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Discover how to efficiently trigger a function in Node.js based on specific output in `stdout` from executing a child process.
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How to Execute a Function in Node.js When Specific Text Appears in stdout

When working with Node.js, one common task is to run a child process that executes a command or script. For instance, you might be running a yarn script that takes some time to finish. This script could be handling a complex build process through tools like Terraform. However, you might want to perform certain actions only when specific text appears in the output of that command. In this guide, we will explore how to achieve this effectively.

The Problem: Detecting Specific Output in stdout

Imagine you're launching a yarn script to execute a Terraform build, and this build can take several minutes. You want to know when the process has finished without having to wait idly. A particular phrase in the output indicates that the build has completed, and this is exactly when you want to trigger another function in your program. So, how do you go about detecting this phrase and executing the function based on that condition?

The Solution: Using readline with child_process

Node.js's built-in child_process module allows you to create child processes easily. To monitor the output of a child process, you can utilize the readline module, which will help you read data input line by line. Here’s a step-by-step guide on how to implement this:

1. Import Required Modules

First, you'll need to require the necessary Node.js modules: child_process for creating the child process, and readline for reading from stdout.

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2. Create a readline Interface

Next, establish an interface for reading the output from the child process's stdout. This is where you will listen for output from the command you executed.

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3. Define the Function to Execute

Now you’ll need to define the function that you want to run when the specified text appears in the output. This function will be passed the specific line of output.

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4. Set Up the Event Listener

Finally, you can set up an event listener on the readline interface to call your specified function whenever a new line is received from stdout.

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Complete Example

Putting it all together, your final code may look something like this:

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Conclusion

With this approach, you can efficiently monitor stdout from a child process in Node.js and execute a specific function when the desired output appears. This technique can be particularly useful in scenarios where you need to take action based on the results of long-running scripts. By leveraging Node.js's event-driven model, you can keep your application responsive and efficient.

Feel free to adapt this example for your specific needs, and happy coding!