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Apologies, localghost. And the worst part is I remember having read that!

One way to see this is to to add a percent sign at the end of temp.tex. In other words, if temp.tex looks like this: this quote%
That has indeed done the trick. Thank you, frabjous: I would not have thought of that.
Unfortunately, it's not obvious to me how to write a percent sign to an external ...

I've discovered (the hard way) that \input{} appends a space to whatever it reads in from file. This is a major problem as I rely heavily on \input{} .

Consider the following minimal complete example: \documentclass[12pt]{book}
\begin{document}
\immediate\write18{echo -n "this quote" > temp.tex ...