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Chapter Six

Introduction to Shell Script Programming

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Lesson A

Using the UNIX Shell as aScripting Language

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ObjectivesUnderstand the program development cycle using a high-level computer language and UNIX shell scripts

Compare the shells to determine the best choice for creating scripts

Learn about shell variables, operators, and wildcard characters

Write simple shell scripts to illustrate programming logic

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The Program Development Cycle

The program development cycle is the process of developing an application– The first step in the cycle is to create program

specifications– The second step in the cycle is to create the

program design– The third step is developing the code, which is

written, tested, and debugged

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Using High-Level Languages

High-level languages are computer languages that use English-like expressions

Example are; COBOL, C, C++

A program’s high-level language statements are stored in a file called the source file, which programmers creates using editors

In order to execute, high-level source files must be converted into a low-level machine language file

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Using High-Level Languages

A compiler is a program that converts source files into executable machine-language files

The complier reads the lines of code the programmer wrote in the source file and converts them to the appropriate machine language instructions

If a source file contains syntax errors, it cannot be converted into an executable file– A programmer must correct these errors before the

program can be run

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Using UNIX Shell ScriptsUnlike high-level language programs, shell scripts do not have to be converted into machine language by a compiler

The UNIX shell acts as an interpreter when reading script files

Interpreters read statements in script files and immediately translate them into executable instructions and cause them to run

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Using UNIX Shell ScriptsAfter creating shell script, the OS is instructed that the file is an executable shell script via the chmod command

When the file is designated as executable, you may it run in one of many ways:– Type the script name at the command prompt after

updating the path variable– If the script is in the current directory, proceed its

name at the prompt with a dot slash (./)– If not in the current directory, specify the absolute

path at the command prompt

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The Programming Shell

All Linux versions use the Bash shell as the default

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VariablesVariables are symbolic names that represent values stored in memory

Three types of variables are:– Configuration variables store information about the

setup of the OS– Environment variables hold information about your

login session– Shell variables are created at the command prompt or

in shell scripts and are used to temporarily store information

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Variables

Use the printenv command to see a list of environment variables

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VariablesTo set:

example=oneTo see:

echo $exampleTo make part of the environment:

export exampleTo remove:

unsetenv example

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

variable to a value and to check variable valuesThe 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 (>) is an example

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Shell Operators

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More About Wildcard Characters

Shell scripts often use wildcard characters

Wildcard characters are intended to match filenames and words– Question mark (?) matches exactly one

character– Asterisk (*) matches zero or more characters – [chars] defines a class of characters, the glob

pattern matches any singles character in the class

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Shell Logic Structures

Four basic logic structures needed for program development are:– Sequential logic– User input– Decision logic– Looping logic– Case logic

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Sequential Logic

commands are executed in the order in which they appear in the script

break in sequence occurs when a branch instruction changes the flow of execution by redirecting to another location in the script

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User input

Script can read user data

Command: read variable

reads user input and assigns text to variable

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User inputCommand: read var1 var2 var3

reads 3 words and assigns 3 variablesLast variable contains rest of input line

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User input

Command: read –p “enter name: “ name

Prompts user, then reads input and assigns to variable

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User input example

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Decision LogicEnables your script to execute statement(s) only if a certain condition is true

Condition:– result of a command – Comparison of variables or values

if statement

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If statementSyntax:

if [ condition ]then

statementselse

statementsfi

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Decision Logic example

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Nested Decision Logic

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Looping Logic

A control structure repeats until some condition exists or some action occurs

Two common looping mechanisms:– For loops cycle through a range of values until

the last in a set of values is reached– The while loop cycles as long as a particular

condition exists

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For Loop

Syntax

for var in listdo

statementsdone

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For Loop exampleProgram control structures can be entered from the command line

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For loop in script

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Loop with wildcard

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While Loop

Syntax

while [ condition ]do

statementsdone

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Looping LogicThe while loop tests repeatedly for a matching condition

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Looping Logic

While loops can serve as data-entry forms

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While loop to enter data

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Case LogicThe case logic structure simplifies the selection from a list of choices

It allows the script to perform one of many actions, depending on the value of a variable

Two semicolons (;;) terminate the actions taken after the case matches what is being tested

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Case statementSyntax:

case $variable in“pattern1”)

statements;;

“pattern2”)statements;;

esac

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Case example

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Case Logic

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Debugging a Shell ScriptShell script will not execute if there is an error in one or more commands

sh has options for debugging– sh -v

displays lines of script as they are read by the interpreter

– sh -x displays the command and its arguments line by line as they are run

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Debugging a Shell Script

View the script line by line as it is running to help locate errors

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Lesson B

Creating and Completing theCorporate Phone Application

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ObjectivesCreate screen-management scripts

Use the trap command

Enter and test shell scripts to print the phone records, view the contents of the corp_phone file, and add new phone records to the file

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Using Shell Scripting toCreate a Menu

A menu is a good example of a shell script that employs the four basic logic structures

A significant feature of the menu script is the screen presentation which should be as appealing and user-friendly as possible

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tput commandtput clear– clear the screen

tput cup r c– position cursor to row and column– ex: tput cup 0 0

tput cup 20 10bold=`tput smso`offbold=`tput rmso`

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Example

tput clear; tput cup 10 15;echo “Hello”; tput cup 20 0

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Creating a Menu

tput can be used to help create data entry screens

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Creating a Menu

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Creating a Menu

tput can be used to help create user menus

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Creating a Menu

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The trap Command

used to guard against abnormal termination of script– user ^C– OS intervention

normal: remove temporary fileexample:trap ’rm ~/tmp/*’ 2 15

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Creating the corp_phones File

The grep command is useful when building script applications by extracting data from files

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Creating the corp_phones File

Using awk speeds development in that it can select fields from many records and display them in a specified format on the screen

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Creating the phoneadd Shell Script

The phoneadd script allows you to add new records to the corp_phones file

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Chapter Summary

A high-level language uses English-like expressions and must be converted into a low-level language before being executedThe shell interprets shell scriptsUNIX shell script instructions do not need to be written from scratch, they are chosen from an inventory of executable commands Linux shells are derived from the UNIX Bourne, Korn and C shells, and bash is the defaultUNIX employs three types of variables: configuration, environment, and shell

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Chapter SummaryThe shell supports numerous operators, including many for performing arithmetic operationsWildcard characters are used in shell scriptsThe logic structures supported by the shell are sequential, decision, looping and caseThe tput command can be used to manage cursor placement on the screenProgrammers and system administrators often customize the .bashrc file to suit their needs

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Chapter SummaryAliases, used to simplify commonly used commands, can be entered into the .bashrc Use the trap command to remove temporary files after the script exitsThe grep command serves a key role in the development of shell scripts by allowing searching and retrieving data from filesThe awk command serves as an effective and easy-to-use tool for generating reports

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