1. Overview

Sometimes we want to know the memory usage of our Linux system. In this tutorial, we’ll see how to use the free command-line utility to do just that.

2. free Command Examples

The free command is a Linux tool that shows the current RAM and swap space usage:

$ free
              total        used        free      shared  buff/cache   available
Mem:       32882588     4418516    26725724      437164     1738348    27589116
Swap:       2097148           0     2097148

Let’s look at what each column in our output means:

  • total: total memory available in the system
  • used: the memory the system is currently using
  • free: the unused memory
  • shared: memory used by tmpfs
  • buff/cache: the combined memory filled by kernel buffers, page cache, and slabs
  • available: estimated free memory that can be used without starting to swap

2.1. Human Readable Option

The free command, by default, displays all the values in KB, which can be a bit uncomfortable to read, but we can use the -h option to display the numbers in a more readable format:

$ free -h
              total        used        free      shared  buff/cache   available
Mem:            31G        6.2G         23G        541M        1.9G         24G
Swap:          1.5G          0B        1.5G

2.2. Memory Expressed in Megabytes:

To display the values expressed in megabytes we can use the -m option:

$ free -m
              total        used        free      shared  buff/cache   available
Mem:          32111        6200       23000         541        1900       24000
Swap:          1500           0        1500

2.3. Memory Expressed in Gigabytes:

Likewise, we can also show the values expressed in gigabytes using the  -g option:

$ free -g
              total        used        free      shared  buff/cache   available
Mem:             31           6          23           0           1          24
Swap:             1           0           1

2.4. Power of 1000 Formatting

By default, the values are expressed by the power of 1024, but if we want to display the value expressed by the power of 1000, we can use the –si option:

$ free -h --si
              total        used        free      shared  buff/cache   available
Mem:            32G         6.3G         22G        553M        1.8G         25G
Swap:          1.5G           0B        1.5G

2.5. Total Amount of Memory

If we want to know the amount of RAM and swap used/free memory combined we can use the -t option:

$ free -h -t 
              total        used        free      shared  buff/cache   available
Mem:            31G         13G        9.8G        1.1G        8.3G         16G
Swap:          1.5G          0B          0B
Total:        32.5G         13G       11.3G

3. Realtime Display

We can also use free to show memory usage in realtime.

Let’s use the -s option followed by the number of seconds we want between each display:

$ free -h -s 2
              total        used        free      shared  buff/cache   available
Mem:            31G        6.2G         23G        541M        1.9G         24G
Swap:          1.5G          0B        1.5G

              total        used        free      shared  buff/cache   available
Mem:            31G        6.2G         23G        541M        1.9G         24G
Swap:          1.5G          0B        1.5G

              total        used        free      shared  buff/cache   available
Mem:            31G        6.2G         23G        541M        1.9G         24G
Swap:          1.5G          0B        1.5G

In this example, we’ll get the statistics every 2 seconds.

4. Memory Usage Logging

Sometimes we need to log the memory usage for a long time. In this case, a small Bash script can be useful.

We can execute the free command and the date command together and redirect the result to a log file:

while true; do date >> memory.log; free >> memory.log; sleep 1; done

This one-line script will write to a log file the date and the memory usage every second. Of course, we can tweak the number after sleep to change the log frequency.

5. Conclusion

In this quick tutorial, we learned how to use the free command to display and log the system memory usage. This won’t stop us from having memory problems but will help us diagnose them if we do.

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