df and du Commands: Check Disk Usage in Linux

Quick take: Use df -h to see free space on each filesystem and du -sh dir to measure how much a directory is using. Together they answer “is the disk full, and what is filling it?”

Introduction

When a disk fills up, two commands diagnose it: df (disk free) shows how much space each mounted filesystem has, and du (disk usage) shows how much space files and directories occupy. df finds the full partition; du finds what is filling it.

This guide covers reading filesystem usage, measuring directory sizes, finding the biggest offenders, and the inode checks that catch a different kind of “disk full”.

Syntax

The basic syntax of the df and du command is:

df [OPTIONS] [PATH]
du [OPTIONS] [PATH]

Common Options and Parameters

The most useful options and parameters for the df and du command:

Practical Examples

Real df and du commands you can run today:

# Show free space on all filesystems
df -h
# Show free space for one path
df -h /var
# Check inode usage (when df shows space but writes fail)
df -i
# Total size of a directory
du -sh /var/log
# Size of each item in the current directory
du -sh *
# Find the biggest subdirectories
du -h --max-depth=1 /var | sort -h

A Disk-Full Troubleshooting Workflow

When a server reports a full disk, df and du together lead you to the cause in a repeatable sequence. Start broad, then drill down:

# 1. Which filesystem is full?
df -h

# 2. Move into it and find the biggest directories
cd /var
sudo du -h --max-depth=1 | sort -h | tail

# 3. Repeat into the largest one until you find the culprit
sudo du -h --max-depth=1 /var/log | sort -h | tail

# 4. If df shows space but writes still fail, check inodes
df -i

Common culprits are runaway log files, old package caches (/var/cache/apt), and forgotten backups. The inode check in step four catches the subtle case where millions of tiny files exhaust the filesystem before the bytes run out.

Tips and Best Practices

• Start with df -h to find the full filesystem, then du -sh * inside it to drill down to the culprit.
• If df shows free space but you still get “No space left on device”, check df -i — you may be out of inodes, not bytes.
• Pipe du into sort for a ranked view: du -h --max-depth=1 | sort -h puts the largest directories last.

Final Thoughts

df and du are the two commands that solve a full disk. Use df -h to see which filesystem is full and du -sh to track down the directories consuming it, and remember df -i when space looks fine but writes fail. Together they turn a vague “disk full” alert into a precise, fixable answer.