df -h shows disk usage per filesystem. du -sh /var/log shows a directory's size. du -sh * | sort -rh | head shows the biggest items. find / -size +100M finds large files causing disk space issues.
Checking Disk Space
df -h # disk space per filesystem
du -sh /var/log # size of directory
du -sh * | sort -rh | head # largest items
# Find large files
find / -size +500M -exec ls -lh {} \;
# Check inode usage
df -i