Chapter Seven Advanced Shell Programming. 2 Lesson A Developing a Fully Featured Program.

Chapter SevenChapter Seven

Advanced Shell Advanced Shell ProgrammingProgramming

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Lesson ALesson A

Developing aDeveloping a

Fully Featured ProgramFully Featured Program

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ObjectivesObjectives

Use flowcharting and pseudocode toolsUse flowcharting and pseudocode tools

Learn to write scripts that tell the system which Learn to write scripts that tell the system which shell to use as an interpretershell to use as an interpreter

Use the test command to compare values and Use the test command to compare values and validate file existence validate file existence

Use the translate command, tr, to display a Use the translate command, tr, to display a record with duplicate fieldsrecord with duplicate fields

Use the sed command in a script to delete Use the sed command in a script to delete phone recordsphone records

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Analyzing the ProgramAnalyzing the Program

A computer program is developed by A computer program is developed by analyzing the best way to achieve the analyzing the best way to achieve the desired results – standard programming desired results – standard programming tools help with this processtools help with this process

The two most popular and proven analysis The two most popular and proven analysis tools are:tools are:– The program flowchartThe program flowchart– PseudocodePseudocode

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FlowchartingFlowcharting

Each step in the program is represented by a Each step in the program is represented by a symbol in the flowchartsymbol in the flowchart

The shape of the symbol indicates the type of The shape of the symbol indicates the type of operation being performedoperation being performed– Arrows connect the symbols and indicate the direction Arrows connect the symbols and indicate the direction

in which the program flowsin which the program flows– Input and output operations are represented by Input and output operations are represented by

trapezoids and decision structures as diamondstrapezoids and decision structures as diamonds

Flowcharts are manually created using a Flowcharts are manually created using a drawing templatedrawing template

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Writing PseudocodeWriting Pseudocode

After flowcharting, the next step in designing a After flowcharting, the next step in designing a program is to write pseudocodeprogram is to write pseudocode

Pseudocode instructions are similar to actual Pseudocode instructions are similar to actual programming statements and are used to create programming statements and are used to create a model that is later used as the basis for a real a model that is later used as the basis for a real programprogram

Pseudocode is a design tool only, and never Pseudocode is a design tool only, and never processed by the computer; therefore there are processed by the computer; therefore there are no strict rules to followno strict rules to follow

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Ensuring the Correct Shell Ensuring the Correct Shell Runs the ScriptRuns the Script

Since each UNIX user has the freedom to Since each UNIX user has the freedom to choose which shell they will use, it is important choose which shell they will use, it is important to ensure that the correct shell is used to run the to ensure that the correct shell is used to run the scriptscript

This is because all shells do not support the This is because all shells do not support the same commands and programming statementssame commands and programming statements

The first line of the script is where you instruct a The first line of the script is where you instruct a user to run it with a specific shelluser to run it with a specific shell

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Using the test CommandUsing the test Command

The test command makes preliminary checks of The test command makes preliminary checks of the UNIX internal environment and other useful the UNIX internal environment and other useful comparisonscomparisons

Place the test command inside the shell script or Place the test command inside the shell script or execute it directly from the command lineexecute it directly from the command line

The test command can be used to:The test command can be used to:– Perform relational tests with integersPerform relational tests with integers– Test stringsTest strings– Determine if a file exists and what type of file it isDetermine if a file exists and what type of file it is– Perform Boolean testsPerform Boolean tests

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Relational Integer TestsRelational 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 TestsString Tests

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Testing FilesTesting Files

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Performing Boolean TestsPerforming 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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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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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 OutputFormatting Record Output

Record output is formatted using the Record output is formatted using the translate utility (tr)translate utility (tr)

Use tr to:Use tr to:– Change the characters typed at the keyboard, Change the characters typed at the keyboard,

character by charactercharacter by character– Work as a filter when the input comes from Work as a filter when the input comes from

the output of another UNIX commandthe output of another UNIX command– Redirect standard input to come from a file Redirect standard input to come from a file

rather than the keyboardrather than the keyboard

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tr was used to change lowercase characters to uppercase and replace colon characters with spaces

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tr was used to help format the record output of the program

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Deleting Phone RecordsDeleting Phone Records

Bash shell operators are in three groups:Bash shell operators are in three groups:– Defining and Evaluating operators are used to set a Defining and Evaluating operators are used to set a

variable to a value and to check variable valuesvariable to a value and to check variable valuesThe equal sign (=) is an exampleThe equal sign (=) is an example

– Arithmetic operators are used to perform Arithmetic operators are used to perform mathematical equationsmathematical equations

The plus sign (+) is an exampleThe plus sign (+) is an example

– Redirecting and piping operators are used to specify Redirecting and piping operators are used to specify input and output data specificationsinput and output data specifications

The greater than sign (>) is an exampleThe greater than sign (>) is an example

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The menu has been updated to allow for deleting a phone record

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Examine the corp_phones file before deleting a record

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The sed command is behind the delete option

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The record is no longer in the file

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Lesson BLesson B

Completing the Case ProjectCompleting the Case Project

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ObjectivesObjectivesSet up a quick screen-clearing techniqueSet up a quick screen-clearing technique

