1. A Guide to Unix Using Linux Fourth Edition
Chapter 7
Advanced Shell Programming

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
Set defaults for the vi editor
Use the test command for programming functions
A Guide to Unix Using Linux, Fourth Edition

3. Objectives (continued)
Format record output
Delete records using a script
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
A Guide to Unix Using Linux, Fourth Edition

4. Understanding Program Design and Analysis
Design process:
Second step in program development cycle
After creating the specifications for program
Develop computer program by analyzing best way to achieve desired results
Two popular and proven analysis tools:
Flowchart
Pseudocode
A Guide to Unix Using Linux, Fourth Edition

5. Flowcharting Flowchart Logic diagram Uses set of standard symbols
Visually explains sequence of events from start of process to its end point
Documents:
Processes
Procedures
Program sequence
A Guide to Unix Using Linux, Fourth Edition

6. Flowcharting (continued)
A Guide to Unix Using Linux, Fourth Edition

7. Writing Pseudocode After creating a flowchart, you write pseudocode
Creates a model that you can later use as a basis for a real program
Display "What is your favorite vegetable? " on the screen
Enter data into veg_name
If veg_name is equal to "broccoli"
Then
Display "Broccoli is a healthy choice." on the screen
Else
Display "Don’t forget to eat your broccoli also." on the
screen
End If
A Guide to Unix Using Linux, Fourth Edition

8. A Guide to Unix Using Linux, Fourth Edition

9. Ensuring the Correct Shell Runs the Script
UNIX/Linux users can choose their shell of preference
To ensure that the correct shell is used to interpret your script:
Include command that sets the particular shell to use on the first line of the script
#!/bin/bash
A Guide to Unix Using Linux, Fourth Edition

10. Setting the Default Shell
Shell set up by default is established by system administrator
/etc/passwd file
trbrown:x:500:500:Thomas Brown:/home/trbrown:/bin/bash
To edit /etc/passwd file:
Use vi or Emacs
Must be very careful!
Make a backup copy first
Use a GUI
e.g., User Manager tool in GNOME desktop, YaST
A Guide to Unix Using Linux, Fourth Edition

11. Setting the Default Shell (continued)
A Guide to Unix Using Linux, Fourth Edition

12. Using Bash Login and Logout Scripts
In Bash, two scripts run when you log in:
.bash_profile and .bashrc
Edit files with a text editor
Unlike .bash_profile, .bashrc is also run each time you start a subshell
/etc/.bashrc, /etc/bashrc, or /etc/bash.bashrc files set system defaults
Not always available
.bash_logout file (in your home directory) executes commands when user logs out
A Guide to Unix Using Linux, Fourth Edition

13. Using Bash Login and Logout Scripts (continued)
A Guide to Unix Using Linux, Fourth Edition

14. Setting Defaults for Using the vi Editor
.exrc used to automatically set up vi environment
Located in your home directory
Example:
set number
set tabstop=3
set shell=/bin/bash
A Guide to Unix Using Linux, Fourth Edition

15. Using the test Command
A Guide to Unix Using Linux, Fourth Edition

16. Performing Relational Integer Tests with the test Command
Exit status: numeric value that command returns to OS when it finishes
Interpreting exit status for test command:
0 (zero)  test result is true
1  test result is false
Use echo $? to view most recent exit status
A Guide to Unix Using Linux, Fourth Edition

17. Performing Relational Integer Tests with the test Command (continued)
A Guide to Unix Using Linux, Fourth Edition

18. Performing String Tests with the test Command
These tests are useful in scripts to test contents of variables
Example: ensure that a variable contains a specific value
A Guide to Unix Using Linux, Fourth Edition

19. Testing Files with the test Command
A Guide to Unix Using Linux, Fourth Edition

20. Performing Boolean Tests with the test Command
Boolean operator: logical operator that symbolizes AND, OR, or NOT to evaluate a relationship
Result of evaluation is either true or false
Examples:
test expression1 -a expression2
test expression1 -o expression2
test !expression
A Guide to Unix Using Linux, Fourth Edition

