1. Advanced Shell Programming

2. 2 Objectives Use techniques to ensure a script is employing the correct shell Set the default shell Configure Bash login and logout scripts

3. 3 Objectives (continued) Set defaults for the vi editor Use the test command for programming functions Format record output Delete records using a script

4. 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

5. 5 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

6. 6 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

7. 7 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

8. 8 Using Bash Login and Logout Scripts (continued)

9. 9 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

10. 10 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

11. 11 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

12. 12 Performing Relational Integer Tests with the test Command (continued)

13. 13 Performing String Tests with the test Command

14. 14 Testing Files with the test Command

15. 15 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

16. 16 Using the test Command

17. 17 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

18. 18 Formatting Record Output tr was used to change lowercase characters to uppercase and replace colon characters with spaces

19. 19 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

20. 20 Deleting Phone Records The sed command is behind the delete option

21. 21 Deleting Phone Records The record is no longer in the file

22. 22 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

23. 23 Clearing the Screen (continued)

24. 24 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

25. 25 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

26. 26 Creating Program Algorithms Incorrect information has been entered by the user

27. 27 Creating Program Algorithms (continued) The algorithm has encountered a minus sign and moved the cursor to the previous field

28. 28 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

29. 29 Protecting Against Entering Duplicate Data (continued)

30. 30 Protecting Against Entering Duplicate Data (continued) The phoneadd program now does input validation

31. 31 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

32. 32 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)

33. 33 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

34. 34 Summary 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

35. 35 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