Chapter 7: Advanced Shell Programming Guide To UNIX Using Linux Third Edition.

Chapter 7:Advanced Shell Programming

Guide To UNIX Using Linux Third Edition

Guide to UNIX Using Linux, Third Edition 2

Objectives

• Perform program design and analysis using flowcharts and pseudocode

• Use techniques to ensure a script is employing the correct shell

• Set the default shell

• Configure Bash login and logout scripts


Objectives (continued)

• Set defaults for the vi editor

• Use the test command for programming functions

• Format record output

• Delete records using a script


Objectives (continued)

• Set up a quick screen-clearing technique

• Create a program algorithm to solve a cursor-repositioning problem

• Develop and test a program to eliminate duplicate records

• Create shell functions and use them in a program


Understanding Program Design and Analysis

• Program development cycle

– Step 1: Program specification

– Step 2: Design

• Two popular and proven analysis tools:

– Program flowchart

– Pseudocode


Flowcharting

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

• The shape of the symbol indicates the type of operation– Arrows connect the symbols and indicate the direction

in which the program flows– Input and output: represented by trapezoids– Decision structures: represented by diamonds


Flowcharting (continued)


Flowcharting (continued)


Writing Pseudocode

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

• Similar to programming statements • A design tool only and never processed by the computer;

no syntax issues


Writing Pseudocode (continued)


Ensuring the Correct Shell Runs the Script

• Each UNIX/Linux user can choose which shell they use by default

– Ensure the correct shell is used to run a script

– Not all shells support the same commands and programming statements

• The first line of a script should specify which shell to use


Setting the Default Shell

• System administrator establishes the default shell for a user account

– In /etc/passwd file

• File can only be edited (carefully!) by system administrator

• Some systems provide management software to assist in setting default shells


Using Bash Login and Logout Scripts

• When Bash is your default shell, scripts run automatically upon login or re-entry

– .bash_profile

– .bashrc (also runs in a subshell)

• Administrator controls /etc/bashrc and /etc/profile

• .bash_logout runs when user logs out

– Often used to clear the screen


Using Bash Login and Logout Scripts (continued)


Setting Defaults for Using the vi Editor

• To use the vi editor for code development, configure .exrc in your home directory

• Automatically sets up the vi environment

– Set the number of tab spaces to use when nesting lines of code

– Display line numbers


Using the test Command

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

• Some uses of the test command are:

– Perform relational tests with integers

– Test strings

– Determine if a file or directory exists and what type of file it is

– Perform Boolean tests


Performing Relational Integer Tests with the test Command

The test command returns an exit status that indicates the result of the test: 0 (zero) if true and 1 (one) if false


Performing Relational Integer Tests with the test Command (continued)


Performing String Tests with the test Command


Testing Files with the test Command


Performing Boolean Tests with the test Command

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


Using the test Command


Formatting Record Output

• Record output is formatted using the translateutility (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/Linux command

– Redirect standard input to come from a file rather than the keyboard


Formatting Record Output

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


Deleting Phone Records

• The sed command

– Takes the contents of an input file and applies specific actions to the file’s contents

– Sends results to standard output

• The -d option of sed can be used to delete matching records from the output



The sed command is behind the delete option



The record is no longer in the file


Clearing the Screen

• The clear command is useful for clearing the screen, but there is a faster way

– Store the output of the clear command in a variable and then echo the contents of the variable on the screen

– About ten times faster than the actual command since the system does not have to locate and execute the clear command


Clearing the Screen (continued)


Creating an Algorithm to Place the Cursor

• To correct data entered into a previous data entry field, move the cursor back to the erroneous 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


Creating an Algorithm to Place the Cursor (continued)

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

• A good practice for creating an algorithm is to develop both the logic shown in a flowchart and the conditions necessary to carry out the logic described in the pseudocode


Creating Program Algorithms

Incorrect information has been entered by the user


Creating Program Algorithms (continued)

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


Protecting Against EnteringDuplicate Data

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

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


Protecting Against EnteringDuplicate Data (continued)


Protecting Against EnteringDuplicate Data (continued)

The phoneadd program now does input validation


Using Shell Functions

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

• Shell scripts can use function names to execute the commands

• Shell functions isolate reusable code sections; no need to duplicate the same algorithm throughout your program


Defining a Function from the Command Line

• To enter a function from the command line– Enter the name of the function– Enter internal code in brackets

• Shell will prompt with a > for next line until closing bracket } is entered

• Arguments are positional ($1 - $9)


Creating Functions Inside Shell Scripts

• Creating a function inside a shell script supports code reuse

• Functions can be automatically loaded or defined upon login using .bashrc or .bash_profile scripts


Chapter Summary

• Two popular analysis tools are the program flowchart and pseudocode

• Use the first line in a script to tell the OS which shell to use with the script

• test command validates the existence of directories and files and compares numeric and string values

• sed reads a file as its input and outputs the file’s modified content


Chapter Summary (continued)

• translate (tr) changes characters typed at the keyboard and filters input from the output of another UNIX/Linux command

• To speed clearing the screen, assign the output of the clear command to the shell variable CLEAR

• Algorithm: a sequence of instructions or commands that produces a desired result

• Shell functions can isolate program code to be reused in multiple programs

Chapter 7: Advanced Shell Programming Guide To UNIX Using Linux Third Edition.

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