Classic Shell Scripting

What's in a Unix File?

One of the tremendous successes of Unix has been its simple view of files: Unix files are just streams of zero or more anonymous bytes of data.

Most other operating systems have different types of files: binary versus text data, counted-length versus fixed-length versus variable-length records, indexed versus random versus sequential access, and so on. This rapidly produces the nightmarish situation that the conceptually simple job of copying a file must be done differently depending on the file type, and since virtually all software has to deal with files, the complexity is widespread.

A Unix file-copy operation is trivial:

            try-to-get-a-byte
            while (have-a-byte)
            {
            put-a-byte
               try-to-get-a-byte
            }

This sort of loop can be implemented in many programming languages, and its great beauty is that the program need not be aware of where the data is coming from: it could be from a file, or a magnetic tape device, or a pipe, or a network connection, or a kernel data structure, or any other data source that designers dream up in the future.

Ahh, you say, but I need a special file that has a trailing directory of pointers into the earlier data, and that data is itself encrypted. In Unix the answer is: Go for it! Make your application program understand your fancy file format, but don't trouble the filesystem or operating system with that complexity. They do not need to know about it.

There is, however, a mild distinction between files that Unix does admit to. Files that ...