Introduction to Unix (CA263)

Introduction to Shell Script Programming

By

Tariq Ibn Aziz

Objectives

• Understand the program development cycle

• Compare UNIX/Linux shells for creating scripts

• Use shell variables, operators, and wildcard characters

Objectives (continued)

• Use shell logic structures

• Employ shell scripting to create a menu

• Use commands to help debug shell scripts

• Explain ways to customize your personal environment

• Use the trap command

• Develop a menu-based application

Previewing the Application

• Shell scripts can be used to customize your environment

• Will develop a simulated employee information system using shell scripts

• Will gain experience with shell variables, shell script operators, and logic structures

The Program Development Cycle

• The program development cycle is the process of developing an application

– First step is to create program specifications

– Second step is to create the program design

– Third step is developing the code, which is written, tested, and debugged

The Program Development Cycle

Using High-Level Languages

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

• Examples are; COBOL, C, C++• High-level language statements are stored in a source

file, which programmers create using an editor

Using High-Level Languages (continued)

• High-level source files must be converted into a low-level machine language file

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

• 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

Using UNIX/Linux Shell Scripts

• Unlike high-level language programs, shell scripts do not have to be converted into machine language

• The UNIX/Linux shell acts as an interpreter when reading script files

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

Using UNIX/Linux Shell Scripts(continued)

• After creating shell script, the OS is instructed that the file is an executable shell script via the chmod command

• Script files can be run in several ways:– Set the path variable and type the script name at the

command prompt– Type ./filename if script is in current directory– Type the script name preceded by the full path

Prototyping an Application

• A prototype is a running model of your application– Less detail, less design time than a full application– Shows potential without full programming effort

• Shell scripts can be used to prototype applications that will later be moved to compiled languages

Using Comments

• Comments are important!• Provide useful documentation to both the programmer

and to others who need to understand or debug the code• To use, start comment line with a #

The Programming Shell

All Linux versions use the Bash shell as the default

Variables

• Variables are symbolic names that represent values stored in memory

• Three types of variables:– 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

Environment and Configuration Variables

• Environment and configuration variables can be used to set up and personalize your sessions

• The printenv command shows current environment and configuration variables

Environment and Configuration Variables (continued)

Use the printenv variable to see a list of environment variables

Environment and Configuration Variables (continued)

Environment and Configuration Variables (continued)

Environment and Configuration Variables (continued)

Environment and Configuration Variables (continued)

Shell Variables

• Variables that you can define and manipulate for use with program commands in a shell

• Observe basic guidelines for handling and naming shell variables

Shell Operators

• Bash shell operators are in four groups:– Defining – Evaluating– Arithmetic – Redirection

Defining Operators

• Used to assign a value to a variable• Most common is = (equal sign)• Use quotation marks with strings• Backquote says execute the command inside the

backquotes and store the result in the variable

Evaluating Operators

• Used for determining the contents of a variable• echo $variablename will show the value of variablename• Double quotes can be used, but not single quotes

Arithmetic Operators

Arithmetic Operators (continued)

• Regular mathematical precedence rules apply to arithmetic operators

To store arithmetic values in a variable, use let statement

Redirection Operators

• The > redirection operator overwrites an existing file• -o noclobber option of set command will prevent

overwriting

Exporting Shell Variables to the Environment

• Shell scripts cannot automatically access variables created and assigned– On the command line– By other scripts

• Make variables global in your environment by using the export command

Modifying the PATH Variable

• PATH variable controls where your shell will look for shell scripts

• You can add directories to your PATH– Special directories for scripts– Your current working directory

More About Wildcard Characters

• Shell scripts often use wildcard characters• Wildcard characters are called glob characters and

are a part of glob patterns• Glob patterns are intended to match filenames and

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

pattern matches any character in the class

Shell Logic Structures

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

Sequential Logic

• Commands are executed in the order in which they appear in the script or program

• The only break in this sequence comes when a branch instruction changes the flow of execution by redirecting to another location in the script or program

• Used for simple, straightforward command sequences

Decision Logic

• Enables your script or program to execute a statement or series of statements only if a certain condition exists

• In many cases, the condition depends upon the result of a command or on a comparison

• The if statement is the primary decision-making control structure in this type of logic

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

Looping Logic (continued)

The for loop repeats for however many values there are in the specified set of values

Looping Logic (continued)

• Program control structures can be entered from the command line

• Wildcard characters can be used in loops• The while loop is set up to test repeatedly for a matching

condition• The while loop is used when code must be repeatedly

executed an undetermined number of times

Case Logic

• The 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

Using Shell Scripting to Create a Menu

• Often useful to create a menu that branches to specific shell scripts

• The tput command is useful when creating menus

• Can initialize the terminal display to place text and prompts in specific locations and respond to the user

Debugging a Shell Script

• A shell script will not execute if there is an error in one or more commands

• Running a shell script using sh enables quick debugging of problems

– sh -v option displays lines of code in the script as they are read by the interpreter

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

Customizing YourPersonal Environment

• When programming and shell scripting, customizing your environment by modifying the initial settings in the login scripts provides many benefits

• Login scripts run just after logging in

• Setting up personal bin directories and modify editor defaults are common customizations

Customizing Your Personal Environment (continued)

• An alias is a name that represents another command

• The .bashrc file in your home directory is used to establish customizations that take effect at each login

• The .bashrc script is executed each time a shell is generated, such as when shell scripts are run

The trap Command

• The trap command causes a shell program to automatically remove temporary files created when shell scripts run

• Programmers often set up a subdirectory to store temporary files, and when a script file exits, trap removes the files

• Having files removed from a temporary directory like this is considered “good housekeeping”

Putting It All Together in an Application

• Applications require you to:– Assign and manage variables– Use shell operators– Employ shell logic structures– Use additional wildcard characters– Use tput for managing screen initialization– Use trap to clean up temporary files

• Will use these skills to build a shell script application in Hands-on Project

Chapter Summary

• A high-level language uses English-like expressions and must be converted into a low-level language before being executed

• The shell interprets shell scripts

• Linux shells are derived from the UNIX Bourne, Korn and C shells, and bash is the default

• UNIX/Linux uses three types of variables: configuration, environment, and shell

• Shell operators include defining, evaluating, arithmetic, and redirection

• Wildcard characters are used in shell scripts

• The logic structures supported are: sequential, decision, looping and case

Chapter Summary (continued)

• The tput command manages cursor placement on the screen

• Programmers and system administrators often customize the .bashrc file

• Aliases simplify common commands can be entered into the .bashrc

• Use the trap command to remove temporary files after the script exits

Chapter Summary (continued)