Signal handling and process management in Deno - Mini Project: Build & Apply

Signal Handling and Process Management in Deno

📖 Scenario: You are building a simple Deno script that listens for system signals and manages a child process. This is useful when you want your program to clean up resources or stop child tasks gracefully when the user interrupts the program.

🎯 Goal: Create a Deno script that starts a child process, listens for the SIGINT signal (triggered by Ctrl+C), and then stops the child process cleanly before exiting.

📋 What You'll Learn

Create a child process that runs the ping command to localhost.

Set up a signal listener for SIGINT to detect when the user presses Ctrl+C.

When SIGINT is received, stop the child process gracefully.

Ensure the script exits cleanly after stopping the child process.

💡 Why This Matters

🌍 Real World

Signal handling and process management are important for building robust command-line tools and servers that can clean up resources and stop child tasks properly when interrupted.

💼 Career

Understanding how to manage processes and signals is essential for backend developers, system programmers, and anyone working with server-side JavaScript or TypeScript environments like Deno.

1

Create a child process running ping localhost

Write code to start a child process using Deno.run that runs the command ping with argument localhost. Assign this process to a variable called child.

Deno

# Your code here to start the child process

Hint

Use Deno.run with cmd set to an array containing "ping" and "localhost".

2

Set up a signal listener for SIGINT

Add a variable called sigint that listens for the SIGINT signal using Deno.signal.

Deno

const child = Deno.run({ cmd: ["ping", "localhost"] });
// Your code here to listen for SIGINT

Hint

Use Deno.signal(Deno.Signal.SIGINT) to listen for Ctrl+C.

3

Wait for SIGINT and stop the child process

Use await sigint.next() to wait for the SIGINT signal. After receiving it, close the signal listener with sigint.dispose() and then stop the child process by calling child.kill() and await child.status().

Deno

const child = Deno.run({ cmd: ["ping", "localhost"] });
const sigint = Deno.signal(Deno.Signal.SIGINT);
// Your code here to wait for SIGINT and stop the child process

Hint

Use await sigint.next() to pause until Ctrl+C is pressed, then clean up resources.

4

Exit the script cleanly after stopping the child process

Add a line to exit the Deno process cleanly by calling Deno.exit(0) after stopping the child process.

Deno

const child = Deno.run({ cmd: ["ping", "localhost"] });
const sigint = Deno.signal(Deno.Signal.SIGINT);

await sigint.next();
sigint.dispose();
child.kill();
await child.status();
// Your code here to exit the script cleanly

Hint

Use Deno.exit(0) to end the script without errors.

Practice

1. What is the main purpose of signal handling in a Deno program?

easy

A. To respond to system events like Ctrl+C or termination requests

B. To speed up the execution of asynchronous code

C. To automatically restart the program after an error

D. To manage memory allocation for the program

Solution

1. Step 1: Understand what signals are

   Signals are system messages sent to a program, such as when a user presses Ctrl+C.

2. Step 2: Identify signal handling purpose

   Signal handling lets the program catch these signals and respond, for example, to clean up resources before exiting.

3. Final Answer:

   To respond to system events like Ctrl+C or termination requests -> Option A

4. Quick Check:

   Signal handling = respond to system events [OK]

Hint: Signals catch system events like Ctrl+C to handle them [OK]

Common Mistakes:

• Confusing signal handling with error handling
• Thinking signal handling speeds up code
• Assuming signals manage memory automatically

2. Which of the following is the correct way to listen for a SIGINT signal in Deno?

easy

A. Deno.listenSignal('SIGINT', () => console.log('Interrupted'))

B. Deno.signal(Deno.Signal.SIGINT).then(() => console.log('Interrupted'))

C. Deno.on('SIGINT', () => console.log('Interrupted'))

D. for await (const _ of Deno.signal(Deno.Signal.SIGINT)) { console.log('Interrupted') }

Solution

1. Step 1: Recall Deno's signal API usage

   Deno.signal returns an async iterable for the given signal.

2. Step 2: Identify correct syntax to listen continuously

   Using a for-await-of loop over Deno.signal(Deno.Signal.SIGINT) listens repeatedly for SIGINT events.

