Chapter Seven

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Chapter Seven

• Advanced Shell Programming

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Lesson A

• Developing a
• Fully Featured Program

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Objectives

• Use flowcharting and pseudocode tools
• Learn to write scripts that tell the system which
  shell to use as an interpreter
• Use the test command to compare values and
  validate file existence
• Use the translate command, tr, to display a
  record with duplicate fields
• Use the sed command in a script to delete phone
  records

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Analyzing the Program

• A computer program is developed by analyzing the
  best way to achieve the desired results
  standard programming tools help with this process
• The two most popular and proven analysis tools
  are
• The program flowchart
• Pseudocode

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Flowcharting

• Each step in the program is represented by a
  symbol in the flowchart
• The shape of the symbol indicates the type of
  operation being performed
• Arrows connect the symbols and indicate the
  direction in which the program flows
• Input and output operations are represented by
  trapezoids and decision structures as diamonds
• Flowcharts are manually created using a drawing
  template

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Writing Pseudocode

• After flowcharting, the next step in designing a
  program is to write pseudocode
• Pseudocode instructions are similar to actual
  programming statements and are used to create a
  model that is later used as the basis for a real
  program
• Pseudocode is a design tool only, and never
  processed by the computer therefore there are no
  strict rules to follow

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Ensuring the Correct Shell Runs the Script

• Since each UNIX user has the freedom to choose
  which shell they will use, it is important to
  ensure that the correct shell is used to run the
  script
• This is because all shells do not support the
  same commands and programming statements
• The first line of the script is where you
  instruct a user to run it with a specific shell

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Using the test Command

• The test command makes preliminary checks of the
  UNIX internal environment and other useful
  comparisons
• Place the test command inside the shell script or
  execute it directly from the command line
• The test command can be used to
• Perform relational tests with integers
• Test strings
• Determine if a file exists and what type of file
  it is
• Perform Boolean tests

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Relational Integer Tests
The test command returns a value known as an exit
status, which is a numeric value and indicates
the results of the test performed true if 0
(zero) and false if 1 (one)
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String Tests
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Testing Files
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Performing Boolean Tests
AND returns true (0) if both expressions are
true, otherwise returns false (1) OR returns
true if either expression is true, otherwise if
neither is true, returns false ! negates the
value of the expression
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Using the test Command
The content of this file was created in part due
to using the test command to determine if a
directory existed
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Using the test Command
The output shown here was created in part due to
using the test command to determine a value
entered by a user
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Formatting Record Output

• Record output is formatted using the translate
  utility (tr)
• Use tr to
• Change the characters typed at the keyboard,
  character by character
• Work as a filter when the input comes from the
  output of another UNIX command
• Redirect standard input to come from a file
  rather than the keyboard

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Formatting Record Output
tr was used to change lowercase characters to
uppercase and replace colon characters with spaces
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Formatting Record Output
tr was used to help format the record output of
the program
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Deleting Phone Records

• Bash shell operators are in three groups
• Defining and Evaluating operators are used to set
  a variable to a value and to check variable
  values
• The equal sign () is an example
• Arithmetic operators are used to perform
  mathematical equations
• The plus sign () is an example
• Redirecting and piping operators are used to
  specify input and output data specifications
• The greater than sign (gt) is an example

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Deleting Phone Records
The menu has been updated to allow for deleting a
phone record
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Deleting Phone Records
Examine the corp_phones file before deleting a
record
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Deleting Phone Records
The sed command is behind the delete option
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Deleting Phone Records
The record is no longer in the file
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Lesson B

• Completing the Case Project

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Objectives

• Set up a quick screen-clearing technique
• Create a program algorithm to solve a cursor
  repositioning problem
• Develop and test a program to reenter fields
  during data entry
• Develop and test a program to eliminate duplicate
  records
• Create shell functions and use them in a program
• Load shell functions automatically when you log in

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Clearing the Screen

• The clear command is a useful housekeeping
  utility for clearing the screen before new
  screens appear, but there is a faster way
• You can clear the screen faster by storing the
  output of the clear command in a variable and
  then echoing the contents of the variable on the
  screen
• This works about ten times faster than the actual
  command since the system does not have to locate
  and execute the clear command

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Moving the Cursor

• You can allow a user to correct data entered into
  a previous data entry field. This will involve
  moving the cursor from one field to another
• One option would make the minus sign (-) the
  users means to move back a field
• If a user enters a minus and hits enter, the
  cursor is repositioned at the start of the
  previous field
• To accomplish this, the first step is to create a
  program algorithm

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Creating Program Algorithms

• An algorithm is a sequence of commands or
  instructions that produces a desired result
• A good practice for creating an algorithm would
  be to develop both the logic shown in a flowchart
  and the expressed conditions necessary to carry
  out the logic described in the pseudocode

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Creating Program Algorithms
Incorrect information has been entered by the user
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Creating Program Algorithms
The algorithm has encountered a minus sign and
moved the cursor to the previous field
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Protecting Against EnteringDuplicate Phone
Numbers

• Input validation is necessary because users dont
  always enter valid data
• Programs should always check to ensure that users
  enter acceptable information

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Protecting Against EnteringDuplicate Phone
Numbers
The phoneadd program now does input validation
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Using Shell Functions

• A shell function is a group of commands that is
  stored in memory and assigned a name
• Shell scripts can use function names to execute
  the commands
• Shell functions are used to isolate reusable code
  sections, so there is no need to duplicate the
  same algorithm throughout your program

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Reusing Code

• To improve programming productivity, learn to
  reuse code
• This means that functions and programs developed
  should be shared with other programs and
  functions as much as possible
• This practice helps to prevent duplications, save
  time, and reduce errors

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Sorting the Phone List
The code that generated this list includes a
shell script which contains sort functions
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Chapter Summary

• The two most popular and proven analysis tools
  are the program flowchart and pseudocode
• Pseudocode is a model of a program
• Use the first line in a script file to tell the
  OS which shell to use when interpreting the
  script
• Use the test command to validate the existence of
  directories and files as well as compare numeric
  and string values

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Chapter Summary

• The translate utility (tr) changes characters
  typed at the keyboard and also works as a filter
  when input comes from the output of another UNIX
  command
• The sed command reads a file as its input and
  outputs the files modified content
• To speed clearing the screen, assign the clear
  command sequence to the shell variable CLEAR that
  can be set inside the login script

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Chapter Summary

• An algorithm is a sequence of instructions or
  commands that produces a desired result
• Shell functions can simplify program code by
  isolating code that can be reused throughout one
  or many programs

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