UNIX Linux

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UNIX / Linux

• Courtesy B. VachonRevised G. Cooper

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What is an Operating System?

• The OS is part Butler and part Security Guard.
• As a Butler, an OS performs services that would
  be too complicated or dangerous for other
  programs to do for themselves (e.g., printing,
  loading applications, etc.)
• As a Security Guard, the OS protects the users
  and their data.
• Keeps files from interfering with each other.
• Makes sure that users cant read or write to each
  others files unless they have permission.
• It allocates to each user their fair share of the
  computers resources memory, disk space,
  processing time, etc.
• It can restrict computer access to authorized
  users only.

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Layers of an Operating System (OS)

• An Operating System can be divided into three
  parts
• Utilities consist of the file management commands
  and programs associated with the OS.
• (e.g., for DOS CHKDSK, FORMAT, DISKCOPY)
• Shell is what stands in between the user and the
  kernel.
• Enable users to set up aliases, histories, etc.
• Popular UNIX shells include the Bourne Shell, C
  Shell, Kornshell, Bourne Again Shell (bash), and
  T Shell.
• The Kernel is the collection of programs that
  provides the basic capabilities of the OS.

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History of UNIX

• UNIX started out as MULTICS, a collaborative
  effort with GE, ATT and MITT in the late 60's.
• UNIX was officially developed by Ken Thompson and
  Dennis Ritchie of Bell Labs (ATT) in New Jersey
  in 1970.

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History of UNIX

• UNIX was developed to accept short commands and
  display no responses unless something went wrong.
• It was intended to be used by only a small number
  of people and it was assumed that the users were
  to be experienced professionals.

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History of DOS

• MS-DOS and PC-DOS was developed by Bill Gates and
  Paul Allen in the late 70's for Microsoft.

• IBM had approached them for an OS which would
  work with their IBM-PC.
• Because DOS was constructed later than UNIX, its
  programmers could build on existing experience
  with UNIX and borrow many UNIX features.

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Why You Should Investigate UNIX?

• UNIX
• makes it possible to share a system with other
  people without interfering with each other.
• is multitasking enables you to do several things
  at once.
• is a mature system (its been around for 30
  years).
• is extremely customizable, it can be streamlined.
• can run on almost any computer (i.e., from a
  microcomputer to a supercomputer).
• is in demand - companies want to hire people with
  UNIX expertise.
• is an extremely powerful OS.
• E.g. using only one command, backup all of the
  files which were created less than a day ago in
  Windows 95,
• cp (find . -ctime -1)
• it is an extremely stable OS

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The Different Flavours of UNIX

• UNIX System V (originally ATTs)
• Now Novell (SVR4.2 UNIXWare).
• Berkeley BSD (Berkeley Software Distribution)
• AIX (IBM)
• HP/UX (Hewlett-Packard)
• OSF/1 (DEC)
• SCO UNIX (SCO) which also sold XENIX (originally
  Microsofts)
• Solaris (SUN)
• Linux (Slackware, Redhat, Caldera, Debian
  Mandrake)

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The UNIX Philosophy

• Reducing User Effort
• Combining Small Tools into Larger Ones
• Customizing

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UNIX
Dilbert - United Features Syndicate Inc.
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The man pages

• To get help with commands in UNIX we can use the
  man command.
• The man pages (Manual Pages) provide detailed
  descriptions to commands
• Warning to new users man pages are often very
  encrypted and difficult to understand.
• man command-name (e.g., man cp)

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The man pages

• The keystrokes for navigating within a man page
  are
• Spacebar - to advance to the next screen
• Enter - to move one line down
• b - to move one screen up
• q - to quit (exit)
• Note A quicker and sometimes more effective way
  to get help, is to type in the command you wish
  help with and then follow it with --help
• For example cp --help

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Login In

• Whenever you use the UNIX OS you will be
  requested for a login name.
• A login name is also referred to as the userid.
• When prompted for a password, make sure that you
  enter it exactly as before.
• Upper-case vs. Lowercase
• UNIX systems are very case-sensitive.
• Most commands must be typed in lowercase.
• To UNIX, a lowercase command typed in upper-case
  will mean something totally different.
• For example
• ECHO Hello will not work
• echo Hello will display Hello on the screen

