -t


	total used free shared buffers cached
Mem: 8027952 5369012 2658940 0 117228 1634396
-/+ buffers/cache: 3617388 4410564
Swap: 15624188 603788 15020400
Total: 23652140 5972800 17679340

Example 5: I want to see continuous varying of RAM usage for every second, use -s option to mention number of seconds

	free -s 1


	total used free shared buffers cached
Mem: 8027952 5370220 2657732 0 117376 1635144
-/+ buffers/cache: 3617700 4410252
Swap: 15624188 603788 15020400 total used free shared buffers cached
Mem: 8027952 5367244 2660708 0 117392 1635272
-/+ buffers/cache: 3614580 4413372
Swap: 15624188 603788 15020400 total used free shared buffers cached
Mem: 8027952 5367556 2660396 0 117392 1635272
-/+ buffers/cache: 3614892 4413060
Swap: 15624188 603788 15020400 total used free shared buffers cached
Mem: 8027952 5367388 2660564 0 117392 1635272
-/+ buffers/cache: 3614724 4413228
Swap: 15624188 603788 15020400

This is not a good way to display continuous output, we can use watch command to do iterations

	watch free

In out next post we will see other RAM details.

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My name is Surendra Kumar Anne. I hail from Vijayawada which is capital of south Indian state of Andhra Pradesh. I am a Linux evangelist who believes in Hard work, A down to earth person, Likes to share knowledge with others, Loves dogs, Likes photography. I work with ICE systems, Sydney as Sr. Consultant. You can contact me at surendra (@) linuxnix dot com.
  • http://gravatar.com/thomaseichberger Thomas Eichberger

    Thx. The best explanation of “free” I read so far on the web.

    • http://www.linuxnix.com Surendra Anne

      Thanks Thomas..!

  • Mike

    Very impressive explanation. Thank you for taking the time to break-it-down for me.

  • http://www.learnlinuxchennai.com Mansoor Md

    Nice explanation.

  • Naresh

    Great explanation Boss…

  • Usuario

    The RAM is now measured in decimal prefix?

  • surendra

    thank you boss…..nice artical

    • http://www.linuxnix.com Surendra Anne

      Welcome alterego.. :)

  • Mahesh Rayala

    Great explanation boss……..:)

  • https://plus.google.com/113320115731380273876 Niloofar Safiran

    Perfect. Thanks a lot.

  • Ravichand

    Very good explanation. Thanks much :)

  • Hisham Mohammed

    Awesome!. Great Explaination. Thank you very much.

  • http://None Eitan

    I'm using a kernel version 2.6.16 in which the output of "free" is

                    total             used          free       shared      buffers                  

     Mem:         115824        80180        35644            0                0

    Swap:                0                0                0

    Total:       115824        80180        35644

     ? cache is missing…, do you know how can I calculate the actual used/free memory in this case 

    ?Is there some other way to get the cache size 



    • http://www.linuxnix.com Surendra Anne

      Actuall used and free is always the total line ie 80180        35644.

      For cached size you can check in /proc/meminfo.


  • Khaja

    @Surendra…this is the best explanation of memory usage in linux as per my knowledge.

    Thanks a lot bro!!!.

  • Mustufa Hasan

    Awesome explaination