Announcements:

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• Announcements
• Instructor Yifeng Zhu
• Email yzhu_at_cse.unl.edu
• Office 501 Building, Room 2, Cubic 2.2
• Office Hour Monday 400-500, or by appointment
• Phone [phone]
• Grader Olga Komina
• Email okomina_at_cse.unl.edu
• Office 501 Building, Room 5, Cubic 5.7
• Office Hour
• Phone [phone]
• For those who got new accounts, please register.
  Otherwise, your new accounts will be closed after
  two weeks.
• A typo on Syllabus web course link
  http//cse.unl.edu/yzhu/cs251
• Quiz drop only one. (two zeros, only one to be
  dropped)

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Chapter 2 Exploring the UNIX File System

• File and File Systems

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Understanding Files and Directories Objectives

• After studying this lesson, you should be able
  to
• File system structure
• Navigating, creating and deleting
• Unix permissions
• Build your own personal web

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Understanding the UNIX File System

• In UNIX, a file is the basic component for data
  storage
• UNIX considers everything it interacts with as a
  file, even attached devices such as monitors
• A file system is the UNIX systems way of
  organizing files on mass storage devices such as
  hard and floppy disks

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Unix file system structure

• The file system is a hierarchical structure
  resembling a rooted tree, anchored at the root
  (/)

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Unix file system structure

• Every item in the file system is a file (or a
  link).
• Directories can contain file and other
  directories (called subdirectories). The
  subdirectory is considered as the child of the
  parent directory
• Each directory may have only one parent.
• A directory is a special kind of file that can
  contain other files and directories

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Absolute Relative Paths

• To describe an object in the file system you
  specify a path.
• Paths are either absolute, beginning at the root
  level
• /usr/local/bin/howto
• Or they are relative to the current directory
• ../stuff/morestuff/afile

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Navigating the File System Continued

• Any time the \ symbol is the first character in a
  path, it stands for the root directory
• A relative path takes a shorter journey
• You can enter the relative path to begin at your
  current working directory and proceed from there

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Navigating the File System

• cd ltdirgt change to ltdirgt
• cd .. change to parent dir
• pwd print current directory
• ls list directory contents
• ls -l long listing
• ls -a list all files
• ls -al long listing, all files

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Using Dot and Dot Dot Addressing Techniques

• UNIX interprets a single dot character to mean
  the current directory, and dot dot (two
  consecutive dots) to mean the parent directory
• cd . - keeps you in the current directory
• cd .. - returns the user to their home directory

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Tilda () directory

• The tilda () represents your home directory.
• It is a short cut. It means the same thing as
  /home/user_name
• Example
• Enter your home directory cd
• Enter another persons home directory cd yzhu

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Creating Deleting files

• cp copy file or directory
• mv move file or directory
• mkdir create a directory
• rmdir delete a directory
• rm delete a file
• rm -r delete recursively
• rm -rf force recursive delete

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Copying files

• cp copy file or directory
• syntax cp options source-file
  destination-file
• source-file name and path of file to be copied
• destination-file name and path of resulting copy
  of the file
• options are optional not required
• Common options
• -i interactive copy prompts before overwriting
  an existing file
• -r recursive copy copies the entire directory
  structure from the top down and recreates it at
  destination
• Examples
• cp /etc/passwd .
• cp i /etc/passwd
• cp /etc/passwd /fileb
• cp -r /etc/pcmcia

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Renaming files

• mv move file or directory
• Better name could be the rename command.
• Changes the name of one file to another.
• Syntax mv options old file name new file
  name
• Note, if new file name is a directory, you will
  move old file name to that directory and keep
  the original name.

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Removing files

• rm remove file or directory
• Deletes the specified file or files.
• This is destructive!
• They are gone!
• They cannot be retrieved!!!
• Syntax rm options file name
• Note this does not generally work with
  directories.
• rm r directory name

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Unix File Permissions

• Because UNIX is a multi-user system, users can
  set permissions for files they own so that others
  can read, write, or execute their files
• A files owner is the person who created it
• The permissions the owner sets are listed as part
  of the file description
• The first section of file permission specifiers
  indicates the owners permissions

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Unix File Permissions

• Each file and directory has permissions that
  support access control.
• Permissions are defined as
• read (r) view contents
• write (w) change contents
• execute (x) run the file or change to the
  directory
• Permissions are defined in three sets, for the
  owner, group, and all others.
• Directories must be executable to be accessed.

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View Permissions
ls l /etc/passwd -rw-r--r-- 1 root root
1369 Apr 30 2003 /etc/passwd ls l
/bin/ls -rwxr-xr-x 1 root root
46888 Jan 19 2003 /bin/ls
owner
group
size
creation date
permissions 1st character type, - for file,
d directory, s special, l link Next three
are read, write, execute for owner Then, next
three for group, and next three for all
others. Character means permission granted, -
means denied.
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File Permission Specifiers
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Examples file permissions

• -rw-------
• Only owner can read and write
• -rwxrwxrwx
• Everyone can read, write and execute (and delete)
  (not very common!)
• -rw-r--r--
• Everyone can read, but only owner can write

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Examples directory permissions

• drwx------
• Only owner can view, cd into, and delete
• drwxrwx---
• Owner and group can view, write, etc.
• drwxr-xr-x
• Everyone can cd into but only owner can
• modify

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Changing Permissions

• chmod ugoa(/-)rwx ltdirgt
• Where
• u user, g group, o others, a all
• add permission, - remove
• To make a directory readable by others
• chmod gorx ltdirgt

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Making a web page

• 1. Create the directory
• cd (or cd )
• mkdir public_html
• 2. Set the Unix permissions
• chmod gox (or chmod ax )
• chmod gorx public_html (or chmod ax
  public_html)
• 3. Create the index.html file
• cd /public_html
• cp yzhu/public_html/cs251/example.html
  index.html
• chmod gorx index.html
• Feel free to edit or substitute your own html
  file(s).

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Chapter Summary

• In UNIX, a file is the basic component for data
  storage. UNIX considers everything to be a file,
  even attached devices
• A file system is the UNIX systems way of
  organizing files on mass storage devices such as
  hard and floppy disks
• Every file can be located by using a correct and
  unique pathname, that is, a listing of names of
  directories leading to a particular file

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Chapter Summary Continued

• The standard tree structure starts with the root
  (/) directory, which serves as the foundation for
  a nested group of other directories and
  subdirectories
• A path, as defined in UNIX, serves as a map to
  access any file on the system. Special path( .
  (the directory itself), .. (the parent
  directory), (the home directory))
• You can use the chmod command to set permissions
  for files that you own
• Commands ls, cd, pwd, cp, rm, mkdir, rmdir