linux中df命令的功能是用来检查linux服务器的文件系统的磁盘空间占用情况。可以利用该命令来获取硬盘被占用了多少空间，目前还剩下多少空间等信息。
Usage: df [OPTION]... [FILE]...
Show information about the file system on which each FILE resides,
or all file systems by default.
Mandatory强制的 arguments to long options are mandatory for short options too.
-a, --all include dummy file systems 全部文件系统列表
-B, --block-size=SIZE scale sizes by SIZE before printing them; 指定区块大小
e.g.,
'-BM' prints sizes in units of 1,048,576 bytes;
see SIZE format below
--direct show statistics for a file instead of mount point
--total produce a grand total
-h, --human-readable print sizes in human readable format (e.g., 1K 234M 2G)
-H, --si likewise, but use powers of 1000 not 1024
-i, --inodes list inode information instead of block usage
-k like --block-size=1K
-l, --local limit listing to local file systems 只显示本地文件系统
--no-sync do not invoke sync before getting usage info (default) 忽略 sync 命令
--output[=FIELD_LIST] use the output format defined by FIELD_LIST,
or print all fields if FIELD_LIST is omitted.
-P, --portability use the POSIX output format
--sync invoke调用 sync before getting usage info 在取得磁盘信息前，先执行sync命令
-t, --type=TYPE limit listing to file systems of type TYPE <文件系统类型> 只显示选定文件系统的磁盘信息
-T, --print-type print file system type 文件系统类型
-x, --exclude-type=TYPE limit listing to file systems not of type TYPE <文件系统类型> 不显示选定文件系统的磁盘信息
-v (ignored)
--help display this help and exit
--version output version information and exit
Display values are in units of the first available SIZE from --block-size,
and the DF_BLOCK_SIZE, BLOCK_SIZE and BLOCKSIZE environment variables.
Otherwise, units default to 1024 bytes (or 512 if POSIXLY_CORRECT is set).
SIZE is an integer and optional unit (example: 10M is 10*1024*1024). Units
are K, M, G, T, P, E, Z, Y (powers of 1024) or KB, MB, ... (powers of 1000).
FIELD_LIST is a comma-separated list of columns to be included. Valid
field names are: 'source', 'fstype', 'itotal', 'iused', 'iavail', 'ipcent',
'size', 'used', 'avail', 'pcent', 'file' and 'target' (see info page).
GNU coreutils online help: <http://www.gnu.org/software/coreutils/>
For complete documentation, run: info coreutils 'df invocation'