How to check inode usage in Linux

A filesystem can report full while df -h still shows free space, because it has run out of inodes. df -i shows inode usage; then find the directory holding millions of small files.

# Inode usage per filesystem
df -i

# Count files per subdirectory to find the culprit
for d in /var/*; do echo $(find $d 2>/dev/null | wc -l) $d; done | sort -rn | head