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The echo command

• The echo command will display the argument on
  screen.
• echo argument
• By using quotes, UNIX treats the entry as one
  argument.
• The command line can be up to 8,192 characters.
• Command variations

If You Type In . . .
UNIX would . . .
ECHO HI MOM
Bash ECHO command not found
echo Hi Mom
Hi Mom
echo Hi Mom
Hi Mom
echo Hi Mom
Hi Mom
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The passwd and exit commands

• The passwd command will allow the user to change
  passwords.
• The user will be prompted to enter the current
  password (for authorization) and then enter the
  new password twice.
• The exit command is used to exit out of the UNIX
  OS.
• Linux also accepts the logout command.

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Text and Binary Files

• UNIX treats all files as either text or binary.
• Text files contain ordinary lines of text.
• Binary files are all other files.
• They contain any kind of data and are commonly
  created with software packages.
• E.g.,
• spreadsheet
• graphics
• word processing
• executable files

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Rules for UNIX Filenames

• UNIX systems comply to the POSIX portable
  character set standard.
• Filenames can contain
• abcdefghijklmnopqrstuvwxyz
• ABCDEFGHIJKLMNOPQRSTUVWXYZ
• 0123456789
• . (dot or period)
• _ (underscore)
• - (dash or minus symbol)
• Depending on the version of UNIX that you are
  using, the filename could also incorporate other
  characters, however, these are compliant to all
  UNIX systems.

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Rules for UNIX Filenames

• All UNIX systems support filenames of up to 14
  characters in length.
• Other versions of UNIX may go beyond this
  limitation.
• Filenames are not limited to one dot as in the
  DOS environment.
• E.g.,
• .user
• please.read.me
• unit-1.page-2
• Upper-case and lowercase filenames are considered
  separate files. (ie are case-sensitive)
• E.g.,
• sonnet and SONNET are not the same file.

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Slash and Backslash

• Just as in the DOS system, UNIX uses the same
  concept of directory structure.
• However, UNIX uses the / (slash) instead of
  backslash character \ to separate components of
  a filename and to indicate the root directory.

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The more Command

• The more command is used to display text file(s).
• more option filename filename
• Command options (Linux)
• -d displays messages
• -p do not scroll (clear the screen, then display
  the text).
• Command navigation keys (Linux)
• Spacebar Display next screen
• b or B Skip backwards 1 screen (page)
• q Exit

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The less Command

• The less command is also used to display text
  file(s).
• less option filename filename
• More flexible because it allows backward movement
  in the file as well as forward movement.
• Type help when using the less command to see the
  exhaustive list of commands
• Also, it doesnt have to read the entire input
  file before starting, therefore its faster when
  opening large files.

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The cat and ls commands

• The cat command
• Similar to the more command but not as user
  friendly (e.g., doesnt pause when screen is
  full).
• cat filename filename
• Can be used to create text file (similar to the
  copy con command)

• The ls command
• The ls command (short for list) displays a list
  of files in alphabetical order.
• Linux also offers the dir command.

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The cp command

• The cp command lets the user copy files.
• cp options from-filename(s) to-filename
• The cp command requires a destination argument.
• If the last argument in the command is that
• of an existing directory, then the cp command
  will copy the file(s) into it.
• of a file, then the cp copy will create the file.
• of an existing file, then the cp command will
  replace the existing file without confirmation.

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The cp command

• It is an error if
• the last argument is not a directory
• more than two files are given.
• By default, it does not copy directories.
• Handy options include
• -i (interactive)
• -r (recursive)

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The rm command

• The rm command (short for remove) lets the user
  remove or delete file(s).
• rm option filename filename . . .
• Handy options include
• -i (interactive)
• -r (recursive)

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The mv command

• The mv command lets the user move (or rename)
  files.
• Once moved, the original file is no longer found
  and destination file or directory must be
  specified.
• mv options filename(s) new-file or directory
• If the last argument in the command is that of an
  existing file, then the mv command will replace
  the existing file without confirmation.