Create a program algorithm to solve a cursor Create a program algorithm to solve a cursor repositioning problemrepositioning problem

Develop and test a program to reenter fields during Develop and test a program to reenter fields during data entrydata entry

Develop and test a program to eliminate duplicate Develop and test a program to eliminate duplicate recordsrecords

Create shell functions and use them in a programCreate shell functions and use them in a program

Load shell functions automatically when you log inLoad shell functions automatically when you log in

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Clearing the ScreenClearing the Screen

The clear command is a useful housekeeping The clear command is a useful housekeeping utility for clearing the screen before new screens utility for clearing the screen before new screens appear, but there is a faster wayappear, but there is a faster way

You can clear the screen faster by storing the You can clear the screen faster by storing the output of the clear command in a variable and output of the clear command in a variable and then echoing the contents of the variable on the then echoing the contents of the variable on the screenscreen– This works about ten times faster than the actual This works about ten times faster than the actual

command since the system does not have to locate command since the system does not have to locate and execute the clear commandand execute the clear command

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Moving the CursorMoving the Cursor

You can allow a user to correct data entered into You can allow a user to correct data entered into a previous data entry field. This will involve a previous data entry field. This will involve moving the cursor from one field to anothermoving the cursor from one field to another

One option would make the minus sign (-) the One option would make the minus sign (-) the user’s means to move back a fielduser’s means to move back a field– If a user enters a minus and hits enter, the cursor is If a user enters a minus and hits enter, the cursor is

repositioned at the start of the previous fieldrepositioned at the start of the previous field– To accomplish this, the first step is to create a To accomplish this, the first step is to create a

program algorithm program algorithm

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Creating Program Creating Program AlgorithmsAlgorithms

An algorithm is a sequence of commands or An algorithm is a sequence of commands or instructions that produces a desired resultinstructions that produces a desired result

A good practice for creating an algorithm would A good practice for creating an algorithm would be to develop both the logic shown in a flowchart be to develop both the logic shown in a flowchart and the expressed conditions necessary to carry and the expressed conditions necessary to carry out the logic described in the pseudocodeout the logic described in the pseudocode

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Incorrect information has been entered by the user

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The algorithm has encountered a minus sign and moved the cursor to the previous field

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Protecting Against EnteringProtecting Against EnteringDuplicate Phone NumbersDuplicate Phone Numbers

Input validation is necessary because Input validation is necessary because users don’t always enter valid datausers don’t always enter valid data

Programs should always check to ensure Programs should always check to ensure that users enter acceptable informationthat users enter acceptable information

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Protecting Against EnteringProtecting Against EnteringDuplicate Phone NumbersDuplicate Phone Numbers

The phoneadd program now does input validation

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Using Shell FunctionsUsing Shell Functions

A shell function is a group of commands that is A shell function is a group of commands that is stored in memory and assigned a namestored in memory and assigned a name

Shell scripts can use function names to execute Shell scripts can use function names to execute the commandsthe commands

Shell functions are used to isolate reusable code Shell functions are used to isolate reusable code sections, so there is no need to duplicate the sections, so there is no need to duplicate the same algorithm throughout your programsame algorithm throughout your program

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Reusing CodeReusing Code

To improve programming productivity, learn to To improve programming productivity, learn to reuse codereuse code

This means that functions and programs This means that functions and programs developed should be shared with other developed should be shared with other programs and functions as much as possibleprograms and functions as much as possible

This practice helps to prevent duplications, save This practice helps to prevent duplications, save time, and reduce errorstime, and reduce errors

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Sorting the Phone ListSorting the Phone List

The code that generated this list includes a shell script which contains sort functions

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Chapter SummaryChapter Summary

The two most popular and proven analysis The two most popular and proven analysis tools are the program flowchart and tools are the program flowchart and pseudocodepseudocode

Pseudocode is a model of a programPseudocode is a model of a program

Use the first line in a script file to tell the OS Use the first line in a script file to tell the OS which shell to use when interpreting the scriptwhich shell to use when interpreting the script

Use the test command to validate the Use the test command to validate the existence of directories and files as well as existence of directories and files as well as compare numeric and string values compare numeric and string values

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The translate utility (tr) changes characters The translate utility (tr) changes characters typed at the keyboard and also works as a filter typed at the keyboard and also works as a filter when input comes from the output of another when input comes from the output of another UNIX commandUNIX command

The sed command reads a file as its input and The sed command reads a file as its input and outputs the file’s modified contentoutputs the file’s modified content

To speed clearing the screen, assign the clear To speed clearing the screen, assign the clear command sequence to the shell variable command sequence to the shell variable CLEAR that can be set inside the login scriptCLEAR that can be set inside the login script

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An algorithm is a sequence of An algorithm is a sequence of instructions or commands that produces instructions or commands that produces a desired resulta desired result

Shell functions can simplify program Shell functions can simplify program code by isolating code that can be code by isolating code that can be reused throughout one or many reused throughout one or many programsprograms

Chapter Seven Advanced Shell Programming. 2 Lesson A Developing a Fully Featured Program.

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