# DD 命令说明

## 1.一般使用

```bash
# 不固定长度，读到文件结尾自动停止
dd if=\<in file name\> of=/dev/\<out file name\> bs=4M status=progress
# 指定长度（4Mx1024=4G）
dd if=\<in file name\> of=/dev/\<out file name\> bs=4M count=1024 status=progress
```

## 2.指定 Flag

```bash
# iflag 指定 input flag
# iflag 指定 output flag
dd iflag=direct,nonblock if=\<in file name\> oflag=direct,nonblock of=/dev/\<out file name\> bs=4M status=progress
```

多个 Flag 之间可以用逗号分割，含义如下表。

| Flag        | Description                                                       |
|-------------|-------------------------------------------------------------------|
| append      | append mode (makes sense only for output; conv=notrunc suggested) |
| direct      | use direct I/O for data                                           |
| directory   | fail unless a directory                                           |
| dsync       | use synchronized I/O for data                                     |
| sync        | likewise, but also for metadata                                   |
| fullblock   | accumulate full blocks of input (iflag only)                      |
| nonblock    | use non-blocking I/O                                              |
| noatime     | do not update access time                                         |
| nocache     | Request to drop cache.  See also oflag=sync                       |
| noctty      | do not assign controlling terminal from file                      |
| nofollow    | do not follow symlinks                                            |
| count_bytes | treat 'count=N' as a byte count (iflag only)                      |
| skip_bytes  | treat 'skip=N' as a byte count (iflag only)                       |
| seek_bytes  | treat 'seek=N' as a byte count (oflag only)                       |