dd命令的用法

脚本会根据剩余空间的大小来计算一个参数，剩余空间很多时，参数值为A；剩余空间较少时，参数值为剩余空间大小的函数；剩余空间有1G时，总是值A；为了验证参数值为剩余空间大小的函数的情况，需要使获得的剩余空间的值小一些，此时，我们可以使用dd命令来创建一个大小为9000M的空白文件，从而使得剩余空间为1024M这样，然后继续测试。

命令使用示例：

[root@db22 ~]# df -m /homeFilesystem           1M-blocks      Used Available Use% Mounted on/dev/sda3                18043     13072      4055  77% /[root@db22 ~]# dd if=/dev/zero of=/tmp/holdspace.fil bs=1M count=30003000+0 records in3000+0 records out3145728000 bytes (3.1 GB) copied, 8.92085 s, 353 MB/s[root@db22 ~]# df -m /homeFilesystem           1M-blocks      Used Available Use% Mounted on/dev/sda3                18043     16072      1055  94% /

上面的示例命令可以看到可用空间从4055M变成了1055M，是因为我们创建了一个3000M的空白文件holdspace.fil。
参数说明：
- if:[input file]输入文件
- of:[output file] 输出文件
- bs:[bytes]字节
- count:数量

通过man来看一下dd命令的用法

NAME       dd - convert and copy a fileSYNOPSIS       dd [OPERAND]...       dd OPTIONDESCRIPTION       Copy a file, converting and formatting according to the operands.       bs=BYTES              read and write BYTES bytes at a time (also see ibs=,obs=)              每一次读写BYTES个字节       cbs=BYTES              convert BYTES bytes at a time              每一次转换BYTES个字节       conv=CONVS              convert the file as per the comma separated symbol list       count=BLOCKS              copy only BLOCKS input blocks              只拷贝BLOCKS个输入块。       ibs=BYTES              read BYTES bytes at a time (default: 512)       if=FILE              read from FILE instead of stdin       iflag=FLAGS              read as per the comma separated symbol list       obs=BYTES              write BYTES bytes at a time (default: 512)       of=FILE              write to FILE instead of stdout       oflag=FLAGS              write as per the comma separated symbol list       seek=BLOCKS              skip BLOCKS obs-sized blocks at start of output       skip=BLOCKS              skip BLOCKS ibs-sized blocks at start of input       status=noxfer              suppress transfer statistics       BLOCKS  and  BYTES  may  be  followed  by  the following multiplicative suffixes: c =1, w =2, b =512, kB =1000, K =1024, MB       =1000*1000, M =1024*1024, xM =M GB =1000*1000*1000, G =1024*1024*1024, and so on for T, P, E, Z, Y.       Each CONV symbol may be:       ascii  from EBCDIC to ASCII       ebcdic from ASCII to EBCDIC       ...  ...