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Hierarchical File Structure

• All files in the UNIX file system are organized in a multi-leveled hierarchy called a directory tree.
• A family tree is an example of a hierarchical structure that represents how the UNIX file system is organized. The UNIX file system might also be envisioned as an inverted tree or the root system of plant.
• At the very top of the file system is single directory called "root" which is represented by a / (slash). All other files are "descendents" of root.
• The number of levels is largely arbitrary, although most UNIX systems share some organizational similarities. The "standard" UNIX file system is discussed later.
• Example:

               / (root)

               |

     ---------------------

     |         |         |

    /bin      /usr      /tmp

                |

                |

        ------------------------------------------------------------

        |        |        |                                        |

       /public  /misc    /staff                                /students

        |                  |                                       |

   ------------         -------------------       ----------------------        

       

   |          |         |     |     |     |       |      |      |      |    

  /software  /doc      /john /mary /bill /carl   /tom  /dick  /mary  /lisa

File Types

• Ordinary Files
• Used to store your information, such as some text you have written or an image you have drawn. This is the type of file that you usually work with.
• Always located within/under a directory file
• Do not contain other files
• Directories
• Branching points in the hierarchical tree
• Used to organize groups of files
• May contain ordinary files, special files or other directories
• Never contain "real" information which you would work with (such as text). Basically, just used for organizing files.
• All files are descendants of the root directory, ( named / ) located at the top of the tree.
• Special Files
• Used to represent a real physical device such as a printer, tape drive or terminal, used for Input/Ouput (I/O) operations
• Unix considers any device attached to the system to be a file - including your terminal:
• By default, a command treats your terminal as the standard input file (stdin) from which to read its input
• Your terminal is also treated as the standard output file (stdout) to which a command's output is sent
• Stdin and stdout will be discussed in more detail later
• Two types of I/O: character and block
• Usually only found under directories named /dev
• Pipes
• UNIX allows you to link commands together using a pipe. The pipe acts a temporary file which only exists to hold data from one command until it is read by another
• For example, to pipe the output from one command into another command:

 

     who | wc -l

This command will tell you how many users are currently logged into the system. The standard output from the who command is a list of all the users currently logged into the system. This output is piped into the wc command as its standard input. Used with the -l option this command counts the numbers of lines in the standard input and displays the result on its standard output - your terminal.

File Names

• UNIX permits file names to use most characters, but avoid spaces, tabs and characters that have a special meaning to the shell, such as:

     & ; ( ) | ? \ ' " ` [ ] { } < > $ - ! /

• Case Sensitivity: uppercase and lowercase are not the same! These are three different files:

     NOVEMBER       November     november

• Length: can be up to 256 characters
• Extensions: may be used to identify types of files

     libc.a       - archive, library file 

     program.c    - C language source file 

     alpha2.f     - Fortran source file 

     xwd2ps.o     - Object/executable code 

     mygames.Z    - Compressed file

• Hidden Files: have names that begin with a dot (.) For example:

     .cshrc      .login      .mailrc     .mwmrc

• Uniqueness: as children in a family, no two files with the same parent directory can have the same name. Files located in separate directories can have identical names.
• Reserved Filenames:

     /   - the root directory (slash)

     .   - current directory (period)

     ..  - parent directory (double period)

     ~   - your home directory (tilde)

Pathnames

• Specify where a file is located in the hierarchically organized file system
• Must know how to use pathnames to navigate the UNIX file system
• Absolute Pathname: tells how to reach a file begining from the root; always begins with / (slash). For example:

     /usr/local/doc/training/sample.f

• Relative Pathname: tells how to reach a file from the directory you are currently in ( current or working directory); never begins with / (slash). For example:

     training/sample.f  

     ../bin  

     ~/projects/report.001

• For example, if your current directory is /usr/home/johnson and you wanted to change to the directory /usr/home/quattro, you could use either of these commands:

     cd ../quattro           - relative pathname 

     cd /usr/home/quattro    - absolute pathname