1. Introduction to Shell Script Programming
Chapter 6
Introduction to Shell Script Programming

2. Guide to UNIX Using Linux, Third Edition
Objectives
Using shell variables, operators, and wildcard characters
Using shell logic structures
Employ shell scripting to create a menu
Use commands to help debug shell scripts
Customizing your personal environment
Using the trap command
Develop a menu-based application
Guide to UNIX Using Linux, Third Edition

3. The Program Development Cycle
The program development cycle is the process of developing an application
1st step is to create program specifications
2nd step is to create the program design
3rd step is developing the code, which is written, tested, and debugged
Guide to UNIX Using Linux, Third Edition

4. The Program Development Cycle
Guide to UNIX Using Linux, Third Edition

5. 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
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6. Guide to UNIX Using Linux, Third Edition
Using High-Level Languages
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
Guide to UNIX Using Linux, Third Edition

7. 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
Guide to UNIX Using Linux, Third Edition

8. Using UNIX/Linux Shell Scripts
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
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9. 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
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10. Guide to UNIX Using Linux, Third Edition
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 #
Guide to UNIX Using Linux, Third Edition

11. Guide to UNIX Using Linux, Third Edition
The Programming Shell
All Linux versions use the Bash shell as the default
Guide to UNIX Using Linux, Third Edition

12. Guide to UNIX Using Linux, Third Edition
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
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13. Guide to UNIX Using Linux, Third Edition
Variables
Use the printenv variable to see a list of environment variables.
You can also use env
Look at the man pages. What is different between the two commands?
Turn to page 195 (2nd Ed.) or page 269 (3rd Ed.)
Guide to UNIX Using Linux, Third Edition

14. Environment and Configuration Variables (continued)
Use the printenv variable to see a list of environment variables
Guide to UNIX Using Linux, Third Edition

15. Environment and Configuration Variables
Guide to UNIX Using Linux, Third Edition

16. Guide to UNIX Using Linux, Third Edition
Environment and Configuration Variables
Guide to UNIX Using Linux, Third Edition

17. Environment and Configuration Variables
Guide to UNIX Using Linux, Third Edition

18. Environment and Configuration Variables
Guide to UNIX Using Linux, Third Edition

19. Guide to UNIX Using Linux, Third Edition
Shell Variables
Shell Variables are variables that you can define and manipulate for use with program commands in a shell
Observe basic guidelines for handling and naming shell variables
I recommend that you use all UPPERCASE characters when naming your variables
Guide to UNIX Using Linux, Third Edition

20. Guide to UNIX Using Linux, Third Edition
Shell Variables
Variables are handled differently depending on the syntax
Type:
echo $USERNAME
echo “$USERNAME”
echo ’$USERNAME’
echo `$USERNAME`
Guide to UNIX Using Linux, Third Edition

21. Guide to UNIX Using Linux, Third Edition
Shell Operators
Bash shell operators are in four groups:
Defining operators
Evaluating operators
Arithmetic operators
Redirection operators
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22. Guide to UNIX Using Linux, Third Edition
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
Guide to UNIX Using Linux, Third Edition

23. Guide to UNIX Using Linux, Third Edition
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
Guide to UNIX Using Linux, Third Edition

24. Guide to UNIX Using Linux, Third Edition
Arithmetic Operators
Guide to UNIX Using Linux, Third Edition

25. Arithmetic Operators (continued)
Regular mathematical precedence rules apply to arithmetic operators
To store arithmetic values in a variable, use let statement
let x=6+4*2
echo $x
Guide to UNIX Using Linux, Third Edition

26. Redirection Operators
The > redirection operator overwrites an existing file
-o noclobber option of set command will prevent overwriting
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27. 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
Guide to UNIX Using Linux, Third Edition

28. 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
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29. 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
Guide to UNIX Using Linux, Third Edition

30. Shell Logic Structures
Four basic logic structures needed for program development are:
Sequential logic
Decision logic
Looping logic
Case logic
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31. Guide to UNIX Using Linux, Third Edition
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
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32. Guide to UNIX Using Linux, Third Edition
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
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33. Guide to UNIX Using Linux, Third Edition
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
Guide to UNIX Using Linux, Third Edition

34. Looping Logic (continued)
The for loop repeats for however many values there are in the specified set of values
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35. 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
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36. Guide to UNIX Using Linux, Third Edition
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
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37. 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
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38. 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
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39. Customizing Your Personal 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
Guide to UNIX Using Linux, Third Edition

40. Customizing Your Personal Environment
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
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41. Guide to UNIX Using Linux, Third Edition
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”
Guide to UNIX Using Linux, Third Edition

42. 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
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43. Guide to UNIX Using Linux, Third Edition
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
Guide to UNIX Using Linux, Third Edition

44. Chapter Summary (continued)
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
Guide to UNIX Using Linux, Third Edition

45. 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
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