CEST-CE Term_2 GNU/LINUX (Week 10)

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Questions and Answers

What do control structures in shell scripts primarily do?

Execute commands randomly

Simplify the command syntax

Alter the sequence of command execution (correct)

Ignore errors automatically

How does a simple if statement function in shell scripts?

It provides a default command to execute.

It executes a command only if a test condition succeeds. (correct)

It runs a command only if a test condition fails.

It executes a command regardless of its success.

What is the effect of prefixing a command with '!' in a shell script?

It cancels the command from execution.

It repeats the command twice.

It inverts the exit status of the command. (correct)

It executes the command normally.

What is the primary role of the if-else statement in shell scripts?

To provide two alternative command paths based on success or failure. Signup and view all the answers

What is one benefit of proper indentation in shell scripts?

It enhances readability and maintainability. Signup and view all the answers

Which statement accurately describes human interaction with shell commands compared to shell programs?

Humans can respond to errors while shell programs execute commands sequentially. Signup and view all the answers

What is a nested if statement used for in shell scripting?

To handle complex conditions by embedding one if within another. Signup and view all the answers

What is the exit status of a command that succeeds in shell scripts?

It is zero. Signup and view all the answers

What is the purpose of the test command in shell scripts?

To check file system properties and set exit statuses. Signup and view all the answers

What exit status does the command 'test -f "/bin/bash"' return?

0, indicating success. Signup and view all the answers

Which command would you use to check if a string is empty?

-z Signup and view all the answers

Which of the following commands checks if a specified file is a directory?

test -d /path/to/file Signup and view all the answers

What does the comparison '-gt' represent in integer tests?

Tests if a number is greater than another. Signup and view all the answers

What is a warning associated with using the '-n' test?

A single argument is automatically assumed to be non-empty. Signup and view all the answers

Which of the following is NOT a pathname property test?

-ge Signup and view all the answers

What will 'test -ne 10 20' evaluate to?

True, because numbers are not equal. Signup and view all the answers

What does the continue statement do within a loop?

Moves to the next iteration of the loop. Signup and view all the answers

What result occurs after executing the shift command on positional parameters?

$1 becomes $2, $2 becomes $3, etc. Signup and view all the answers

What is the primary purpose of the ErrorExit function in shell scripting?

To display usage information and exit with an error Signup and view all the answers

What does the following function definition do? Foo () { echo "Hello $*" ; }

Prints 'Hello' followed by all passed arguments. Signup and view all the answers

Which statement about short-circuit command operators is correct?

The || operator executes the second command only if the first command fails. Signup and view all the answers

Why might functions in shell scripts be defined in .bashrc?

To maintain function definitions across sessions. Signup and view all the answers

What will be the output of the script running './example.sh one two three four five six' after multiple shifts?

'$1' becomes empty after six shifts. Signup and view all the answers

Which syntax correctly reads user input into a variable named num1?

read -p "Enter a number:" num1 Signup and view all the answers

How can you ensure error handling is centralized in a script?

Using functions to manage errors. Signup and view all the answers

Why is it important for error messages to appear on standard error?

To prevent errors from being lost in file outputs. Signup and view all the answers

What must be included in a good error message according to shell scripting best practices?

The name of the program from the variable $0. Signup and view all the answers

What happens to parameters passed to functions in shell scripts?

They become the positional parameters within the function. Signup and view all the answers

What does the command 'echo "The sum is $(( num1 + num2 ))"' do in a shell script?

Adds num1 and num2 and prints the result with a textual message. Signup and view all the answers

In the given scripted loop, what does the command 'if [ ! -e "$name" ]; then' check for?

If the file indicated by $name exists. Signup and view all the answers

How is input from users handled using the 'read' command in shell scripts?

It splits input into variables based on whitespace. Signup and view all the answers

In the context of the discussed error handling, which term should be avoided in error messages?

Parameter Signup and view all the answers

What key information must good error messages include?

