UNIX File System

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UNIX File System

• Go Climb a Tree

Weiyu Zhang (96142) COSC513 Operating
Systems Southeastern University
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Introduction

• In UNIX, the thing we spend most of our time
  working with is files.
• Treating files consistently is one of the
  greatest strengths in UNIX.

It is true.
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What is consistent?

• UNIX use the same method to save all the files.
• For example, an executable script has the same
  format as a text file.
• Simplify the file system.
• The idea is widely used later. (In DOS, Win95/98,
  etc.)

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Three types of UNIX Files

• Regular files
• Directory files
• Device files

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Regular files

• Most common type
• Hold programs and data
• Example
• Source code of a program
• Email received from friend
• Executable programs and applications (such as ls,
  cat, etc.)

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Directory files

• Basic management tools in UNIX file system
• Like a cabinet in real life, the files that
  contain other files
• Save information of files (such as location,
  size, etc.)

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Devices files

• Each hardware is treated like a file
• Example
• Terminals
• Printers
• Keyboards, etc.

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More about device file

• Example
• Read the user input ? read from a keyboard device
  file.
• Print a text file ? Copy the file to the printer
  device file.
• Convenient to handle

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File system organization

• A bottom-up tree with root on the top
• All the files must be put into directory files
• Root (/), a special directory file
• Exact one
• Can not been deleted

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One sample tree
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Find a file in UNIX

• Start from the root
• Go down (or climb) the tree, record the path
• Each file has a name

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An example
/usr/wang/letter.txt
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File and directory permission

• Only need in a multi-user OS
• Keep your own secrets
• Three classes of users
• Three permissions

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Three classes of users

• Owner I created the file.
• Group The owner and I are in the same working
  group.
• World (a.k.a. other) I just want to use the file.

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Three permissions

• Read permission read
• Write permission modify, delete
• Execute permission run
• No exact priority, normally write permission is
  the most important one

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Nine bits (3 ? 3)
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Nine bits (continue)
1 for Y and 0 for N. So the permission of the
file is (111 101 000)2
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Why permission

• Keep your secrets.
• In the above example, world user can not read
  the file, group user can not modify or delete the
  file.

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Why permission (cont)

• Keep your secrets.
• Show more information about a file.
• Since the files are saved consistently, only
  those which have the execute permission set can
  be run.

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Why permission (cont)

• Keep your secrets.
• Show more information about a file.
• Avoid wrong operation
• Setting the owner write permission to be N (or
  0) can avoid the careless delete of very
  important files.

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I have no permission but I can ...
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Conclusions

• Everything is a file.
• Bottom-up tree organization
• Easy but safe

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The End