---
title: free — Check memory usage
date: 2026-10-18
time: 12:00
model: admin
category: programs
author_type: human
summary: A command that shows physical memory, swap, and buffer/cache usage at a glance.
tags: program, Service and process management, free
---

**💬 "free command"**

free shows **memory and swap usage**. The available value is the indicator used to judge actual available memory.

buff/cache is reclaimable, so a low free value can still be fine.

- **total**: Total physical memory.
- **used**: Memory used by processes.
- **free**: Completely unused memory. This value alone is misleading.
- **available**: An estimate of what a new process can use, so **look at this value**.
## Commonly used options

| Option | Description |
| --- | --- |
| -h | Output in human-readable units |
| -m / -g | Output in MB / GB units |
| -s <seconds> | Repeat output every specified number of seconds |
| -t | Add a total line |

## 3 essential command combinations

- `free -h` — Check current memory status (most basic)
- `free -h -s 5` — Repeat every 5 seconds to check trends
- `free -m | awk 'NR==2{print $7}'` — Extract the available value for monitoring
**💬 "Show me just one example of checking a long-running server"**

Here are situations commonly encountered on real production servers, organized in order.

### Step 1 · Current status

Check available memory and swap usage.

```
free -h
```

### Step 2 · Check trends

Use repeated output to see the decreasing trend.

```
free -h -s 5
```

### Step 3 · Identify the cause

Find processes using a lot of memory.

```
ps -eo pid,%mem,rss,comm --sort=-%mem | head
```

## How it is used in practice

### 1. OOM prevention check

**Situation** — Processes keep dying because memory is low.

```
free -h && dmesg -T | grep -i oom | tail -3
```

**Result** — If available is very low and swap usage is high, the system is short on memory.

**Point** — Review application memory limits (php-fpm pm.max_children, etc.) or scale the server.

### 2. Don't panic over cache

**Situation** — free was nearly 0, so it was mistaken for a failure.

```
free -h
```

**Result** — Confirm that buff/cache accounts for a large portion and that available is sufficient.

**Point** — Linux uses spare memory as cache, so judge by available.

## Important notes

- A low free value does not immediately mean memory is short. Use available as the basis.
- Heavy swap usage sharply degrades performance (increased iowait).
- In container environments, host memory and cgroup limits differ, so be careful.
## Actual output

This is the result of checking memory and swap usage.

![free command output: memory and swap usage](https://games.aidebatehub.com/assets/free-demo.png)

An actual production server

---
Source (HTML): https://games.aidebatehub.com/programs/linux/services/free.html
This file is an auto-generated Markdown twin of the DevHub document.