(4) Files

• In ordinary language "file" refers to, e.g., a Word document, an MP3, a digital photograph, etc.
• To a computer a file is simply an abstraction for some arbitrary chunk of information
• A file has (at least) a name, a size (in bytes), permissions (who can read, write, or execute it), and a type
  • Most basic type distinction: text vs. binary
  • Filename suffixes (e.g. .doc, .mp3, .jpg) are only hints - changing them does not change a file's type

(5) Filesystems and Directory Structure

• There's an indirect relationship between files and storage media
• This relationship is managed by the file system
• The interface the filesystem presents to users is the directory structure
• Directory structure is (mainly) hierarchical: directories can contain files and other directories
• Different ways of visualizing directory structure
  • Folders in a graphical user interface
  • Nested lists in command line interface
• Directory structure is increasingly de-emphasized in user interfaces (example: iPhone)

(6) Graphical View of Directory Structure

(7) Textual View of Directory Structure

(9) File Paths

• A path is a sequence of symbols for uniquely describing a file or directory's location in a directory structure
• A path consists of a sequence of directory names, optionally followed by a file name if the path is to a file
• The elements in the sequence are separated by a delimiter character (/ on Mac and Unix, \ on Windows)
• Example file path on Linux or Mac:

  /home/dilanm/Desktop/notes.txt

• Example file path on Windows:

  C:\Documents and Settings\Dilan Mahendran\Desktop\notes.txt

(10) Absolute vs. Relative File Paths

• File paths may be absolute or relative
• An absolute path starts at the "root directory" and always refers to the same file or directory
  • Root directory on Mac, Unix, and Linux is /
  • Root directory on Windows is (for example) C:\
• A relative path is interpreted relative to some current location
• Relative paths may use some special characters
  • . refers to the current directory (the directory we're in)
  • .. refers to the parent directory (the directory above the one we're in)

(11) Absolute vs. Relative File Paths

• Assuming we're in the directory named 'Desktop', the following paths are all equivalent:
  • Absolute:

    /home/dilanm/Desktop/notes.txt

  • Relative:

    notes.txt

    ./notes.txt

    ../Desktop/notes.txt

(12) Why Relative Paths?

• Directory structure varies from computer to computer
• Absolute paths name every directory from the root
• An absolute path that points to a file on one computer may not point to anything on another computer
• A relative path usually only names directories beside or below around the current directory
• Using relative paths, files in or below a certain directory can refer to each other without making assumptions about the directory structure of the whole computer
• Thus relative paths are more portable
• This is important for web sites, which are usually moved around to different computers

(14) Web Servers

• "Web server" is used to mean two things:
  1. Software that receives requests from web browsers and sends back the files that compose a web page
  2. A computer running such software
• A web server is not a special kind of computer
  • If you have a Mac, you have a web server [http://docs.info.apple.com/article.html?path=Mac/10.6/en/8236.html] (Apache)
• In particular, (computers running) web servers have file systems and directory structures just like any other computer
• Note that Unix is far more common among computers being used as web servers than it is among computers being used for personal computing
  • So we'll always use Unix (and Mac) -style paths in this course

(15) URL Paths

• A web URL has two main parts: the domain and the path

  http://[domain]/[path]

• For example:

  http://courses.ischool.berkeley.edu/i153/s11/

  • The domain is "courses.ischool.berkeley.edu"
  • The path is "/i153/s11/index.shtml"
• (URLs have more parts than this, as we'll learn in late February)

(16) URL Paths vs. File Paths

• URL paths might be the same as file paths, but usually they aren't
• Web server software is responsible for translating URL paths into file paths
• Some URL paths are not translated into file paths at all, but are interpreted in other ways (e.g. looking up something in a database)
• Example of a URL path and its corresponding file path:
  • URL path:

    /i153/s11/index.shtml

  • (Absolute) file path:

    /courses/i153/public_html/s11/index.shtml

• Like file paths, URL paths can be absolute or relative