Introduction to UNIX

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Introduction to UNIX

• Thara Angskun

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Introducing the UNIX Operating System

• UNIX is a multi-user, multitasking operating
  system with built-in networking functions
  
• It can be used on systems functioning as
• Dedicated serves in a server-based network
• Client workstations connected to a server-based
  network
  
• Client/server workstations connected to a
  peer-to-peer network
  
• Standalone workstations not connected to a network

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Introducing the UNIX Operating System Continued

• UNIX is a multi-user system, which lets many
  people simultaneously access and share the
  resources of a server computer
  
• A multitasking system lets one user execute more
  than one program at a time
  
• UNIX is also a portable operating system

4
Introducing the UNIX Operating System Continued

• Its portability means it can be used in a variety
  of computing environments
  
• UNIX runs on a wider variety of computers than
  any other operating system
  
• It also runs on the Internet, regulating popular
  programs such as File Transfer Protocol (FTP), an
  Internet protocol used for sending files and
  Telnet, an Internet terminal emulation program

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A Brief History of UNIX

• A group of programmers at Bell Labs originally
  developed UNIX in the early 1970s
  
• Bell Labs distributed UNIX in its source code
  form, so anyone who used UNIX could customize it
  as needed
  
• Eventually, two standard version of UNIX
  evolved
  
• ATT produced System V
• University of California at Berkeley developed BSD

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UNIX Concepts

• Microsoft DOS and Microsoft Windows adopted
  original UNIX design concepts, such as the idea
  of a shell--an interface between the user and the
  operating system--and the hierarchical structure
  of directories and subdirectories
• The kernel is the base operating system, which
  interacts directly with the hardware and services
  the user programs

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Layers of a UNIX System
8
UNIX Concepts Continued

• The kernel is only accessible through kernel
  mode, which is reserved for the system
  administrator
  
• This prevents unauthorized commands from invading
  the foundation layer or the hardware that
  supports the entire UNIX structure
  
• User mode provides access to higher layers where
  all application software resides

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Linux and UNIX

• Linux is a UNIX-like operating system
• Linus Torvalds, who released it to the public,
  free of charge, in 1991, originally created
  Linux
  
• Linux offers all the complexity of UNIX at no
  cost
  
• You can install Linux on your PC where it can
  coexist with other operating systems, and test
  your UNIX skills

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Introducing UNIX Shells

• The shell is a UNIX program that interprets the
  commands you enter from the keyboard
  
• UNIX provides several shells, including the
  Bourne shell, the Korn shell, and the C shell
  
• Steve Bourne at ATT Bell Laboratories developed
  the Bourne shell as the first UNIX command
  processor

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Introducing UNIX Shells Continued

• The Korn shell includes many extensions, such as
  a history feature that lets you use a keyboard
  shortcut to retrieve commands you previously
  entered
• The C shell is designed for C programmers use
• Linux uses the freeware Bash shell as its default
  command interpreter

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Shells Relationship to the User and the Hardware
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Choosing Your Shell

• You choose a shell when the system administrator
  sets up your user account
  
• Most users choose the Bash shell, although you
  can choose any of these
  
• Bourne
• Korn
• C shell
• Bash
• tcsh (a freeware shell derived from the C shell)
• zsh (a freeware shell derived from the Korn shell)

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Switching from Shell to Shell

• After you choose your shell, the system
  administrator stores your choice in your account
  record, and it becomes your assigned shell
  
• UNIX uses this shell any time you log on
• However, you can switch from one shell to another
  by typing the shells name (such as tcsch, bash,
  or zsh) on your command line

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Entering Commands

• UNIX is case-sensitive, that is, it distinguishes
  between uppercase and lowercase letters, so that
  John differs from john
  
• You type most UNIX commands in lowercase
• You must know a commands syntax to enter I
  properly
  
• Syntax refers to a commands format and wording,
  as well as the options and arguments you can use
  to extend and modify its functions

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Command-line Editing

• Shells support certain keystrokes for performing
  command-line editing
  
• For example, Bash (which is the default Linux
  shell) supports the left and right arrow keys,
  which move the cursor on the command line
  
• Not all shells support command-line editing in
  the same manner

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Multiple Command Entry

• You may type more than one command on the command
  line by separating each command with a
  semicolon()
  
• When you press Enter, UNIX executes the commands
  in the order you entered them

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Command-line Editing Continued

• The clear Command
• You can use the clear command to clear your
  screen it has no options or arguments
  
• The Command-line History
• You can access the command history with the up
  and down arrow keys
  
