df (disk free space) in Linux CLI - Deep Dive

Overview - df (disk free space)

What is it?

The df command in Linux shows how much disk space is used and available on your file systems. It lists each mounted file system with details like total size, used space, and free space. This helps you understand storage usage on your computer or server. It is a simple way to check if your disks are running out of space.

Why it matters

Without df, you would not easily know how much storage space is left on your disks. This could lead to running out of space unexpectedly, causing programs to fail or data loss. Knowing disk usage helps you manage files, clean up space, and plan upgrades. It keeps your system healthy and prevents crashes caused by full disks.

Where it fits

Before learning df, you should understand basic Linux commands and the concept of file systems and storage. After mastering df, you can learn related commands like du (disk usage) for folder sizes, and tools for monitoring disk health and performance.

Mental Model

Core Idea

df reports the current storage usage and availability for each mounted file system on your Linux machine.

Think of it like...

df is like a fuel gauge for your car's gas tank, showing how much fuel (disk space) you have left before you need to refill (free up space).

┌───────────────┬───────────┬───────────┬───────────┬─────────────┐
│ Filesystem    │ Size      │ Used      │ Avail     │ Mounted on  │
├───────────────┼───────────┼───────────┼───────────┼─────────────┤
│ /dev/sda1     │ 100G      │ 60G       │ 40G       │ /           │
│ tmpfs         │ 2G        │ 0G        │ 2G        │ /run        │
└───────────────┴───────────┴───────────┴───────────┴─────────────┘

Build-Up - 7 Steps

1

Foundation

Basic df command usage

Concept: Learn how to run df to see disk space usage for all mounted file systems.

Open a terminal and type: df This shows a table with columns for filesystem, size, used, available space, and mount point.

Result

You see a list of mounted file systems with their total size, used space, and free space.

Understanding the default output of df is the first step to monitoring disk space on your system.

2

Foundation

Understanding filesystem and mount points

Concept: Learn what filesystems and mount points mean in the context of df output.

Each line in df output represents a filesystem, which is a storage area formatted to hold files. The mount point is where that filesystem is attached in your directory tree, like / or /home.

Result

You can identify which physical or virtual disk corresponds to which part of your system.

Knowing what filesystems and mount points are helps you interpret df output correctly.

3

Intermediate

Using human-readable sizes

🤔Before reading on: do you think df shows sizes in bytes or a more readable format by default? Commit to your answer.

Concept: Learn to use the -h option to show sizes in KB, MB, or GB for easier reading.

Run: df -h This changes the size columns to use units like G for gigabytes, M for megabytes, making it easier to understand at a glance.

Result

Output shows sizes like 20G instead of large numbers like 21474836480.

Using human-readable sizes prevents mistakes caused by misreading large numbers.

4

Intermediate

Checking specific filesystem usage

🤔Before reading on: do you think df can show info for just one filesystem or directory? Commit to your answer.

Concept: Learn to check disk usage for a specific mount point or directory.

Run: df -h /home This shows disk usage only for the filesystem mounted at /home, helping focus on one area.

Result

Output shows size, used, and available space only for /home filesystem.

Targeting specific filesystems helps when managing space on multi-disk systems.

5

Intermediate

Understanding filesystem types with df

Concept: Learn to display the type of each filesystem using the -T option.

Run: df -T This adds a column showing filesystem types like ext4, tmpfs, or nfs, which tells you about the storage format or network mount.

Result

Output includes a new column listing filesystem types for each entry.

Knowing filesystem types helps diagnose issues and understand performance or compatibility.

6

Advanced

Interpreting reserved space and root usage

🤔Before reading on: do you think all free space shown by df is available to all users? Commit to your answer.

Concept: Learn about reserved blocks that only root can use and how df reports them.

Linux filesystems reserve a small percentage of space for root to prevent full disk errors. df shows available space for normal users, which excludes reserved blocks.

Result

You understand why df might show less free space than expected and why root can still write when others cannot.

Knowing reserved space prevents confusion about disk full errors and system stability.

7

Expert

How df calculates disk usage internally

🤔Before reading on: do you think df reads file sizes directly or queries the filesystem metadata? Commit to your answer.

Concept: Understand that df queries filesystem metadata, not individual files, for fast disk usage reporting.

df uses system calls to get filesystem statistics like total blocks, free blocks, and block size. It does not scan files, so it is fast even on large disks.

Result

You realize df is efficient and why it might differ from du, which scans files.

Understanding df's internal method explains its speed and limitations compared to other tools.

Under the Hood

df uses the statfs or statvfs system calls to ask the operating system for filesystem statistics. These calls return data like total blocks, free blocks, and block size. df then calculates used and available space by multiplying blocks by block size. It does not read individual files, so it is very fast. The output reflects the current state of the filesystem as reported by the kernel.

