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The importance of an ordinary space in a Unix shell command line

In the sidebar to yesterday's entry I (originally) made a Unix command line mistake by unthinkingly leaving out an ordinary, innocent looking space (it's corrected in the current version of the entry after it was noted by Emilio in a comment). This innocent looking mistake and its consequences are an illustration of something in Unix shell command lines, although I'm not sure of just what, so I'm going to write it up.

So when I was writing the original command line in yesterday's entry, I absently mashed these two things together in my mind and wrote: It only means an argument that is an empty string if it occurs on its own; this is a special interpretation added by the shell, and programs don't actually see the ''s. In the process of writing this entry, I realized that there's a more interesting way to make what is fundamentally the same mistake, although it goes deeper into Unix arcana and doesn't look half as obvious.

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