There’s nothing more crucial to a computer or server than memory.
Information on memory installed, memory used and memory free are indispensable data to any Linux system administrator.
Should memory on your system be inadequate for the load, the results will be most unpleasant to users.
The best way to understand memory use and availability on your Linux system is via the ‘free‘ command, a feature that comes with most Linux installations.
Understanding Memory
Before jumping into the nuances of the “free” command, let’s spend a few seconds to understand the basic memory concepts.
* Physical Memory – Amount of memory installed and currently used
* Real Memory – How much memory is used by the applications
* Swap Memory – Portion of virtual memory on hard disk that’s used when RAM is full (high swap memory usage is not good)
* Virtual Memory – Combination of RAM and disk space that running processes can leverage
Free – Basics
On the command line, we use free to get information about memory use on a Linux desktop or server.
In this post, let’s consider the various options for free.
usage: free [-b|-k|-m|-g] [-l] [-o] [-t] [-s delay] [-c count] [-V]
-b,-k,-m,-g show output in bytes, KB, MB, or GB
-l show detailed low and high memory statistics
-o use old format (no -/+buffers/cache line)
-t display total for RAM + swap
-s update every [delay] seconds
-c update [count] times
-V display version information and exit
Free – Various Commands
Let’s start with the basic command.
jason@ChristyPC ~ $ free
total used free shared buffers cached
Mem: 3968848 3352196 616652 200544 225512 2040400
-/+ buffers/cache: 1086284 2882564
Swap: 4022268 2016 4020252
In the above example from my Linux Mint desktop, the three lines represent physical memory, real memory and swap memory respectively.
The “used” memory in the first line is misleading because Linux systems take into account even buffer and cache.
So when you run the free command, the second line representing real memory is the most important line in understanding memory use on your computer.
In the above example, there’s still 2.88GB of free memory available, which means I’m in a comfortable zone on my Linux Mint desktop.
Display Free Output in Human Understandable Format
Sometimes command line outputs are needlessly complicated.
Thankfully, there are simple ways out.
jason@ChristyPC ~ $ free -h
total used free shared buffers cached
Mem: 3.8G 3.4G 387M 217M 219M 2.0G
-/+ buffers/cache: 1.1G 2.6G
Swap: 3.8G 68K 3.8G
The above command worked fine on Ubuntu and Linux Mint and by extension should work on all Debian systems.
Note: Unfortunately, on Red Hat Linux systems the free -h command does not work.
I got an an error message (see below) when I ran the free -h command on a Red Hat system:
jason@ChristyPC ~ $ free -h free: invalid option -- h
Display Memory in Gigabytes
You have the option to display in gigabytes, megabytes etc.
Let’s look at the command for free in gigabyte format since it’s easy to understand at just a glance.
jason@ChristyPC ~ $ free -g
total used free shared buffers cached
Mem: 3 3 0 0 0 1
-/+ buffers/cache: 1 2
Swap: 3 0 3
Display Memory in Megabytes
The below command will output memory in megabytes format.
jason@ChristyPC ~ $ free -m
total used free shared buffers cached
Mem: 3875 3269 606 196 220 1993
-/+ buffers/cache: 1055 2820
Swap: 3927 1 3926
Display Memory in Kilobytes
I doubt many people still look at memory in kilobytes any more since gigabyte is more the norm.
jason@ChristyPC ~ $ free -k
total used free shared buffers cached
Mem: 3968848 3344556 624292 201112 226092 2041088
-/+ buffers/cache: 1077376 2891472
Swap: 4022268 2016 4020252
Display Memory in Bytes
I’d be surprised if even one in a thousand Linux users look at memory in bytes any more.
jason@ChristyPC ~ $ free -b
total used free shared buffers cached
Mem: 4064100352 3429953536 634146816 202133504 231030784 2086174720
-/+ buffers/cache: 1112748032 2951352320
Swap: 4118802432 2064384 4116738048
Display Total Line
The below command combines physical memory and swap memory to give you total memory.
jason@ChristyPC ~ $ free -t
total used free shared buffers cached
Mem: 3968848 3346840 622008 200360 226304 2040372
-/+ buffers/cache: 1080164 2888684
Swap: 4022268 2016 4020252
Total: 7991116 3348856 4642260
Remove Display of Buffer Line
The below ‘free’ command gets rid of the buffer/cache line.
jason@ChristyPC ~ $ free -o
total used free shared buffers cached
Mem: 3968848 3353580 615268 205088 226360 2045116
Swap: 4022268 2016 4020252
Display Memory Status for Regular Intervals
In the below ‘free’ command, I’m checking memory use every five seconds (hence -s 5).
Since I waited for 15 seconds, you’ll see three results below.
jason@ChristyPC ~ $ free -s 5
total used free shared buffers cached
Mem: 3968848 3347544 621304 201800 226424 2041840
-/+ buffers/cache: 1079280 2889568
Swap: 4022268 2016 4020252
total used free shared buffers cached
Mem: 3968848 3345948 622900 199856 226432 2039896
-/+ buffers/cache: 1079620 2889228
Swap: 4022268 2016 4020252
total used free shared buffers cached
Mem: 3968848 3346072 622776 199876 226440 2039920
-/+ buffers/cache: 1079712 2889136
Swap: 4022268 2016 4020252
Display Low and High Memory Statistics
jason@ChristyPC ~ $ free -g
total used free shared buffers cached
Mem: 3968848 3349400 619448 200772 226548 2040840
Low: 855884 671920 183964
High: 3112964 2677480 435484
-/+ buffers/cache: 1082012 2886836
Swap: 4022268 2016 4020252
Check Version of Free on your computer
I first ran the free -V below command on my Linux Mint desktop:
jason@ChristyPC ~ $ free -V free from procps-ng 3.3.9
Then I ran the same command on a Red Hat server:
jason@ChristyPC ~ $ free -V procps version 3.2.7
The above output shows it’s time to update “free” on the Red Hat system.
Free – Bottom Line
I cannot overemphasize how important memory is to the smooth running of your computer and ultimately your business, non-profit, web site, blog etc.
Folks, I hope the above ‘free‘ cheat sheet proves helpful to you as you newbies go about mastering the command line.
Now go ahead and try these “free” commands on your Linux computer.
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