# dd

_Peios / Using Peios / Peiosutils / Files and directories_

> Copy data block by block between files, optionally converting and reshaping it on the way.

`dd` copies data from one place to another, a block at a time, and can transform the data as it goes. It is the tool for low-level copying — writing a disk image to a device, extracting an exact byte range out of a file, copying from or to a raw device.

```
dd [operand]...
```

```
$ dd if=disk.img of=/dev/mydisk bs=4M status=progress
```

## How dd is told what to do

`dd` does not take options in the usual `-x` form. It takes **operands**, each written `name=value`, in any order. With no `if=` operand it reads from standard input; with no `of=` operand it writes to standard output.

## The operands

| Operand | Meaning |
|---|---|
| `if=FILE` | Read input from `FILE`. |
| `of=FILE` | Write output to `FILE`. |
| `bs=BYTES` | Read and write `BYTES` at a time. Sets both the input and output block size at once. |
| `ibs=BYTES` | Input block size (default 512). |
| `obs=BYTES` | Output block size (default 512). |
| `cbs=BYTES` | Conversion block size — used by the `block` and `unblock` conversions. |
| `count=N` | Stop after `N` input blocks, rather than reading to the end. |
| `skip=N` | Skip `N` input blocks before copying. Also spelled `iseek=N`. |
| `seek=N` | Skip `N` output blocks before writing. Also spelled `oseek=N`. |
| `conv=LIST` | Apply the comma-separated conversions below. |
| `iflag=LIST` | Comma-separated input flags — how the input is opened and read. |
| `oflag=LIST` | Comma-separated output flags — how the output is opened and written. |
| `status=LEVEL` | How much to report: `progress` (periodic stats while copying), `noxfer` (final counts only), `none` (silent). |

A size value accepts a unit suffix (`K`, `M`, `G`, …). So `bs=4M` is four mebibytes per block.

## Conversions (`conv=`)

| Value | Effect |
|---|---|
| `ucase` / `lcase` | Convert the data to upper-case / lower-case. |
| `swab` | Swap every adjacent pair of bytes. |
| `sync` | Pad each input block out to its full size — with zeros, or with spaces alongside `block`/`unblock`. |
| `block` / `unblock` | Convert between newline-terminated lines and fixed-size `cbs` records. |
| `sparse` | Where an output block is all zeros, seek past it instead of writing it. |
| `excl` | Fail if the output file already exists. |
| `nocreat` | Fail if the output file does *not* already exist. |
| `notrunc` | Do not truncate the output file when opening it. |
| `noerror` | Continue past read errors instead of stopping. |
| `fdatasync` / `fsync` | Flush the data — or the data and metadata — to storage before finishing. |

When `dd` **creates** the output file — an `of=` file that does not already exist — the new file needs a [security descriptor](/peios/security-fundamentals/security-descriptors/overview.md), and it inherits one from the directory it is created in.

## Input and output flags (`iflag=`, `oflag=`)

| Flag | Applies to | Effect |
|---|---|---|
| `count_bytes` | input | Interpret `count=N` as a number of bytes, not blocks. |
| `skip_bytes` | input | Interpret `skip=N` as a number of bytes. |
| `fullblock` | input | Wait for a full `ibs` of data on each read. |
| `seek_bytes` | output | Interpret `seek=N` as a number of bytes. |
| `append` | output | Open the output in append mode. |
| `direct` | both | Use direct I/O, bypassing the cache. |
| `dsync` / `sync` | both | Use synchronised I/O — for data, or for data and metadata. |
| `nonblock` | both | Use non-blocking I/O. |
| `noatime` | both | Do not update the file's access time. |
| `nocache` | both | Ask the system to drop the file's cached pages. |
| `directory` | both | Fail unless the file is a directory. |

## What dd prints

Unless `status=none` is set, `dd` prints a summary when it finishes:

```
16+0 records in
16+0 records out
67108864 bytes (67 MB, 64 MiB) copied, 1.234 s, 54.4 MB/s
```

The `records in` / `records out` counts are written `complete+partial` — full-sized blocks plus any short final block. `status=progress` prints the last line periodically during a long copy.

## Exit status

| Code | Meaning |
|---|---|
| `0` | The copy completed. |
| `1` | The copy failed — a bad operand, an I/O error, or a file that could not be opened. |

Related content:

- [cp](/peios/using-peios/peiosutils/files-and-directories/cp.md)
- [Files and directories](/peios/using-peios/peiosutils/files-and-directories/overview.md)