Signal handling and process management in Deno

Introduction

Signal handling lets your program listen for system messages like stop or restart. Process management helps control running programs.

You want to clean up resources before your program stops.

You need to restart a task when a signal is received.

You want to handle user interruptions like Ctrl+C gracefully.

You want to monitor or control child processes your program starts.

Syntax

Deno

const signal = Deno.signal(signalName);
for await (const _ of signal) {
  // handle signal
}
signal.dispose();

Use Deno.signal() with signal names like 'SIGINT', 'SIGTERM'.

Always call dispose() to clean up when done listening.

Examples

This listens for Ctrl+C (SIGINT) and exits cleanly.

Deno

const sigint = Deno.signal("SIGINT");
for await (const _ of sigint) {
  console.log("Got SIGINT, exiting...");
  sigint.dispose();
  Deno.exit();
}

This handles termination signals to clean up before exit.

Deno

const sigterm = Deno.signal("SIGTERM");
for await (const _ of sigterm) {
  console.log("Received SIGTERM, cleaning up...");
  sigterm.dispose();
  Deno.exit();
}

This runs a child process and waits for it to finish.

Deno

const p = Deno.run({ cmd: ["echo", "hello"] });
const status = await p.status();
p.close();
console.log(`Process exited with code ${status.code}`);

Sample Program

This program waits for you to press Ctrl+C. When you do, it prints messages, cleans up, and exits.

Deno

const sigint = Deno.signal("SIGINT");
console.log("Program started. Press Ctrl+C to stop.");

for await (const _ of sigint) {
  console.log("Signal SIGINT received. Cleaning up...");
  sigint.dispose();
  console.log("Cleanup done. Exiting now.");
  Deno.exit();
}

Important Notes

Signals are messages from the operating system to your program.

Always dispose signal listeners to avoid resource leaks.

Use Deno.run() to start and manage child processes.

Summary

Signal handling lets your program respond to system events like Ctrl+C.

Process management helps you run and control other programs from your code.

Always clean up signal listeners and child processes to keep your program healthy.

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