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Comment by amelius | Hacker News Reader

So I suppose that the command

    pv < image.iso > /dev/sdb

would actually need to buffer the complete file, before commencing to write to the device (and showing the progress), which would defeat the whole idea of showing progress.

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If `pv` doesn't know the input size, it doesn't show it. In your case, it can determine that its stdin is a file by looking at the `/proc/self/fd/0` symlink:

   $ ls -l /proc/self/fd/0 < /tmp/x
   lr-x------ 1 user users 64 <date> /proc/self/fd/0 -> /tmp/x

It wouldn't. It just won't be able to predict a file size.

From what I can tell, bash uses 4K buffers for pipelines.

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