Unix File System

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Unix File System

• CSCI 2467 System Programming Concepts
• Computer Science Department
• University of New Orleans
• Instructor Michael Ruth
• mruth_at_cs.uno.edu

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Topics

• Before We Get Started
• Quick Review
• Trees
• File System
• Files And Directories
• Quick Definitions
• Filenames - Absolute vs Relative
• Guarded Access through Permissions

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Before We Get Started

• I neglected to mention one of the key things
  youll need to know as Unix users
• RTM (Read the manual)
• If you want to know what a command does (standard
  commands) you use the manual command to do so
• Ex
• man man
• man cd
• man mv
• This becomes even more important when you cant
  remember the option that make this a one time
  command

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Quick Review From Last Week

• Everything is a file
• Allows for every I/O subsystem to be handled
  uniformly
• Mouse, printer, text file, etc
• Unix files have neither record structure nor
  attributes (not really, but we said that too)
• Everything can and should be treated as ASCII
• The file system is hierarchal
• Tree structure

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First, some math (graph theory)

• Graph consist of a set of nodes and a set of
  edges that establish relationships (connections)
  between the nodes
• Trees are special cases of graphs
• The relationship is children/parent
• Each node has zero or more child nodes, which are
  below it in the tree (by convention, trees grow
  down, not up as they do in nature).
• A node that has a child is called the child's
  parent node (or ancestor node, or superior).
• A node has at most one parent.
• The bottommost nodes are called leaves and the
  topmost node is called the root
• A subtree is a portion of a tree data structure
  that can be viewed as a complete tree in itself

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Now a picture pictures help I hope
B is a child of A, therefore A is a parent of B
A
A
Root Node
B
R
B
tree with root node B highlighted here is a
subtree of the Tree with root A
C
D
E
T
Leaf Nodes
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Recursively, Treewise

• A tree is either empty (having no nodes with no
  children) or is made up of a single node which
  has any number child trees
• This recursive definition allows us to consider
  how this definition makes everything uniform

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File System

• File System
• method for storing and organizing computer files
  and the data they contain to make it easy to find
  and access them
• More formally, a file system is a set of abstract
  data types (ADTs) that are implemented for the
  storage, hierarchical organization, manipulation,
  navigation, access, and retrieval of data.
• In Unix this concept is produced in the form of a
  tree.
• The tree itself is how the ADTs are stored in the
  more concrete file system
• The files themselves are the ADTs
• We interact with the kernel to move around in the
  system as well as move things around the system

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Files and Unix

• Files are the center point to any Unix system and
  for purposes of the Unix File System (or UFS)
  they are identified by an inode data type
• More about inodes later (after weve discussed
  what the information contained in them actually
  is)
• The only important point is that inodes have
  identifying numbers (unique for each file on each
  device)
• file is just a sequence of binary digits
• It is up to the program using the file to
  understand the meaning and internal layout of
  information in the file and present it to a user
  as a document, image, song, or program

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Directories

• In computing, a directory is an entity in a file
  system which contains a group of files and/or
  other directories.
• A typical file system contains thousands of
  files, and directories help organize them by
  keeping related files together.
• A directory contained inside another directory is
  called a subdirectory of that directory (think
  subtree)
• Together, the directories form a hierarchy, or
  tree structure
• The root of the UFS tree is denoted by /
• However, in Unix Directories themselves are files
  which do not contain the files themselves, rather
  they contain links to the files along with the
  file names

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Pictorially
/
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Filenames

• Every file has a filename
• Current versions of Unix allow up to 255
  characters, older versions limit to 14
• Almost any character can be used in a filename
• Sticking with the following will save many
  problems
• Uppercase letters (A-Z)
• Lowercase letters (a-z)
• Numbers (0-9)
• Period ( . ) can be used as any other character
  and may denote special files
• Exception is the root directory, which is always
  named /
• No other file can use this name

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More Naming Concerns

• Other characters may be used by escaping them
  with a \ character
• Willie\Weasil - displays as WillieWeasil
• Much confusion can result from this
• File names are case sensitive
• my_file, My_File, and MY_FILE are all unique
  names

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Getting around in UFS

• Working directory?
• Where you are in a Unix system
• pwd
• Provides absolute path
• Home Directory
• Where you started in a Unix System (upon login)
• Change your working directory?
• cd directory path
• No argument changes the working directory to home
  directory
• With argument changes the working directory to
  specified directory
• Can be either a relative or absolute path

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Pathnames
/

• Every file has a pathname
• Can be either absolute or relative
• Absolute
• A pathname is built by tracing a path from the
  root directory, through all intermediate
  directories, to the file
• String all the filenames in the path together,
  separating them with slashes ( / ) and proceeding
  them with the root directory ( / )
• Example /home/mruth/Work1.c

home
stu
mruth
research
grads
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Relative pathnames

• Relative
• The pathname is built by tracing a path from the
  current working directory to where the file
  exists
• Every directory has two intrinsic directories
• . (this directory)
• .. (the parent directory)
• These make using relative pathnames easy

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Relative Pathnames Examples
/
mruth/research/Work1.c
../mruth/research/Work1.c
home
research/Work1.c
stu
mruth
./research/Work1.c
research
grads
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Other Useful commands (ls)

