Monitoring Disk Space with df

In Plain English

The `df` (disk free) command displays the amount of available and used storage space on mounted filesystems. Understanding partition utilization, mount points, and inode capacity (`df -i`) is vital for preventing server outages caused by full disks.

The Mental Model

"`df` is like checking the fuel gauge on your car dashboard: it tells you what percentage of your storage tank is full before the engine stalls."

Deep Dive: How It Works

Core Flags: `-h` (human-readable GB/MB), `-T` (show filesystem type e.g. ext4, xfs, apfs), `-i` (inode usage stats), `-x tmpfs` (exclude virtual memory filesystems).

The 100% Inode Trap: A disk can run out of space even with 500GB free if 100% of available inodes are consumed by millions of tiny 1-byte files.

Root Reserve: Linux ext4 filesystems reserve 5% of space by default for root to prevent system lockups.

Syntax Blueprint

df -h                     # Human-readable disk usage
df -hT                    # Include filesystem type (ext4, xfs)
df -i                     # Inode availability check
df -h /                   # Check root partition only

Use df -h to inspect disk partition capacity and df -i to monitor inode usage.

Core Rules to Remember

Human-Readable -h: Instantly translates raw 1K blocks into GB, TB, and percentages.

Inode Capacity: Every file consumes one inode; monitoring df -i prevents "No space left on device" errors.

Partition Targeting: `df -h /var` specifically checks the volume hosting logs and databases.

Common Beginner Traps & How to Fix Them

Mistake:Deleting a large open log file with `rm` but `df -h` still shows 100% full.

Why it happens: If a running process still has the file open, disk blocks are not freed until the process is restarted or file truncated.

How to fix: Truncate the file in place with `: > /var/log/app.log` or restart the holding process.

Pro Tip: Use `df -h / | awk 'NR==2 {print $5}' | tr -d '%'` to extract the root partition percentage for alert scripts.

Live Interactive Example

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Parsing Disk Utilization with df and awk

Git 2.45 / CLI

Output Console

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How it works: df / queries the root mount, and awk extracts the 5th column (usage percentage).

Your Turn: Micro Challenge

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Micro Exercise

Check Disk Space on Root Mount

Run `df -h /` and pipe to `head -n 2` to print filesystem headers and root partition stats.

Sandbox Output

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