• Pressing the up arrow key once recalls the most
  recently used command
  
• Each time you press the up arrow key, you recall
  an older command

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Redirecting Output

• In UNIX, the greater than sign () is called the
  redirection symbol
  
• You can use the redirection symbol to create a
  new file or overwrite an existing file by
  attaching it to a command that produces output
  
• You can also use the redirection symbol with the
  cal command to save a calendar in a text file
  
• You can also use the cat command to create files
  from information you type at the keyboard

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The man Program

• For reference, UNIX includes an online manual
  that contains all commands, including their
  options and arguments
  
• The man program in UNIX displays this online
  manual, called the man pages, for command-line
  assistance

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Man Pages List the Following Items

• Name the name of the command and a short
  statement describing its purpose
  
• Synopsis a syntax diagram showing the usage of
  the command
  
• Description a more detailed description of the
  command than the name item gives
  
• Options a list of command options and their
  purposes
  
• See Also other commands or man pages that
  provide related information
  
• Bugs a list of the commands known bugs

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Manual Page
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Elementary Command

• man, whatis , apropos
• pwd, ls, cd, mkdir, rmdir, mv, rm, chattr
• chmod, chown
• time, date, cal, sleep
• cat, more, less, head, tail
• tr, look, grep, sed,
• sort, cut,crypt, tar, find, locate
• hostname, uname, who, w, finger,echo

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Elementary Command

• tee, script
• ps,kill,killall,

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User Interaction with the Shell

• User logs in
• shell shows the prompt
• User types a command
• shell executes the appropriate program
• User interacts with the program
• User logs off

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Command Interpretation Cycle

• 1. Display the prompt
• 2. Receive and parse (analyze the syntax of) the
  next command
  
• 3. Perform input processing, transforming the
  command
  
• 4. Carry out or initiate the requested
  operations
  
• 5. Wait for initiated operations to finish
• 6. go to step 1

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Metacharacters - Special Characters (1)

• ? Matches any single character
• Matches any string, including the empty
  string
  
• list Matches any one of the characters
  specified in the list
  
• !list Matches any one of the characters not
  specified in the list
  

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Metacharacters - Special Characters (2)

• Executing the commands
• Sequencing the commands
• Grouping the commands ( )
• Background processing
• Chaining the commands

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Command Execution

• Once a executable file has been found, the shell
  creates a separate process to run the program
  following three steps
  
• 1. A new (child) process is created that is a
  copy of the shell.
  
• 2. The child process is overlaid with the
  executable file, then the command name together
  with any arguments are passed to it.

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Command Execution (cont)

• 3. a. The interactive shell waits for the child
  process to terminate before returning to receive
  the next command, unless the command line
  includes a trailing ampersand ().
• b. If the command line ends with , the shell
  returns without waiting, and the commands is run
  in the background.

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Introduction of UNIX shell Programming Language

• Shell Script (Shell Procedure)
• A program written in shell programming language
  is known as a shell script or shell procedure.

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Introduction of UNIX shell Programming Language

• Shell Programming Language
• The shell programming language is a command
  language with a lot of features common to many
  computer programming languages, including the
  structured language constructs sequence,
  selection, and iteration.
• Command Languages
• Command languages are interpreted language.

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The First UNIX Shell Program - foo1

• foo1 display of users currently logged in
• A at the beginning of a line indicated that
  the line is a
  
• remark line and is for documentation.
• who wc -l

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Executing a Script

• Invoking Scripts with sh
• To use the sh command.
• sh won01
• Making Files Executable
• To make the script file an executable file.
• chmod ux won01
• won01

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sh Command

• sh Command
• The sh command invokes a new copy of the shell.
  You can run your script files using sh command.

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sh Command

• Options of sh Command
• -n Read commands, but does not execute them.
• -v Prints the input to the shell as the shell
  reads it.
  
• -x Prints command lines and their arguments as
  they are executed. This option is used mostly
  for debugging.
  

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Scripts in shells

• You can place any command or sequence of commands
  in a file, make the file executable, and then
  execute its contents simply typing its name at
  the shell prompt.
• Most UNIX systems include more than one shell.
  Use the man command to see how a particular
  command is used in your version of the shell.

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Scripts in shells

• You can change your current shell by typing sh,
  ksh, or csh to invoke any of the three shells,
  Bourne shell, Korn shell, or C shell. This does
  not change your login shell. Typing exit
  terminates the current shell and you are back to
  your login shell.

