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The example is right in practice. The book's original explanation is an accurate description of what the POSIX standard says, but the POSIX-like shells I have handy (<code>bash</code> and <code>dash</code>, the only ones I believe are commonly seen on Linux) are not that picky. The POSIX standard <a href="http://pubs.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#tag_02_07_05" rel="nofollow">says the same thing as the book</a> about input and output descriptors, and goes on to say this: for <code>n<&word</code>, "if the digits in <code>word</code> do not represent a file descriptor already open for input, a redirection error shall result". So if you want to be careful about POSIX compatibility, you should avoid this usage. The bash documentation also <a href="http://www.gnu.org/software/bash/manual/html_node/Redirections.html" rel="nofollow">says the same thing</a> about <code><&</code> and <code>>&</code>, but without the promise of an error. Which is good, because it doesn't actually give an error. Instead, empirically <code>n<&m</code> and <code>n>&m</code> appear to be interchangeable. The only difference between <code><&</code> and <code>>&</code> is that if you leave off the fd number on the left, <code><&</code> defaults to 0 (stdin) and <code>>&</code> to 1 (stdout). For example, let's start a shell with fd 1 pointing at a file <code>bar</code>, then try out exactly the <code>exec 4<&1</code> example, try to write to the resulting fd 4, and see if it works: <pre><code>$ sh -c 'exec 4<&1; echo foo >&4' >bar; cat bar foo </code></pre> It does, and this holds using either <code>dash</code> or <code>bash</code> (or <code>bash --posix</code>) for the shell. Under the hood, this makes sense because <& and >& are almost certainly just calling <a href="http://linux.die.net/man/2/dup2" rel="nofollow">dup2()</a>, which doesn't care whether the fds are opened for reading or writing or appending or what. [EDIT: Added reference to POSIX after discussion in comments.]
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1. POexec n<&m versus exec n>&m -- based on Sobell's Linux book
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1. USGreg Price
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1. USGreg Price
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