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Comment by WhyNotHugo | Hacker News Reader

This isn't portable though, it's a GNU-specific "smart" things. Elsewhere, this overwrites the target device file.

Redirection doesn't overwrite device files, so

  cat foo.img > /dev/sda

works and should be portable. No control over the buffer size, though.

And no printing of progress either.

How exactly is cp not portable?

Whatever behaviour you’re describing is not how POSIX mandates cp to work. In fact you can do this with busybox too.

cp will open("/dev/sda", O_WRONLY | O_TRUNC) and simply start writing to that file descriptor.