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From: ark@alice.UUCP (Andrew Koenig)
Newsgroups: net.sources.bugs
Subject: Re: /dev/stdin for SVR2
Message-ID: <3879@alice.UUCP>
Date: Thu, 20-Jun-85 21:04:42 EDT
Article-I.D.: alice.3879
Posted: Thu Jun 20 21:04:42 1985
Date-Received: Mon, 24-Jun-85 06:26:22 EDT
References: <334@umcp-cs.UUCP>
Organization: Bell Labs, Murray Hill
Lines: 32

> This would appear to give write access to read-only files.
> What happens if I say

>	cat </tmp/foo >/dev/stdin

> as root?

If you are root, you already have write access to read-only files.
If not root,

	cat </etc/passwd >/dev/stdin

elicits the error message:

	cat: output - is input

whereas:

	cat >/dev/stdin </etc/passwd

prints the password file on my terminal.

This behavior appears a little less mysterious when you realize that
(a) the standard input is normally connected to the terminal and is
opened for both input and output, and (b) the Shell processes
redirection requests one at a time from left to right.  Thus in the
first case, the Shell first opens /etc/passwd as file descriptor
1, then opens its standard output as dup(1), which results in both
file descriptors being connected to the same file, which cat detects.
In the second case, the Shell connects the standard output to dup(1)
and then (re-) opens the standard input.