Master Linux Memory Monitoring: The Complete Guide to the free Command

This article provides a thorough guide to the Linux free command, which shows statistics about physical memory and swap usage.

Metric Explanation

total : total physical memory (total = used + free)

used : memory in use, including applications, buffers, and cache

free : amount of idle memory

shared : shared memory amount

buffers : cache used by block devices

cached : cache of regular file data

available : total memory readily available for new applications (does not include swap)

For the available field, kernels ≥ 3.14 read it from /proc/meminfo (MemAvailable); older kernels simulate it, otherwise it equals the free value.

Adjusted values –/+ buffers/cache

used : memory used by applications after subtracting buffers and cache

free : true free memory, i.e., idle memory plus buffers and cache

Swap usage statistics

total : total swap space

used : swap currently in use

free : unused swap space

Normal conditions (no cause for alarm)

Free memory ( free) is close to 0.

Used memory ( used) is close to total.

Available memory ( free+buffers/cache) exceeds 20% of total.

Swap usage remains unchanged.

Low‑memory warnings

Available memory ( free+buffers/cache) is very low, approaching 0.

Swap usage ( swap used) increases or fluctuates.

Output of dmesg | grep oom-killer shows an Out‑Of‑Memory killer running.

Common options

-b/k/m/g : display results in bytes, kilobytes, megabytes, or gigabytes.

-h : human‑readable format.

-t : add a total line.

-o : omit the second line with adjusted buffers/cache values.

-s : refresh output every specified number of seconds.

-c : limit the number of refreshes (used with -s).

-l : show detailed low and high memory statistics.

-a : display available memory.

-V : show version information.

Reference example

# free -t -a -g