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Checking Disk Usage (df, du)

1. Introduction to Disk Usage Monitoring​

Disk space is a critical resource in Linux systems. Proper monitoring ensures that systems remain functional and avoid unexpected downtime caused by insufficient storage. It is especially important for:

  • Preventing system failures: Running out of disk space can halt important processes.
  • Maintaining performance: Low disk space can lead to slower I/O operations.
  • Analyzing and optimizing storage usage: Identifying large or unnecessary files helps free up space.

Linux provides two essential commands for monitoring disk usage:

  • df (Disk Filesystem): Provides an overview of disk usage for mounted filesystems.
  • du (Disk Usage): Examines detailed usage for specific files and directories.

2. Using the df Command – Disk Filesystem Overview​

The df command is primarily used to display the total, used, and available disk space on all mounted filesystems.

Syntax:

df [OPTION] [FILE]

Examples:​

1. Display Disk Space Summary for All Mounted Filesystems:​
df

Expected Output:

Filesystem     1K-blocks    Used Available Use% Mounted on
/dev/sda1       20511356 12456928   7042560  64% /
tmpfs            1024000        0   1024000   0% /dev/shm
  • Filesystem: The device or partition name.
  • 1K-blocks: Total space in 1KB blocks.
  • Used: Space currently utilized.
  • Available: Free space remaining.
  • Use%: Percentage of used space.
  • Mounted on: Directory where the filesystem is attached.
2. Human-Readable Format (Sizes in KB, MB, GB):​
df -h

Expected Output:

/dev/sda1        20G   12G  6.7G  64% /
  • The -h option converts units to human-readable formats.
3. Analyze a Specific Directory or Partition:​
df /home
  • This provides information only for the /home directory, useful for troubleshooting specific storage issues.
4. Include Filesystem Type Information:​
df -T

Expected Output:

Filesystem     Type   Size  Used Avail Use% Mounted on
/dev/sda1      ext4    20G   12G  6.7G  64% /
  • Displays the type of filesystem, such as ext4 or xfs.
5. Check Inode Usage (File System Metadata):​
df -i

Expected Output:

Filesystem       Inodes  IUsed   IFree IUse% Mounted on
/dev/sda1       1024000   50200  974800    5% /
  • Useful when there is space available, but no more files can be created due to inode exhaustion.

3. Using the du Command – Detailed Disk Usage Analysis​

The du command provides detailed information about disk usage for files and directories.

Syntax:

du [OPTION] [FILE/DIR]

Examples:​

1. View Disk Usage of a Directory:​
du /var/log

Expected Output:

512     /var/log/mail
1024    /var/log/syslog
2048    /var/log
  • Lists usage in KB for all subdirectories.
2. Human-Readable Sizes (MB, GB):​
du -h /var/log

Expected Output:

512K    /var/log/mail
1.0M    /var/log/syslog
2.0M    /var/log
3. Summarize Total Usage of a Directory:​
du -sh /home/user

Expected Output:

arduino
Copy code
2.0M    /home/user
  • -s summarizes only the total usage of the directory.
4. Sort Files by Size – Find Large Files:​
du -ah /home | sort -rh | head -10
  • Displays the 10 largest files/directories in /home.
5. Exclude Specific File Types:​
du -h --exclude='*.log' /var/log
  • Ignores files ending in .log.

4. Combining df and du for Effective Monitoring​

Examples​

1. Check Overall Usage and Analyze Details:​
df -h
du -sh /var/log
2. Find Large Files When Space Is Low:​
df -h
du -ah /home | sort -rh | head -n 5
3. Redirect Output for Reports:​
df -h > disk_report.txt
du -h /var/log >> disk_report.txt

5. Automating Disk Usage Monitoring with Cron Jobs​

Schedule Disk Usage Checks with Cron​
crontab -e

Add the following line:

0 8 * * * df -h > /home/user/disk_report.txt
  • This command runs every day at 8 AM and saves the report in /home/user/disk_report.txt.

Key Takeaways​

df Command:

  • Best for high-level monitoring of disk usage across mounted filesystems.
  • Use df -h for human-readable output.
  • Check inode usage with df -i to detect metadata issues.

du Command:

  • Ideal for detailed analysis of directory and file sizes.
  • Use du -sh to get the total size of a directory.
  • Combine with sort to find large files quickly.

Combining Commands:

  • Use df to detect space issues and du to pinpoint causes.

Automation:

  • Regular disk monitoring can be automated using cron jobs to avoid surprises.

Practical Use Cases:

  • Cleaning up logs or cache files when space runs low.
  • Preparing storage reports for audits.
  • Monitoring disk usage growth trends.
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