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[vox] cat and >> - how does it know?
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[vox] cat and >> - how does it know?

Not paying attention to tab-completion, I just accidentally did this:

  cat somefile >> somefile

Luckily, "cat" noticed, and complained:

  cat: somefile: input file is output file

But - how does it know?  Using just argv/argc, all "cat" would know
is that you want to throw "somefile" to stdout.

The shell is the one worrying about the ">>" and everything after it.

I'm guessing the shell makes a note of what it's doing (redirecting/appending)
and makes that info. available to the process (in this case, cat),
who can notice and, in this case, complain.

How, exactly, does it do this?

(too lazy to dig up source to /bin/cat ;) )
PS - This was under both tcsh and bash
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