Why does dd dislike taking input from process substitution?

Question

Is there something about process substitution (which I think is implemented with unnamed pipes) that is incompatible with dd?

For example, this fails:

$ dd if=<(unzip -p raspbian.zip) of=/dev/sdb status=progress        
dd: unrecognized operand ‘/dev/fd/4’
Try 'dd --help' for more information.

But this works fine:

$ unzip -p raspbian.zip | dd of=/dev/sdb status=progress       
108458496 bytes (108 MB) copied, 19.446285 s, 5.6 MB/s

Is the use of if somehow implicitly telling dd that it should be able to seek?

Answer 1

This seems to be caused by your shell adding leading whitespace (or otherwise performing argument splitting) when expanding the process substitution. We can demonstrate this:

$ echo if=<(:)
if= /dev/fd/4

Assuming that this behaviour is reproducible on the most recent version you can get, I think there's nothing you can do here other than submit a bug report, including the justifying case of dd to demonstrate that the bug causes real problems.

You might consider switching shells, but I can understand if that's unpalatable to you!