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

3. Objectives (continued)
Set defaults for the vi editor
Use the test command for programming functions
Format record output
Delete records using a script
Guide to UNIX Using Linux, Third Edition

4. 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
Guide to UNIX Using Linux, Third Edition

5. Understanding Program Design and Analysis
Program development cycle
Step 1: Program specification
Step 2: Design
Two popular and proven analysis tools:
Program flowchart
Pseudocode
Guide to UNIX Using Linux, Third Edition

6. 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
Guide to UNIX Using Linux, Third Edition

7. Flowcharting (continued)
Guide to UNIX Using Linux, Third Edition

8. Flowcharting (continued)
Guide to UNIX Using Linux, Third Edition

9. 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
Guide to UNIX Using Linux, Third Edition

10. Writing Pseudocode (continued)
Guide to UNIX Using Linux, Third Edition

11. 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
Guide to UNIX Using Linux, Third Edition

12. 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
Guide to UNIX Using Linux, Third Edition

13. 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
Guide to UNIX Using Linux, Third Edition

14. Using Bash Login and Logout Scripts (continued)
Guide to UNIX Using Linux, Third Edition

15. 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
Guide to UNIX Using Linux, Third Edition

16. 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
Guide to UNIX Using Linux, Third Edition

17. 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
Guide to UNIX Using Linux, Third Edition

18. Performing Relational Integer Tests with the test Command (continued)
Guide to UNIX Using Linux, Third Edition

19. Performing String Tests with the test Command
Guide to UNIX Using Linux, Third Edition

20. Testing Files with the test Command
Guide to UNIX Using Linux, Third Edition

21. 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
Guide to UNIX Using Linux, Third Edition

22. Using the test Command
Guide to UNIX Using Linux, Third Edition

23. Formatting Record Output
Record output is formatted using the translate utility (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
Guide to UNIX Using Linux, Third Edition

24. Formatting Record Output
tr was used to change lowercase characters to uppercase and replace colon characters with spaces
Guide to UNIX Using Linux, Third Edition

25. 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
Guide to UNIX Using Linux, Third Edition

26. Deleting Phone Records
The sed command is behind the delete option
Guide to UNIX Using Linux, Third Edition

27. Deleting Phone Records
The record is no longer in the file
Guide to UNIX Using Linux, Third Edition

28. 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
Guide to UNIX Using Linux, Third Edition

29. Clearing the Screen (continued)
Guide to UNIX Using Linux, Third Edition

30. 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
Guide to UNIX Using Linux, Third Edition

31. 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
Guide to UNIX Using Linux, Third Edition

32. Creating Program Algorithms
Incorrect information has been entered by the user
Guide to UNIX Using Linux, Third Edition

33. Creating Program Algorithms (continued)
The algorithm has encountered a minus sign and moved the cursor to the previous field
Guide to UNIX Using Linux, Third Edition

34. Protecting Against Entering Duplicate Data
Input validation is necessary because users don’t always enter valid data
Programs should always check to ensure that users enter acceptable information
Guide to UNIX Using Linux, Third Edition

35. Protecting Against Entering Duplicate Data (continued)
Guide to UNIX Using Linux, Third Edition

36. Protecting Against Entering Duplicate Data (continued)
The phoneadd program now does input validation
Guide to UNIX Using Linux, Third Edition

37. 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
Guide to UNIX Using Linux, Third Edition

38. 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)
Guide to UNIX Using Linux, Third Edition

39. 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
Guide to UNIX Using Linux, Third Edition

40. 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
Guide to UNIX Using Linux, Third Edition

41. 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
Guide to UNIX Using Linux, Third Edition