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The cd command

• The cd command (short for change current
  directory) lets the user change the current
  directory.
• cd directory-name
• Command variations

If You Type In . . .
UNIX would change to the . . .
cd
The users home directory
cd /
The Root directory
cd /bin
The bin directory (branching from the root)
cd ..
The parent directory (one level up)
cd ../dirname
The sibling directory
cd -
The last directory you were in. Use it again and
it switches back to the directory you were in.
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The mkdir and rmdir commands

• The mkdir command is used to create directories.
• mkdir directory-name
• A user can only create directories in directories
  they have write permission to (e.g., their home
  area).

• The rmdir command is used to remove an empty
  directory.
• rmdir directory-name
• The directory cannot be the working directory
  (directory they are in).

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The pwd command

• The pwd command (present working directory) asks
  your system to tell you what directory youre
  currently in.
• It displays the absolute path as opposed to the
  relative path

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Unix File System

• Unlike Windows which have drive letters to
  contend with, Unix only has one single top level
  root directory.
• Other drives appear as directories.
• The process of telling the file system that the
  drive exists and what directory it should appear
  at is called mounting the drive.

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Files and Directories

• UNIX employs the same concept of directory
  structures as DOS.
• Most UNIX systems will have the following
  structure

• Its the highest level directory
• Should not be cluttered with files or too many
  directories.
• Home directory for root (superuser)

/ root
home
bin
dev
etc
tmp

• Binary directory contains the essential utilities
  and commands

• Device directory contains device files

user2
user3
user1

• Temporary holding area for programs

• Users personal directories

• Administrative, configuration and other system
  files are here

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File Characteristics

• The UNIX file system records the following
  characteristics for each file including
• the size of the file
• the date and time it was last changed
• and who is permitted to use the file and how are
  they permitted to use it.

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Three Sets of File Permissions

• Owner
• The person who created the file is the owner of
  the file.
• Group
• The system supervisor can create groups to help
  teams work together.
• Each person on the system is assigned to one
  primary group and could be assigned to several
  other secondary groups.
• When a file is created, the owners primary group
  automatically receives their rights to that file.
• Other
• Everyone else on the system who is not the files
  owner or in the files group.

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Type of File Permissions

• UNIX has three types of file permissions and each
  is represented by a code letter.
• r Lets you read the file.
• w Lets you write to the file.
• x Lets you execute the file.
• They are always written in the following order
• rwx

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Type of File Permissions

• A file will list the permissions to it as
  rwxrwxrwx
• The file permissions are divided into three
  sections
• The first sequence of rwx are the owners rights
• The second sequence of rwx are the groups rights
• The last sequence of rwx is for the others
  rights.
• If a letter is substituted by a dash (-), then
  that right is not available.

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Type of File Permissions continued . . .

• For example

These rights . . .
Indicate . . .
rwxrwxrwx
Everybody has rights to the file/directory
rwxrw-r--
Owner (rwx) all rights Group (rw-) read and
write rights Others (r--) read right only
r--r--r--
Owner (r--) read right only Group (r--) read
right only Others (r--) read right only
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Types of Directory Permissions

• UNIX has a similar system of permissions for
  directories.
• Each is represented by a code letter.
• r Lets you read the files/ subdirectories in the
  directory.
• w Lets you create new files/subdirectories in
  that directory.
• x Lets you search through the directory.

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Listing File Characteristics

• The ls command can also be used to list file
  characteristics.
• ls -l filename (e.g., ls -l sonnet)

A - Indicates a file or directory (- for file, d
for directory) B - File permissions for owner
(read and write) C - File permissions for group
(read and write) D - File permissions for others
(read) E - of hard links to the file F - Owners
name G - Owners primary group name H - File size
(bytes) I - Date and time since it last
saved J - The name of the file or directory.
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The chmod command

• The chmod (change mode) command is used to set
  file and directory permissions.
• chmod permissions name
• Specify u for owner, g for group, o for others,
  and a for all.
• More than one permission can be specified at a
  time.
• To activate a permission use a and to
  deactivate a permission use a - symbol.

This command
Indicates that for the nursery file
chmod o-w nursery
takes away write permission from others
chmod gorw nursery
gives read write to the group and others
chmod urwx nursery
gives read, write and execute to you