The script's name, expected input type, and user-supplied input values Signup and view all the answers

When should usage messages be displayed in scripts?

Only when the script usage is incorrect Signup and view all the answers

What is an important characteristic of user input in error messages?

Input should be reported with specific formatting Signup and view all the answers

What is a common misconception regarding arithmetic in shell scripts?

Shell commands must be used in if statements for comparison Signup and view all the answers

Which of the following is a proper syntax example for square bracket expressions in shell scripts?

if [1 -eq 1] Signup and view all the answers

What should be avoided when using test operators in shell scripts?

Using redirection operators for comparisons Signup and view all the answers

What is a recommended practice to avoid common script problems?

Use debug options like -x or -v to aid debugging Signup and view all the answers

Why is it important to specify the expected input type and count in error messages?

To provide clarity and improve user experience Signup and view all the answers

Study Notes

Control Structures / Control Statements

Control structures modify the command execution sequence in shell scripts, crucial for managing errors and flow.
Humans can react to errors manually, but shell scripts execute commands sequentially without error intervention.
Control statements allow for conditional execution and repetition (loops) based on success or failure of commands.

Branch Conditional Shell Control Structures

if, then, else, fi structures rely on command exit statuses for branching.
A simple if statement executes commands only if a test condition succeeds.
The ! operator inverts command exit status, allowing commands to run on failure.
if-else statements facilitate execution of alternative commands based on success or failure.
Nested if statements enable handling complex conditions through layered testing.

Indentation in Shell Scripts

Proper indentation improves readability and maintainability, although it is not syntactically necessary in shell environments.

The `test` Helper Program

The test command is essential for evaluating file existence, string comparisons, and numeric values.
For file existence:
- test -f "/bin/bash" returns 0 (success); test -f "/bin/nosuchfile" returns 1 (failure).

Categories of Tests

Pathname Properties: -e, -f, -d, -r, -w, -x, -s
String Comparisons: -z, -n, =, !=
Integer Comparisons: -eq, -ne, -lt, -le, -gt, -ge

Using `test` for Various Checks

Check for file type and existence using -f (file) and -d (directory).
String checks include -z for empty, -n for non-empty, and equality or inequality checks with = and !=.
Numeric checks facilitate mathematical comparisons using -eq, -ne, -lt, etc.

Using `continue` Statement

The continue statement allows skipping to the next iteration of a loop, helpful for ignoring inaccessible or non-existent items.

Shifting Positional Parameters

The shift command moves positional parameters down by one, enabling flexible argument handling in scripts.

Shell Functions

Functions encapsulate reusable code with parameters, enhancing script clarity and maintaining functionality across executions.
External functions can be defined in .bashrc to persist between sessions.

ErrorExit Function

Centralizes error handling to improve script maintenance, providing systematic output on failure.

Short-Circuit Command List Operators

&& and || are used for conditional command execution based on success or failure.
Compact error handling is achievable with these operators, although they may lack detailed error messaging compared to traditional if statements.

Reading Input with `read`

The read command obtains user input from standard input, allowing variable assignment from the command line.

Good Error Messages

Error messages should be sent to standard error (stderr) to ensure visibility.
Messages must include the script name, expected input description, and user input values for clarity.

Best Practices for Usage Messages

Provide clear usage instructions following identification of a usage error.
Display usage messages only for actual usage errors to avoid confusion.

Avoiding Common Script Problems

Scripts should produce clear error messages; write incrementally, and use debugging options for easier error identification.
Arithmetic operations must employ commands correctly within conditions, avoiding incorrect syntax involving square brackets.
Always ensure necessary spaces exist around operators for accurate command execution.

Summary of Common Script Issues

Do not mix square brackets with commands or neglect space around conditional operators.
Avoid using redirection operators in numerical comparisons for correct functionality.

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Description

This quiz covers the key concepts of control structures and statements in shell scripting. You'll learn about conditional execution, branching using 'if' statements, and the significance of indentation for readability. Understand how these elements enhance error management and flow control in your scripts.