Disk Space: df & du
df shows free space on your disks; du shows how much space files and folders use.
df -h
du -sh [path]Running out of disk is a common server problem. Two commands diagnose it.
df — free space per disk
df -h lists each mounted filesystem with its size, used and available space. The -h gives human-readable units. Watch the Use% column.
du — space used by files
du reports how much space specific files and folders occupy. du -sh gives a single total for a folder.
Together they answer "the disk is full — what is filling it?"
Example
# Free space on each disk
df -h
# Filesystem Size Used Avail Use% Mounted on
# /dev/sda1 40G 22G 16G 59% /
# Total size of a folder
du -sh /var/log
# 512M /var/log
# Biggest subfolders here, sorted
du -sh * | sort -hWhen to use it
- A sysadmin runs 'df -h' to check disk usage across all mounted filesystems before a large database migration.
- A developer uses 'du -sh /var/log/*' to identify which log files are consuming the most space on a nearly full server.
- A DevOps engineer adds 'df -h | awk NR>1{if($5+0>80)print $0}' to a monitoring script to alert when any filesystem exceeds 80% usage.
More examples
Check filesystem disk usage
Shows df -h for human-readable sizes across all mounted filesystems, including Use% which triggers alerts.
df -h
# Filesystem Size Used Avail Use% Mounted on
# /dev/sda1 50G 32G 16G 67% /
# /dev/sdb1 200G 145G 45G 77% /varFind large directories
Uses du -sh with a glob to measure directory sizes and sort -rh to rank them largest first.
# Show size of each item in /var
du -sh /var/*
# 1.2G /var/cache
# 3.8G /var/log
# 512M /var/lib
# Find top 10 largest directories from /
du -sh /* 2>/dev/null | sort -rh | head -10Alert on high disk usage
Pipes df into awk to filter and print only filesystems whose Use% exceeds the threshold, ready for a cron alert.
#!/bin/bash
# Alert if any filesystem exceeds 85% usage
df -h | awk 'NR>1 && $5+0 > 85 {
print "ALERT: " $6 " is at " $5
}'
Discussion