Why is stdin always a pipe?

[João Eiras]

Hi.

I'm trying to figure out the best way to detect when my program is
being piped data.

So far I've used in linux often "[[ -p /dev/stdin ]]". During an
interactive session, /dev/stdin will be a character device, not a
pipe, and if stdin is closed ( command 0<&- ) then it's nothing.

But in cygwin, /dev/stdin is ALWAYS a pipe. So, I could use "[[ -t 0
]]" to check if stdin is a tty, but that would not cover when running
my script in a background job or service with a closed stdin.

So, a) what is the correct way to detect if stdin is being piped data
and b) is this a bug or design limitation ?

Thank you.

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