Open Source OS Lab 6: The Shell

Questions and Answers

What must be true about the name of a function in a shell script?

It can be the same as existing command names.

It must be unique within the shell or script. (correct)

It should start with a number.

It can be any sequence of characters without restrictions.

Which of the following describes how a function's commands are executed?

They run in a new process created for each execution.

They are executed like regular commands in the current shell. (correct)

They only run if called recursively.

They are executed in an external interpreter.

What is a required feature of function syntax regarding curly braces?

Curly braces must be separated from the body by spaces. (correct)

Curly braces can be omitted if there is only one command.

Curly braces must be enclosed in parentheses.

Curly braces must be placed on the same line as the function name.

What will happen if a function body does not end with a semicolon or a newline?

<p>It may lead to unexpected behavior or errors.</p> Signup and view all the answers

What role does the exit status of a function play?

<p>It represents the exit status of the last command executed in the body.</p> Signup and view all the answers

Study Notes

The Shell

Acts as an interface between the user and the kernel of the operating system.
Prompts for input, processes commands, and executes actions before prompting again.

Shell Features

Combining Commands: Use ; to execute multiple commands in a single line.
Command Line Completion: Pressing Tab after typing an unambiguous prefix autocompletes the command.
Command Line Edition: Navigate and edit commands using arrows and key combinations like Ctrl+A (to start) and Delete to erase characters.

Command History

Stores previously executed commands for easy access using Up and Down arrow keys.
Command history displays the list of past commands, each with an associated event number.
Re-execute commands using !event_number or !! for the last command.

Running Commands in Background

Add & at the end of a command to run it in the background without waiting for it to complete.

Command Substitution

Use $(command) or backticks `command` to use output of one command as an argument in another.
Example: ls -l $(locate manual.html) lists all manual.html files.

Using Pipes

Use | to send the output of one command to another, allowing commands to be executed concurrently.
Example: echo Happy Birthday! | rev reverses the string output.

Redirecting Input and Output

Standard Output: Redirect using > to save output to a file; use >> to append.
Standard Error: Redirect using 2> to capture error messages to a file.
Example: cat nonexistent 2>errors saves error messages to the file named errors.

Adding Aliases

Simplify complex commands by creating aliases with alias name='command'.
Example: alias p='pwd; ls --CF' enables p to execute both commands in sequence.

Shell Functions

Group commands for repetitive tasks, executed with a single name.
Function syntax: function FUNCTION { COMMANDS; } or FUNCTION () { COMMANDS; }.
Example: u() { ls -l; } allows you to use u as a shortcut for ls -l.

Common Shell Command Usage

History: Use history to display commands; press Up/Down for navigation.
Background Commands: Use & to run tasks simultaneously.
Pipes: Create command chains for more efficient output handling.
Input/Output Redirection: Efficiently manage where command outputs/errors are stored.
Aliases and Functions: Streamline command use for efficiency and simplicity.

Description

Explore the fundamentals of shell programs in this lab quiz. Test your knowledge on command features like combining commands, command line completion, and using pipes. Learn to manage command history, background processes, and input/output redirection effectively.