• List (ls) list all files in directory
• ls options names
• If no names are given, list the files in the
  current directory
• If names we use metacharacters and text to list
  the files that match
• Important options
• -a list all files (including pesty hidden files
  which begin with .
• -i list the inode for each file
• -l long format listing
• Note about options if we wish to use more than
  one we simply concatenate the options like so ls
  -ai

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Directory creation and removal

• mkdir creates a directory with a given name
• mkdir options directories
• Options are -m for mode, and -p for
  autocreate things in between
• mkdir p mycourses/2467/tests (in empty
  directory)
• rmdir removes a directory with a given name
  (directory must be empty)
• rmdir options directories
• I have never ever used any of the options (rarely
  even use this command)

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Unix Security

• The Unix system treats everything as a file
• The Unix System Provides security capable of
  handling
• Group access
• Individual access
• System access
• Of course, this is handled at the file level

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Files and Owners

• Every file has a single owner as well as a
  permissions (or mode) associated with it
• The only person who can access everything by
  default is the root user
• The root is generally the system administrator
• Everyone one else can access what they have
  permission to access
• This permission is set at the individual file
  level

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Access Types

• Individual Access same level of access provided
  by the login/logout scheme on a Unix System
• Each user is given both a username and a UID to
  uniquely identify them on the system
• Group Access there are a number of groups on
  any given system which form some sort of cohesive
  unit (all accountants on 3rd floor)
• Eases IT management
• Each group is given both a name and a GID to
  uniquely identify them on the system
• Each user can be a member of several groups and
  each group can be comprised of several members

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Three levels of Access

• Read allows examination of a files contents
• Directory contents of the given directory can be
  listed
• Write allows altering or adding to the files
  contents also allows deletion
• One can add files to the given directory
• Execute allows execution of the file as a
  program
• One can change their working directory to the
  given directory

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Access levels and types

• Each level may be selected added or removed for
  each access type for each file
• This provides a fair amount of flexibility in
  deciding how to allow/deny access to what it is
  that you are trying to protect

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chmod

• chmod takes two forms
• chmod perms filename
• Perms is specified in octal
• Ex chmod 0755 grants rwx-r-x-r-x
• chmod who-what filename
• Ex
• chmod u-x ? removes execute for user
• chmod x ? adds execute for everyone (all three)
• Three types are user (u), group (g), and others
  (o)

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chown/chgrp/su

• chown allows one to change the individual
  ownership of the file
• chown options newowner filename
• chgrp allows one to change the group ownership of
  the file
• chgrp options newgroup filename
• Su allows one to change (temporarily) a users
  user id
• su username
• Switch user NOT super user
• If no username is given, it defaults to root

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Umask

• Umask is the default file creation mode mask
• Every file you create or will be created with
  have a set of default permissions associated with
  it
• You can change that using umask command
• umask value
• If no value is specified it returns current value
• If value is present, set it to that value

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Revisiting Inodes

• Earlier I said Unix files have neither record
  structure nor attributes
• Unix files are said neither to have structure nor
  attributes in any normal sense
• No a priori knowledge of
• What kind of file it is (text/binary)
• What kind of program opens it
• Is this archived?
• However, even though the inode structures in Unix
  holds information about each file they were
  designed to be extremely generic so that it can
  be used for any type of file

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Inode properties

• Device ID device inode resides on
• Inode ID inode number
• Mode permission mode
• Link Count number of hard links to the file
• UID user id of the owner
• GID group id of the owner
• Device Type the device type of the device the
  inode resides on
• Size size in bytes of the total size of file
• Three dates
• Last access
• Last modification
• Last status change
• Optimal block size for the system
• Links to the device that actually holds the data

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Links???

• Unix provides both hard and symbolic links to
  files.
• Hard links
• must reside on the same filesystem. A hard link
  is
• essentially a direct reference to the file by a
  another name.
• Both files share the same inode and the inode
  increases the reference count of the file by one
  for each link.
• The ln command creates hard links. The format of
  the command is ln existing-file new-file.
• A symbolic link is a
• file that indirectly points to another file.
• While the affect is the same as a hard link a
  symlink does not increase the reference count of
  a file.
• A symbolic link also can point to files on other
  file systems and files that don't even exist!
• The ln -s command creates a symbolic link. The
  format of the command is ln -s existing-file
  new-file.

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File types

• Upon running ls l (or similar commands) we find
  that the system does keep track of certain system
  file types
• The first letter of the command shows us what
  file type it is
• Regular files have a prefix of
• Directories have a prefix of d
• Links have a prefix of l
• There are a few more, but well talk about them
  as we get there

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Important file commands

• cp options source target
• Most important option -r recursively copy
• mv options source target
• file options files
• Tries to determine what kind of file is present
• rm options target
• Most important options
• -f doesnt ask if anythings okay it just forces
  it
• -r recursively does its job

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Multiple Disks and UFS
/
/
home
homework
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Mounting a drive (into the tree)
/
This is what we want
/
home
homework
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Mount command

• mount -t vfstype device dir
• Do NOT use the t option unless not using it is
  NOT an option
• Ex mount /dev/fd0 /home/meruth

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Future Classes

• Text/File Processing
• Processes and Job Control
• Shell Programming Regular Expressions
• Introduction to C Programming

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Questions?