Directories

Figure 10: (a) Single-Level Directory Systems (b) Two-Level Directory Systems (c) Hierarchical Directory Systems.

• A directory is a symbol table, which can be searched for information about the files. Also, it is the fundamental way of organizing files. Usually, a directory is itself a file
• A typical directory entry contains information (attributes, location, ownership) about a file. Directory entries are added as files are created, and are removed when files are deleted.
• Provides mapping between file names and the files themselves
• Goals in Organization of Directory
  • Efficiency; locate file quickly
  • Naming; convenient to users,
    • 2 users can use same name for different files
    • Same file can have several different names
  • Grouping; logical grouping of files by attributes, (e.g., all Java programs, all games, ...)

  Figure 11: Example to (a) Single-Level Directory Systems (b) Two-Level Directory Systems (c) Hierarchical Directory Systems.
• Single-Level Directory Systems (see Figs. 10,11)
  • List of entries, one for each file
  • Sequential file with the name of the file serving as the key
  • Provides no help in organizing the files
  • Forces user to be careful not to use the same name for two different files
• Two-Level Directory Systems (see Figs. 10,11)
  • One directory for each user and a master directory
  • Master directory contains entry for each user; Provides access control information
  • Each user directory is a simple list of files for that user
  • Still provides no help in structuring collections of files
• Hierarchical, or Tree-Structured Directory Systems (see Figs. 10,11)
  • Files can be located by following a path from the root, or master, directory down various branches; This is the absolute pathname for the file
  • Can have several files with the same file name as long as they have unique path names