Signal handling and process management in Deno - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to listen for the SIGINT signal and log a message.

Deno

Deno.addSignalListener([1], () => {
  console.log("Received SIGINT, exiting...");
  Deno.exit();
});

Drag options to blanks, or click blank then click option'

A"SIGTERM"

B"SIGINT"

C"SIGHUP"

D"SIGUSR1"

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong signal name like "SIGTERM" or "SIGHUP".

Forgetting to put the signal name in quotes.

The "SIGINT" signal is sent when you press Ctrl+C. Listening to it allows graceful shutdown.

2fill in blank

medium

Complete the code to spawn a subprocess that runs the 'echo' command.

Deno

const process = Deno.run({
  cmd: ["echo", [1]],
  stdout: "piped"
});

Drag options to blanks, or click blank then click option'

A"Hello, Deno!"

Cecho

D"run"

Attempts:

3 left

💡 Hint

Common Mistakes

Not using quotes around the argument string.

Passing the command name again as argument.

The command arguments must be strings. So the message should be in quotes.

3fill in blank

hard

Fix the error in the code to correctly wait for the subprocess to finish.

Deno

const process = Deno.run({ cmd: ["ls"], stdout: "piped" });
const status = await process.[1]();
process.close();

Drag options to blanks, or click blank then click option'

Await

Bcomplete

Cfinish

DwaitFor

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect method names like waitFor or finish.

Not awaiting the promise returned by wait().

The correct method to wait for a subprocess to finish is wait().

4fill in blank

hard

Fill both blanks to create a signal listener that stops the process on SIGTERM and logs a message.

Deno

Deno.addSignalListener([1], () => {
  console.log("[2] received, stopping process.");
  Deno.exit();
});

Drag options to blanks, or click blank then click option'

A"SIGTERM"

B"SIGINT"

CSIGTERM

D"Process termination signal"

Attempts:

3 left

💡 Hint

Common Mistakes

Using signal name without quotes.

Using wrong signal name like SIGINT here.

The signal name must be a string like "SIGTERM". The message can be any string describing the event.

5fill in blank

hard

Fill all three blanks to spawn a subprocess that runs 'cat' on a file, waits for it, and closes it.

Deno

const process = Deno.run({ cmd: [[1], [2]], stdout: "piped" });
const status = await process.[3]();
process.close();

Drag options to blanks, or click blank then click option'

A"cat"

B"file.txt"

Cwait

D"run"

Attempts:

3 left

💡 Hint

Common Mistakes

Not quoting the command or filename.

Using wrong method name to wait.

The command is "cat", the argument is the filename "file.txt", and the method to wait is wait().

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