Permission denied using pv for copying image to USB drive

Question

I am attempting to copy an Arch Linux ISO image directly to a USB drive. Previously, I have simply used either cp or dd to do this, such as:

sudo cp image.iso /dev/sdb

where /dev/sdb is the USB device. This has worked fine, but I want something that shows the progress while it is copying. I learned about the pv utility, which can show progress. However, I get permission denied whenever attempting to run the following:

sudo pv image.iso > /dev/sdb
bash: /dev/sdb: Permission denied

I thought that maybe it wasn't working because of an existing file system on the device, so I did:

sudo dd if=/dev/zero of=/dev/sdb bs=1G

to clear everything off the device, then retried. But I get the same error. What could be causing this? I'm pretty sure that I'm using pv correctly. I have tested using pv by copying the ISO to another location and it works:

sudo pv image.iso > test.iso

works fine. I am running these commands on a fully-updated x86-64 Arch Linux system.

NOTE: I am aware that dd also has progress state with the status=progress option, but this does not seem to work either.

Answer 1

Your problem with

sudo pv image.iso > /dev/sdb
bash: /dev/sdb: Permission denied

is that > it is a redirect of the output of sudo and therefore runs as the original user, not as root.

You need to ensure that whatever is doing the write is also running as root.

For example:

sudo pv image.iso | sudo dd of=/dev/sdb

Now the write is being done by dd, which is running as root, so no issues.

There are various general ways to solve this problem, as it occurs regularly in shell scripting. For simple text, you may like:

sudo my_root_command | sudo tee path/to/output/file > /dev/null

... and there many other ways too. There are better ways to do what you're doing, as suggested by @forest, but the above is why you are getting permission denied.

Answer 2

You cannot use redirection with sudo and expect unprivileged bash to be able to open a root-writable file for writing. See How to run a command that involves redirecting or piping with sudo?

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You need to stop using such a large block size. Try this instead:

dd if=image.iso of=/dev/sdb bs=1M status=progress

In some situations, you can also improve performance by enabling direct I/O mode which bypasses the page cache. This will also increase accuracy of the progress report:

dd if=image.iso of=/dev/sdb bs=1M status=progress oflag=direct

Beware that using direct I/O takes all optimizations out of the kernel's hands, and thus block size can have a tremendous impact on performance. Try a few different block sizes.