来自 man 页面的解释：
df - report file system disk space usage
而来自 wikipedia 的解释更加清楚地阐释了其含义：
df (abbreviation for disk free) is a standard Unix command used to display the amount of available disk space for file systems on which the invoking user has appropriate read access. df is typically implemented using the statfs or statvfs system calls.
-- df (Unix) - Wikipedia
df 表示 disk free，主要用来查看还剩多少可用空间。
CentOS 下的输出示例：
$ df Filesystem 1K-blocks Used Available Use% Mounted on /dev/vda1 103079828 5742136 93043356 6% / devtmpfs 3968704 0 3968704 0% /dev tmpfs 3983076 24 3983052 1% /dev/shm tmpfs 3983076 280220 3702856 8% /run tmpfs 3983076 0 3983076 0% /sys/fs/cgroup tmpfs 796616 0 796616 0% /run/user/0 /dev/vdb 309504832 495948 293263860 1% /data