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Questions and Answers
What must be true about the name of a function in a shell script?
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?
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?
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?
What will happen if a function body does not end with a semicolon or a newline?
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What role does the exit status of a function play?
What role does the exit status of a function play?
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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
historydisplays the list of past commands, each with an associated event number. - Re-execute commands using
!event_numberor!!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 allmanual.htmlfiles.
Using Pipes
- Use
|to send the output of one command to another, allowing commands to be executed concurrently. - Example:
echo Happy Birthday! | revreverses 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>errorssaves error messages to the file namederrors.
Adding Aliases
- Simplify complex commands by creating aliases with
alias name='command'. - Example:
alias p='pwd; ls --CF'enablespto execute both commands in sequence.
Shell Functions
- Group commands for repetitive tasks, executed with a single name.
- Function syntax:
function FUNCTION { COMMANDS; }orFUNCTION () { COMMANDS; }. - Example:
u() { ls -l; }allows you to useuas a shortcut forls -l.
Common Shell Command Usage
- History: Use
historyto 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.
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