Content from 2015-08

Limits of Unix shell
posted on 2015-08-20 14:54:10

Let's go back to the GNU Coreutils list of tools. ls for example. Usually the user will have set some alias to ls instead of the plain invocation, either to enable highlighting (--color), sorting (--sort), or to add more information than just the filenames (e.g. --format). There is even integration with Emacs (--dired).

The question then is: How much of the functionality of ls is actually devoted to secondary formatting instead of listing files? And shouldn't this functionality be moved into separate tools? Since output is intended for multiple kinds of recipients, additional data creeps in and complicate tools a lot.

Alternatively, we could imagine using ls only to get unformatted and unsorted output. Which would then be passed through to a sort command and a fmt command of sorts. Of course this all takes some more time, re-parsing of output etc., so it's understandable in the interest of performance not to do this in the traditional Unix shell.

However, let's assume a more sophisticated shell. Assuming ls is limited to listing files, then the user will alias ls to a pipeline instead, namely something akin to ls | sort | fmt. Then again, formatting is part of the user interface, not the functionality, so it should rather be part of the internal shell formatting, possibly exposed as separate filters as well.

The result of ls is a (possibly nested) directory listing. Regardless of post-processing, this "object" should still be available for further investigation. Which means that while sorting may be applied destructively, formatting may not, unless specifically requested, in which case the result would be a kind of "formatted" object (text, GUI widget) instead.

In other terms, the user should be able to refer to the last results immediately, instead of rerunning the whole pipeline. E.g. coming from Common Lisp, variables like * to *** will store the last three results for interactive use. In the shell then, ls would set * to the generated directory listing; since the listing is also most likely printed to the screen, the full listing will also be stored (in that object) to be used again if e.g. * is requested again. Rerunning the command, on the other hand, will possibly generate a different directory listing as files may have been changed, so there is an immediate difference between the two forms.

Examples

The pipeline ls | wc -l is (at least for me) often used to get the number of files in the (current) directory. Unfortunately there is no direct way to get this number directly except to enumerate the entries in a directory (under Linux that is).

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