free

Understanding the Basics

The simplest form of the command, free, provides a snapshot of your system’s memory usage. Let’s break down a typical output:

              total        used        free      shared  buff/cache   available
Mem:          1996        162        1677          11         157        1786
Swap:         2047          0        2047

• Mem: Represents physical memory (RAM).
  • total: Total amount of RAM installed.
  • used: Memory currently in use by processes and the kernel.
  • free: Memory not currently used by any process.
  • shared: Memory shared between processes (often used by libraries).
  • buff/cache: Memory used for buffering and caching. This memory is not necessarily “free” in the sense that applications can directly use it, but it’s readily available.
  • available: This is arguably the most important metric. It represents the amount of memory readily available to new processes. It accounts for “free” memory and memory used for buffers/cache that can be easily reclaimed.
• Swap: Represents swap space (a portion of your hard drive used as an extension of RAM).
  • total: Total amount of swap space.
  • used: Amount of swap space currently in use.
  • free: Amount of swap space available.

Adding Detail with Options

The free command offers many useful options to customize the output:

• -h (or --human-readable): Displays the output in a more human-friendly format, using units like KB, MB, and GB. This is highly recommended for easier interpretation.

free -h

• -m (or --mega): Displays the output in megabytes.

free -m

• -g (or --giga): Displays the output in gigabytes.

free -g

• -b (or --bytes): Displays the output in bytes.

• -s <interval> (or --interval <interval>): Displays the memory usage information repeatedly at the specified interval (in seconds). This is very useful for real-time monitoring.

free -h -s 2  # Display memory usage every 2 seconds.

• -t (or --total): Displays the total memory usage (sum of Mem and Swap).

free -h -t

• -o (or --only-mem): Shows only the memory information without the Swap information.

free -h -o

• -w (or --width <width>): Use this to set the output width

free -h -w 60

Interpreting the Output for Troubleshooting

High used memory and low available memory could indicate a memory leak, a process consuming excessive resources, or insufficient RAM. High Swap usage could point to insufficient RAM, requiring you to either upgrade your RAM or optimize your processes to consume less memory. The free command provides the foundational data to investigate these scenarios further. By combining free with other commands like top (for monitoring processes) and ps (for listing processes), you can effectively diagnose and resolve memory-related issues.

Beyond the Basics: Working with Output

The output of free can be further processed using other commands like awk to extract specific values or create custom reports. For example, to get just the available memory in megabytes:

free -m | awk '/Mem:/ {print $7}'

This example uses awk to filter the output for the line containing “Mem:” and then print the 7th column (available memory in MB). Similar techniques can be used to extract and manipulate any other data field from the free command’s output.