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Escape Characters

• \b
• \c
• \n
• \r
• \t
• \0n

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echo and Escape Characters

• echo and escape characters
• The echo command recognizes escape characters.
  Escape characters all start with the backslash (
  \ ), and you can use them as part of the echo
  command argument string.
• echo \07\07Warning
• echo \nHello\n

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The dot Command(.)

• The dot command ( . ) is a build-in shell command
  that lets you execute a program in the current
  shell and prevents the shell from creating the
  child process.
• Example
• . bye

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read Command (kb_read, read_test)

• The read command can be used to read the user
  input and save it in a user-defined variable.
  
• The features of read command
• - When read is executed, the shell waits until
  the user enters a line of text.
  
• - The variables are listed after the read on
  the command line.

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read Command (kb_read, read_test)

• The read command reads one line from the input
  device. The first input word is stored in the
  first variable, the second word in the second
  variable, and so on. If input string contains
  more words than the number of the variables, the
  left-over words are stored in the last variable.

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Comments - shell Programming

• symbol
• The shell recognizes as the comment
  symbol therefore, characters after the are
  ignored.
  

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Variables - shell Programming Basic (2)

• variablevalue
• Spaces are not permitted on either side of the
  equal sign.
  
• unset variable
• Embedded White Space
• If you value string contains embedded white
  space, then you must place it between quotation
  marks.
  
• variableThe Main Menu

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shell Positional Variable

• 0
• 1,2,...9
• _at_
• ?

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if-then and if-then-else

• The if-then Construct
• if condition
• then
• true-command(s)
• fi

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if-then and if-then-else

• The if-then-else Construct
• if condition
• then
• true-commands
• else
• false-command(s)
• fi

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if-then-elif

• if condition1
• then
• commands_1
• elif condition2
• then
• commands_2
• else
• commands_n
• fi

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test Command

• The test Command
• The test command is a built-in shell command
  that evaluates the expression given to it as an
  argument and return true if the expression is
  true, if otherwise, false.
• Invoking test with Brackets
• You can use square brackets ( ) instead of
  the word test.
  

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test Command

• Example
• if test variable value
• if variable value
• Two statements have the same function.

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Logical Operators

• Logical Expression
• test expression_1 logical operator
  expression_2
  

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Logical Operators

• Logical and Operator ( -a )
• Logical or Operator ( -o )
• Logical not operator ( ! )

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Numeric Test Operators

• test number1 numeric test operator number2
• -eq Is number 1 equal to number2 ?
• -ne Is number1 not equal to number2 ?
• -gt Is number 1 great than number2 ?
• -ge Is number1 great than or equal to number2 ?
• -lt Is number 1 less than number2 ?
• -le Is number1 less than or equal to number2 ?

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String Test Operators

• string1 string2 Does string1 match
  string2?
  
• ! string1 ! string2 Does string1 not match
  string2?
  
• -n -n string Does string
  contain characters (nonzero length)?
  
• -z -z string Is string an empty
  string
  
• (zero length?

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File Test Operators

• -r filename
• -w filename
• -s filename
• -f filename
• -d filename

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shell Variable Evaluation Operators

• variable
• variable
• variable-string
• variablestring
• variablestring
• (variable?string

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Arithmetic Operations

• Arithmetic Operator
• The Bourne shell does NOT include a simple
  built-in operator for arithmetic operations.

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The expr Command

• The expr Command
• - In the Bourne shell (sh), you can use the expr
  command to evaluate expressions.
  

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Arithmetic Operators of expr

• Arithmetic Operators
• Addition operator
• - Subtraction operator
• / Division operator
• Multiplication operator
• Remainder operator

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Arithmetic Operators of expr

• Example
• x10
• xexpr x 1
• echo x
• 11

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Relational Operators of expr

• Relational Operators of expr
• equal to
• ! not equal to
• greater than
• greater than or equal to

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For Loop

• The Basic Format of For Loop
• for variable
• in list-of-value
• do
• command(s)
• done

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The While Loop

• The Format of While Loop
• while condition
• do
• command(s)
• done

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The Until Loop

• The Format of the Until Loop
• until condition
• do
• command(s)
• done

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Debugging Shell Programs

• Debugging Shell Programs
• We can use the sh command with one ot its
  options to make the debugging of the script files
  easier.

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Debugging Shell Programs

• Options of sh Command
• -n Read commands, but does not execute them.
• -v Prints the input to the shell as the shell
  reads it.
  
• -x Prints command lines and their arguments as
  they are executed. This option is used mostly
  for debugging.

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THE END