Why do we use cp to copy files and not dd? (in unix-derivatives)

Question

For normal file copying in *nix, I've only ever seen people use cp (eg. cp /mnt/mydevice/myfile ~/myfile), but I eventually ran into dd, which appears to do the exact same thing (dd if=/mnt/mydevice/myfile of=~/myfile). I do see that they have some different parameters available (dd seems better at fine-tuned copying), but they appear redundant. Do these tools really do the same thing?

Solution 1

To answer your main question, no, they do not do the same thing.

dd works on the file you specify, making it able to copy data between devices, or from a device to a file. This is commonly used for moving data if devices specifically are involved (create an iso image from a cd-rom disc for example: dd if=/dev/cdrom of=mycdrom.iso), or backup raw devices (sometimes used in RAC databases: dd if=/dev/raw/raw1 of=device_raw1)

cp is used for duplicating file content to a new file or to a new location. things you specifically want there are preservation of ownership, timestamp and mode (rights), and being able to recurse the operation (=being able to copy directories).

Source

Solution 2

They do the same thing UNLESS you are specifying one of the options to dd which limits which bytes are copied, such as seek or skip or count or if you use the dd options to mutate bytes such as conv. If you aren't using one of these options to dd and are just using the more commonly seen options like if, of, bs then both utilities do the same thing: open both files, read from the input, write to the output until either the input is exhausted or the output cannot accept more bytes.

There is a lot of superstition about reading and writing "device" files stating that you must use dd for these, but it is just that, superstition. dd isn't doing anything different, we are just opening files and reading and writing bytes.

Comments

[Eddy_Em]

They do the same things only for regular files. Try for example to copy a directory with dd.

[jpaugh]

...or a device (like /dev/sda) with cp.

[Kruug]

Was your question answered?

[Pacerier]

@Eddy_Em, What's wrong with that?

[user232105]

Ah! Okay, so dd copies the raw file, whereas cp copies the contents (which allows it to for instance copy directories without breaking the filesystem). Thanks!

[Pacerier]

@Kruug, I must be missing something because I still don't see what cp can do that dd cannot. dd is also able to "duplicating file content to a new file or to a new location" and "preservation of ownership, timestamp and mode" right?

[Kruug]

@Pacerier from what I can understand, cp is a more focused utility whereas dd is more general. dd can do all of what cp can do, but cp can only do some of what dd can do.

[gorn]

I do not think that the difference is explained clearly enoug. What cp can not do and dd can? And the other way round?

[Sergio Abreu]

More or less this... if you use of=destiny depending on the device it is, some problems may occur, ex. USB flash disk. I discover that I have to use >> destiny and remove of=xxx parameter for it to work. If I use of=destiny strange problems occur because I oper with skip and iflags=skip_bytes flag... so, no so much superstition. Needs care and tests because of=destiny may not work correctly in some conditions where >>destiny goes smooth. I wrote a script to save large file to pendrive with steps. Full 400MB copy used to mess sdcard disk partition.

[MestreLion]

@Pacerier: dd cannot copy multiple files, recurse in directories - cp *.txt dest/ and cp -R mydir/ dest/ are 2 simple examples of things cp can do that dd cannot.