Linux dd Command

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The Linux dd command is used to read, convert, and output data.

dd can read data from standard input or a file, convert the data according to the specified format, and then output to a file, device, or standard output.

Parameter description:

  • if=file name: input file name, defaults to standard input. That is, specify the source file.
  • of=file name: output file name, defaults to standard output. That is, specify the destination file.
  • ibs=bytes: read in bytes bytes at a time, i.e., specify a block size of bytes bytes.
    obs=bytes: output bytes bytes at a time, i.e., specify a block size of bytes bytes.
    bs=bytes: simultaneously set the input/output block size to bytes bytes.
  • cbs=bytes: convert bytes bytes at a time, i.e., specify the conversion buffer size.
  • skip=blocks: skip blocks blocks from the beginning of the input file before starting to copy.
  • seek=blocks: skip blocks blocks from the beginning of the output file before starting to copy.
  • count=blocks: copy only blocks blocks, with block size equal to the number of bytes specified by ibs.
  • conv=<keyword>, the keywords can be the following 11 types:
    • conversion: convert the file with the specified parameters.
    • ascii: convert EBCDIC to ASCII
    • ebcdic: convert ASCII to EBCDIC
    • ibm: convert ASCII to alternate EBCDIC
    • block: convert each line to a length of cbs, padding the insufficient part with spaces
    • unblock: make each line's length cbs, padding the insufficient part with spaces
    • lcase: convert uppercase characters to lowercase characters
    • ucase: convert lowercase characters to uppercase characters
    • swap: swap each pair of input bytes
    • noerror: do not stop when an error occurs
    • notrunc: do not truncate the output file
    • sync: pad each input block to ibs bytes, filling the insufficient part with null (NUL) characters.
  • --help: display help information
  • --version: display version information

Examples

To create a boot disk under Linux, you can use the following command:

dd if=boot.img of=/dev/fd0 bs=1440k

Convert all English letters in the testfile file to uppercase, then convert it into the testfile_1 file, use the following command at the command prompt:

dd if=testfile_2 of=testfile_1 conv=ucase

The content of testfile_2 is:

$ cat testfile_2 #testfile_2的内容  
HELLO LINUX!  
Linux is a free unix-type opterating system.  
This is a linux testfile!  
Linux test

After the conversion is completed, the content of testfile_1 is as follows:

$ dd if=testfile_2 of=testfile_1 conv=ucase #使用dd 命令，大小写转换记录了0+1 的读入  
记录了0+1 的写出  
95字节（95 B）已复制，0.000131446 秒，723 KB/s  
cmd@hdd-desktop:~$ cat testfile_1 #查看转换后的testfile_1文件内容  
HELLO LINUX!  
LINUX IS A FREE UNIX-TYPE OPTERATING SYSTEM.  
THIS IS A LINUX TESTFILE!  
LINUX TEST #testfile_2中的所有字符都变成了大写字母

Read a string from the standard input device, convert the string to uppercase, and then output it to the standard output device. The command used is:

dd conv=ucase

After entering the above command, press the Enter key, input a string, press the Enter key again, and press the combination key Ctrl+D to exit. The following result appears:

$ dd conv=ucase 
Hello Linux! #输入字符串后按回车键  
HELLO LINUX! #按组合键Ctrl+D退出，转换成大写结果  
记录了0+1 的读入  
记录了0+1 的写出  
13字节（13 B）已复制，12.1558 秒，0.0 KB/s

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