3. Final Answer:

   for await (const _ of Deno.signal(Deno.Signal.SIGINT)) { console.log('Interrupted') } -> Option D

4. Quick Check:

   Use for-await-of with Deno.signal for signals [OK]

Hint: Use for-await-of loop with Deno.signal to handle signals [OK]

Common Mistakes:

• Using .then() which only handles one signal once
• Trying to use event emitter style which Deno does not support
• Using non-existent Deno.listenSignal method

3. What will the following Deno code output when you press Ctrl+C once?

const sig = Deno.signal(Deno.Signal.SIGINT);
for await (const _ of sig) {
  console.log('Signal received');
  sig.dispose();
}

medium

A. Signal received (printed once), then program stops listening for SIGINT

B. SyntaxError because dispose() cannot be called inside loop

C. Signal received printed repeatedly on every Ctrl+C press

D. Signal received (printed once), then program continues running

Solution

1. Step 1: Understand the for-await-of loop over signal

   The loop waits for SIGINT signals and runs the body each time one occurs.

2. Step 2: Effect of calling sig.dispose()

   Calling dispose() stops the signal listener, so after the first Ctrl+C, the loop ends and no more signals are handled.

3. Final Answer:

   Signal received (printed once), then program stops listening for SIGINT -> Option A

4. Quick Check:

   dispose() stops signal listening after first event [OK]

Hint: dispose() stops signal listening; loop ends after first signal [OK]

Common Mistakes:

• Thinking dispose() only pauses temporarily
• Assuming the loop continues printing on every Ctrl+C
• Believing dispose() causes syntax errors

4. Identify the error in this Deno code snippet that tries to spawn a child process and handle its exit:

const p = Deno.run({ cmd: ["ls", "-l"] });
p.status().then(status => {
  console.log(`Process exited with code ${status.code}`);
});
p.close();

medium

A. Deno.run does not exist; should use Deno.spawn instead

B. Missing await before p.status() causes unhandled promise

C. Calling p.close() before the process finishes causes resource leaks

D. The cmd array should be a string, not an array

Solution

1. Step 1: Understand process lifecycle in Deno

   Deno.run starts a child process and returns a process handle.

2. Step 2: Analyze order of close() call

   Calling p.close() immediately releases resources before the process finishes, which can cause issues.

3. Final Answer:

   Calling p.close() before the process finishes causes resource leaks -> Option C

4. Quick Check:

   Close process handle only after process ends [OK]

Hint: Call close() only after process completes to avoid leaks [OK]

Common Mistakes:

• Calling close() too early before process ends
• Confusing Deno.run with Deno.spawn (spawn is newer API)
• Thinking cmd should be a string instead of array

5. You want to write a Deno script that runs a child process and gracefully handles Ctrl+C to kill the child before exiting. Which approach correctly combines signal handling and process management?

hard

A. Call child.kill() immediately after spawning, then listen for SIGINT to exit

B. Use for-await-of on Deno.signal(Deno.Signal.SIGINT) to catch Ctrl+C, then call child.kill() and await child.status() before exiting

C. Use Deno.run with cmd and ignore signals; rely on OS to kill child on exit

D. Spawn child process with Deno.spawn and never call kill(), just exit on SIGINT

Solution

1. Step 1: Understand graceful shutdown

   On Ctrl+C, the program should catch SIGINT and clean up child processes before exiting.

2. Step 2: Combine signal listening and child process control

   Using for-await-of on Deno.signal(Deno.Signal.SIGINT) allows catching Ctrl+C; then calling child.kill() stops the child, and awaiting child.status() ensures it ends cleanly.

3. Final Answer:

   Use for-await-of on Deno.signal(Deno.Signal.SIGINT) to catch Ctrl+C, then call child.kill() and await child.status() before exiting -> Option B

4. Quick Check:

   Signal handling + kill child + await status = graceful exit [OK]

Hint: Catch SIGINT, kill child, await status for clean exit [OK]

Common Mistakes:

• Killing child before signal received
• Ignoring signals and leaving child running
• Not awaiting child.status() causing premature exit