xion.log » Unintended Consequences

Unintended Consequences

2012-10-02 12:13

In Unix-like systems, files and directories with names starting from ‘.’ (dot) are “hidden”: they don’t appear when listed by ls and are not shown by default in graphical file browsers. It doesn’t seem very clear why there is such a mechanism, especially when we have extensive chmod permissions and attributes which are not tied to filename. Actually, that’s one of distinguishing features of Unix/Linux: there are neither .exe nor .app files, just chmod +x.

But, here it is: name-based visibility control for files and directories. Why such a thing was ever implemented in the first place? Well, it turns out it was purely by accident:

Long ago, as the design of the Unix file system was being worked out, the entries . and .. appeared, to make navigation easier. (…) When one typed ls, however, these entries appeared, so either Ken [Thompson] or Dennis [Ritchie] added a simple test to the program. It was in assembler then, but the code in question was equivalent to something like this:

  1. if (name[0] == '.') continue;

That test was meant to filter out . and .. only. Unintentionally, though, it ruled out a much bigger class of names: all that start with a dot, because that’s what it actually checked for. Back then it probably seemed like an innocuous detail. Fast-forward a couple of decades and it’s a de facto standard for storing program-specific data inside user’s home path. They can grow quite numerous over time, too:

  1. $ ls -A1 ~ | grep '^\.' | wc -l
  2. 113

That’s over 100 entries, a vast majority of my home directory. It’s neither elegant nor efficient to have that much of app-specific cruft inside the most important place in the filesystem. And even if GUI applications tend to collectively use a single ~/.config directory, the tradition to clutter the root $HOME path is strong enough to persist for foreseeable future.

Heed this as a warning. In the event your software becomes a basis for many derived solutions, future programmers will exploit every corner case of every piece of logic you have written. It doesn’t really matter what you wanted to put into your code, but only what you actually did.

Tags: , , ,
Author: Xion, posted under Programming »


3 comments for post “Unintended Consequences”.
  1. Kos:
    October 2nd, 2012 o 15:19

    In other news, I always wonder about the history of ‘.’ and ‘..’ entries. They seem a bit awkward when the filesystem links don’t form a tree

  2. Xion:
    October 3rd, 2012 o 11:04

    I’m not really sure what you mean by “filesystem links not forming a tree”. As far I know, the structure of hierarchical directories in contemporary filesystems is a tree :)

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