Understanding the free Command in Linux

Understanding the free Command in Linux

One of the most common things that Linux users and admins do is keep an eye on system memory. The free command is one of the easiest and most useful tools you can use to find out how your system uses memory or fix performance problems.

In this post, I’ll talk about what the free command does, how to read its output, and give some real-life examples.

What is the free command?

The Linux command “free” shows information about how much memory your system is using, such as:

• Total memory available
• Used and free memory
• Buffers and cache memory usage
• Swap space statistics

It provides a quick snapshot of how your RAM and swap are being utilized.

Basic Syntax

free [options]

If you simply run:

free

You’ll get an output similar to this:

               total        used        free      shared  buff/cache   available
Mem:          972160      120344      631688         448      220128      711300
Swap:        2097148           0     2097148

Let’s break it down.

Understanding the Output Columns
 

1. total

The total amount of physical RAM or swap memory in your system.

2. used

The memory that is currently being used by running processes and the system cache.

3. free

The memory that is completely unallocated (not used at all).

4. shared

Memory that is shared between processes (commonly used by tmpfs).

5. buff/cache

Memory used by the kernel for buffers and cache.

Linux uses free memory for caching files to improve performance – but this memory is released automatically when needed by applications.

6. available

This is the most important value – it shows how much memory is actually available for new applications without swapping.

Example with Human-Readable Format
 

To make the output easier to read, use the -h (human-readable) option:

free -h

Output example:

              total        used        free      shared  buff/cache   available
Mem:            15Gi        4.4Gi       2.9Gi       521Mi       8.2Gi        10Gi
Swap:          2.0Gi           0B       2.0Gi

Now you can easily see memory sizes in GB or MB.

Other Useful Options
 

1. Display in Megabytes or Gigabytes

You can use the -s option to update the output continuously:

free -m   # Display in MB
free -g   # Display in GB

2. Show Memory Every 2 Seconds

You can use the -s option to update the output continuously:

free -h -s 2

This refreshes the memory statistics every 2 seconds — useful for monitoring memory changes in real time.

3. Display the Total at the End

# free -mt
               total        used        free      shared  buff/cache   available
Mem:             949         117         616           0         215         694
Swap:              0           0           0
Total:           949         117         616

This shows a total line summarizing RAM and swap usage.

Understanding Buffers and Cache

One common confusion is when used memory seems high.

For example:

used: 14GB
free: 1GB

But your system isn’t actually running out of memory – that’s because Linux uses free RAM to cache frequently accessed data.

You can get a truer sense of available memory by focusing on the “available” column.

Combine with Other Commands

You can combine free with tools like top or vmstat for deeper analysis:

• top or htop → for live process and memory monitoring
• vmstat → for virtual memory statistics
• cat /proc/meminfo → for detailed system memory info

Summary

Final Thoughts

free command is a simple yet essential Linux utility for system monitoring. Understanding how Linux manages memory – especially the buffers/cache concept – helps you make smarter decisions when diagnosing system performance or resource usage.
When you know how to read free output correctly, you’ll never misinterpret “high memory usage” again.