Why designed this way?

df was designed to provide a quick snapshot of disk usage without scanning files, which would be slow on large filesystems. Using system calls to get metadata is efficient and reliable. This design balances speed and accuracy for everyday monitoring needs. Alternatives like scanning files (du) exist but serve different purposes.

┌───────────────┐
│ df command    │
└──────┬────────┘
       │ calls statfs/statvfs
       ▼
┌───────────────┐
│ Kernel FS info│
│ (blocks, size)│
└──────┬────────┘
       │ returns stats
       ▼
┌───────────────┐
│ df calculates │
│ used/avail    │
└──────┬────────┘
       │ displays output
       ▼
┌───────────────┐
│ User sees disk│
│ usage summary │
└───────────────┘

Myth Busters - 4 Common Misconceptions

Quick: Does df show the exact disk space used by files or just filesystem metadata? Commit to yes or no.

Common Belief: df scans all files and sums their sizes to show disk usage.

Reality: df reads filesystem metadata about blocks and sizes, not individual files.

Why it matters: Believing df scans files leads to confusion when df and du outputs differ, causing misinterpretation of disk usage.

Quick: Does df show all free space available to every user? Commit to yes or no.

Common Belief: df shows all free space on the disk available to all users.

Reality: df excludes reserved space set aside for root, so normal users see less free space.

Why it matters: Not knowing about reserved space can cause surprise when users get disk full errors even though df shows free space.

Quick: Does df report space for unmounted or hidden filesystems? Commit to yes or no.

Common Belief: df shows disk usage for all disks and partitions, mounted or not.

Reality: df only reports on mounted filesystems visible in the directory tree.

Why it matters: Assuming df shows all disks can cause missed monitoring of unmounted or hidden partitions.

Quick: Can df show disk usage for network filesystems like NFS? Commit to yes or no.

Common Belief: df only works for local disks, not network mounts.

Reality: df works for all mounted filesystems, including network filesystems like NFS or SMB.

Why it matters: Knowing df works on network mounts helps monitor remote storage usage consistently.

Expert Zone

1

df output can differ depending on block size used internally; using -h or -B options controls this for clarity.

2

Some filesystems support compression or snapshots, which can make df's reported usage differ from actual file sizes.

3

When multiple mount points share the same device, df shows usage per mount, which can be confusing without understanding bind mounts or overlays.

When NOT to use

df is not suitable when you need detailed folder or file size information; use du instead. Also, df does not detect disk errors or fragmentation; use tools like smartctl or fsck for health checks.

Production Patterns

System administrators use df in scripts to monitor disk space and trigger alerts when free space is low. It is combined with cron jobs for regular checks and integrated into monitoring tools like Nagios or Prometheus exporters.

Connections

du (disk usage)

complements df by showing disk usage at directory and file level, while df shows filesystem-level usage

Understanding both df and du helps manage disk space from overall system to individual files.

Filesystem metadata

df relies on filesystem metadata to report usage without scanning files

Knowing how filesystems store metadata clarifies why df is fast and how it can differ from actual file sizes.

Inventory management

both track available resources and usage to prevent shortages

Just like a store tracks stock levels to avoid running out, df tracks disk space to prevent system failures.

Common Pitfalls

#1 Misreading df output as exact file sizes

Wrong approach:

df
Filesystem 1024-blocks Used Available Use% Mounted on
/dev/sda1 20971520 10485760 10485760 50% /
# Assuming Used means sum of all file sizes exactly

Correct approach:

df -h
Filesystem Size Used Avail Use% Mounted on
/dev/sda1 20G 10G 10G 50% /
# Understand this is metadata-based usage, not file-by-file sum

Root cause:

Confusing filesystem block usage with actual file sizes leads to wrong assumptions about disk usage.

#2 Ignoring reserved space for root

Wrong approach:

df -h
Filesystem Size Used Avail Use% Mounted on
/dev/sda1 20G 19G 1G 95% /
# User tries to write 2G more and gets disk full error

Correct approach:

Understand reserved blocks exist; root can write when normal users cannot.

Root cause:

Not knowing reserved space causes surprise disk full errors despite apparent free space.

#3 Running df on unmounted partitions expecting output

Wrong approach:

df /dev/sdb1
# No output or error because /dev/sdb1 is not mounted

Correct approach:

mount /dev/sdb1 /mnt
df /mnt
# Now df shows usage for that filesystem

Root cause:

df only reports mounted filesystems; unmounted devices are invisible.

Key Takeaways

df is a quick tool to check disk space usage and availability on mounted filesystems.

It reports usage based on filesystem metadata, not by scanning files, making it fast but sometimes different from file sizes.

Reserved space for root users means normal users see less free space than total disk space.

Using options like -h and -T makes df output easier to read and understand.

df is essential for monitoring disk health and preventing storage-related system problems.