Classic Shell Scripting

The Unix Hierarchical Filesystem

Large collections of files bring the risk of filename collisions, and even with unique names, make management difficult. Unix handles this by permitting files to be grouped into directories: each directory forms its own little name space, independent of all other directories. Directories can also supply default attributes for files, a topic that we discuss briefly in Section B.6.1, later in this Appendix.

Filesystem Structure

Directories can be nested almost arbitrarily deep, so the Unix filesystem forms a tree structure. Unix avoids the synonym folder because paper file folders do not nest. The base of the filesystem tree is called the root directory, and is given a special and simple name: / (ASCII slash). The name /myfile then refers to a file named myfile in the root directory. Slash also serves another purpose: it acts as a delimiter between names to record directory nesting. Figure B-1 shows a tiny portion of the top-level structure of the filesystem.

Figure B-1. Filesystem tree

Unix directories can contain arbitrary numbers of files. However, most current Unix filesystem designs, and filesystem programming interfaces, assume that directories are searched sequentially, so the time to find a file in a large directory is proportional to the number of files in that directory, even though much faster lookup schemes are known. If a directory contains ...