[Solved] What is the function of cp debian.iso /dev/sdX?

[Free_Alan]

Hi:

Since I used Rufus or balenaEtcher to create USBMedia before, I recently saw the official Debian document(https://wiki.debian.org/DebianInstaller/CreateUSBMedia) , which uses Linux commands to create USBMedia.

The Linux commands used in this document are as follows:

Code: Select all

# cp debian.iso /dev/sdX
# sync

A warning statement is included below the Linux instruction as follows:
The image must be written to the whole-disk device and not a partition, e.g. /dev/sdb and not /dev/sdb1.

I would like to ask why the copy operation is performed on the entire hard disk space. Will the file system be changed and all data originally on the hard disk deleted after this operation?

Thanks for any help.

[steve_v]

I would like to ask why the copy operation is performed on the entire hard disk space.

Because it's a hybrid bootable disk image, so it includes partition table, boot sector(s), and filesystems. That's how disk images generally work.
A cp to a block device node is roughly equivalent to a dd to the same, and writes directly to the raw disk or partition.

Will the file system be changed and all data originally on the hard disk deleted after this operation?

Of course. As the wiki states:

Be aware, that the methods described here will destroy anything already on the device!

[kent_dorfman766]

FWIW, I would not use the cp command, but would instead do "dd if=debian.iso of=/dev/sdx bs=512" since cp is meant for filesystem operations, not raw devices.

[sunrat]

FWIW, I would not use the cp command, but would instead do "dd if=debian.iso of=/dev/sdx bs=512" since cp is meant for filesystem operations, not raw devices.

Poppycock. It's less characters to type than dd! But you are free to use whatever you wish.
As OP stated, it is also what is advised to use in the Debian wiki - https://wiki.debian.org/DebianInstaller/CreateUSBMedia