21. Formatting Record Output
To format record output, use translate utility, tr
Two examples:
1) tr [a-z] [A-Z] < counters
Sends contents of counters file as input to tr
Then, converts lowercase characters to uppercase
2) cat names | tr ":" " "
Sends output of cat to tr
Pipes (|) contents of names file to tr
tr replaces each occurrence of “:” with a space
A Guide to Unix Using Linux, Fourth Edition

22. Deleting Phone Records
sed takes contents of input file and applies actions to file’s contents
Actions provided as options and arguments
Results are sent to standard output device
Simple way to delete a phone record:
Use -d option
Enter phone number
Use sed -d to delete the matching phone number and output to
a temporary file, f
Confirm acceptance
If the output is accepted, copy the temporary file f back to
corp_phones (overlaying it)
A Guide to Unix Using Linux, Fourth Edition

23. Clearing the Screen
A Guide to Unix Using Linux, Fourth Edition

24. Creating an Algorithm to Place the Cursor
Read information into field2
While field2 equals "-"
Move cursor to position of previous field, field1
Clear current information displayed in field1
Read new information into field1
If field1 = "q"
Then
Exit program
End If
Move cursor to position of field2
End While
tput cup 5 18; read lname
while test "$lname" = "-"
do tput cup 4 18; echo " "
tput cup 4 18; read phonenum
done
A Guide to Unix Using Linux, Fourth Edition

25. Protecting Against Entering Duplicate Data
A program should always check its input to ensure the user has entered acceptable information
Input validation
A Guide to Unix Using Linux, Fourth Edition

26. A Guide to Unix Using Linux, Fourth Edition

27. Using Shell Functions
Shell function: group of commands stored in memory and assigned a name
Shell scripts can use function name to execute the commands
Use shell functions to isolate reusable code sections
Reduce typing and debugging time
Example:
datenow() {
date }
A Guide to Unix Using Linux, Fourth Edition

28. Defining a Function from the Command Line
You can define functions from the command line:
However, functions are usually stored in script files
Loaded into memory when you log in
Arguments are passed to functions in the same manner as any other shell procedure:
~]$ datenow() <Enter>
> { <Enter>
> date <Enter>
> } <Enter>
~]$
~]$ datenow "Today’s date and time are:"
Today’s date and time are:
Mon Feb 9 21:49:45 MST 2009
A Guide to Unix Using Linux, Fourth Edition

29. Creating Functions Inside Shell Scripts
Example: a shell script called .myfuncs
You may start .myfuncs from your .bash_profile or .bashrc login script, or the command line
sort_name() {
sort -k 2 -t: corp_phones }
sort_job()
sort -k 6 -t: corp_phones
sort_dept()
sort -k 5 -t: corp_phones
A Guide to Unix Using Linux, Fourth Edition

30. Troubleshooting a Shell Script
Some tips to help you troubleshoot a script:
Ensure script has execute permissions
Be certain first line of script specifies shell to use
Use the sh -n, -v, and -x troubleshooting options
Look for typographic errors
Look for errors in the use of particular characters
;, ‘, `, ‘’, “”, #, <, >
Check for syntax errors in the use of commands
Look for the use of command options that are not supported in your distribution of UNIX/Linux
Check initial and exit value of looping logic
A Guide to Unix Using Linux, Fourth Edition

31. Summary
Two most popular and proven analysis tools: flowchart and pseudocode
Have the first line in a script file specify the shell
Use test to validate the existence of directories and files as well as to compare numeric and string values
tr changes characters typed at keyboard
sed reads a file as its input and outputs the file’s modified contents
Shell functions can make programmer more efficient
A Guide to Unix Using Linux, Fourth Edition

32. Command Summary
A Guide to Unix Using Linux, Fourth Edition

33. Command Summary (continued)
A Guide to Unix Using Linux, Fourth Edition