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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?

jhfrontz
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  • Hmm... standard input over a pipe isn't seekable either. – Kusalananda Nov 14 '19 at 15:09
  • @Kusalananda right -- that's why I'm wondering if if is implying that the input should be seekable. – jhfrontz Nov 14 '19 at 15:12
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    Your first example lists an error from dd, which doesn't show up under bash. What shell do you use? – user3840170 Nov 14 '19 at 15:16
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    I can't reproduce this with cat replacing unzip (I don't have Zip archives) in bash nor zsh on OpenBSD using either of GNU dd or OpenBSD dd. Did you originally have a space before the process substitution? That would make GNU dd complain as you show. – Kusalananda Nov 14 '19 at 15:17
  • Simple test: dd if=<(echo foo) of=/dev/null works for me, but dd if= <(echo foo) of=/dev/null gives an error dd: unrecognized operand ‘/dev/fd/63’ similar to that described in the question. I don't know which shell will pick fd 4 for its first process substitution. Try also echo if=<(:) to see if your shell is introducing a space where it shouldn't. – Toby Speight Nov 14 '19 at 15:43
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    @TobySpeight you got it! I'm using ksh (specifically "Version AJM 93u+ 2012-08-01"). Trying $ echo if=<(:) produces if= /dev/fd/4 (with a rogue leading space). ARGH! – jhfrontz Nov 14 '19 at 17:29
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    I'll make that an answer, then. – Toby Speight Nov 14 '19 at 17:31
  • In any case, you don't want to use dd on a pipe or to write to block devices. Just use unzip -p raspbian.zip > /dev/sdb or unzip -p raspbian.zip | pv > /dev/sdb if you want a progress status. – Stéphane Chazelas Nov 14 '19 at 17:46
  • @StéphaneChazelas thanks. I left off some of the extras in my pipeline -- e.g., I use the bs parameter of dd to (perhaps vainly) optimize the drive I/O. – jhfrontz Nov 14 '19 at 18:57

1 Answers1

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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!

Toby Speight
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