参考man reference可以看出，-b(作用相同-P), -k, -m or -g这些参数分别对应以不同的块大小(Block Size),512-bytes-block, kilobytes, megabytes or gigabytes来计算disck free space.
从-h 和 -H两个参数选项中可以看出，这些不同的BLOCKSIZE只是为了以不同的基数计算出磁盘可用空间，以用户友好的方式显示出来。
BLOCKSIZE的大小按照一定的优先级来确定：
如果在本次命令中设置了BLOCKSIZE的大小，例如-b -k -h等等，则使用相应的大小；
否则，本次程序运行在SHELL的环境变量中看是否设置了BLOCKSIZE的大小，如果有，则使用；
e.g.
ME-Macbook-Pro:Downloads me$ export BLOCKSIZE=728
ME-Macbook-Pro:Downloads me$ df
Filesystem 728-blocks Used Available Capacity Mounted on
/dev/disk0s2 291989320 289193340 2475980 100% /
devfs 364 364 0 100% /dev
map -hosts 0 0 0 100% /net
map auto_home 0 0 0 100% /home
最后，默认使用512-bytes-block；
-b Use (the default) 512-byte blocks. This is only useful as a way to override an BLOCKSIZE specification from the environment.
-g Use 1073741824-byte (1-Gbyte) blocks rather than the default. Note that this overrides the BLOCKSIZE specification from the environment.
-k Use 1024-byte (1-Kbyte) blocks, rather than the default. Note that this overrides the BLOCKSIZE specification from the environment.
-m Use 1048576-byte (1-Mbyte) blocks rather than the default. Note that this overrides the BLOCKSIZE specification from the environment.
-P Use (the default) 512-byte blocks. This is only useful as a way to override an BLOCKSIZE specification from the environment.
-H "Human-readable" output. Use unit suffixes: Byte, Kilobyte, Megabyte, Gigabyte, Terabyte and Petabyte in order to reduce the number of digits to three or less using base 10 for sizes.
-h "Human-readable" output. Use unit suffixes: Byte, Kilobyte, Megabyte, Gigabyte, Terabyte and Petabyte in order to reduce the number of digits to three or less using base
2 for sizes.
