Why Senior Engineers Disagree About standard streams (stdin, stdout, stderr)

To a programmer, standard streams (stdin, stdout, stderr) are as fundamental as it gets. Yet, mention them in a room of senior engineers, and you might spark a surprisingly heated debate. Here’s why this seemingly settled topic is anything but.

The Textbook Definition We All Learned

Let's start with the basics everyone agrees on. In Unix-like operating systems, every process starts with three open communication channels, known as standard streams. Standard Input (stdin) is for feeding data into a program. Standard Output (stdout)

is where a program sends its successful results. And Standard Error (stderr) is where a program sends its error messages, warnings, and other diagnostic information. For decades, this separation was sacred: stdout was for the program's 'real' output, the data that another program might use. stderr was for everything else—the chatter, the warnings, the cries for help—intended for human eyes.

The Classic Philosophy: A Sacred Separation

The traditional Unix philosophy prizes small, focused programs that do one thing well and can be chained together. The `pipe` operator (`|`) is the magic that makes this work, sending the stdout of one program to the stdin of the next. Keeping diagnostic messages on stderr was crucial because it prevented them from 'polluting' the stdout data stream. Imagine trying to pipe a list of filenames from one command to another, only to have it contaminated with a random "Warning: disk space is low" message. The pipeline would break. This clean separation of data (stdout) from diagnostics (stderr) is a cornerstone of powerful command-line workflows and is considered gospel by many experienced engineers.

The New Reality: Containers and the 12-Factor App

Then, the world changed. The rise of cloud computing, microservices, and containerization platforms like Docker and Kubernetes brought a new way of thinking. A highly influential methodology called the "Twelve-Factor App" emerged to define best practices for modern, cloud-native applications. One of its most debated principles concerns logging. Factor XI, "Logs," instructs developers to treat logs as event streams. A 12-Factor app, it declares, should never concern itself with routing or storing its own logs. Instead, it should write its event stream, unbuffered, to stdout. The execution environment—the container runtime or cloud platform—is then responsible for capturing this stream and routing it to a centralized logging system.

The Great Debate: All Logs to stdout?

This is where the disagreement ignites. The 12-Factor approach effectively says: forget the distinction and dump everything, including errors and structured logs, into stdout. Proponents argue this radically simplifies application code. The developer just writes to one place, and the complex job of log collection, aggregation, and storage is handled by the platform. It makes apps more portable and scalable, as they don't need to know anything about the logging infrastructure of the environment they're running in.

But traditionalists are horrified. To them, this breaks a fundamental, time-tested principle. They argue that stdout is for data, and logging is not data. By combining them, you lose the ability to easily pipe the actual output of a program without also getting a firehose of log messages. It conflates two different types of output, forcing downstream tooling to parse a messy, combined stream. They contend that stderr is the correct, purpose-built channel for logs and diagnostics, and that modern platforms are perfectly capable of capturing both stdout and stderr separately.

So Who Is Right?

As with most engineering debates, the answer is: it depends. The 'all to stdout' camp prioritizes application simplicity and statelessness, key virtues in a containerized world. For a microservice that primarily communicates via APIs and whose 'output' is just structured logs, sending everything to stdout is pragmatic and efficient. The execution environment is designed to handle this firehose. The 'preserve stderr' camp prioritizes composability and the Unix philosophy of sharp, interoperable tools. For command-line utilities or any program whose primary output is meant for other programs, keeping stdout clean is non-negotiable. Polluting it with logs breaks the contract that has made shell scripting so powerful for half a century. The disagreement isn't about being right or wrong; it's a clash of two valid but competing philosophies, one born from the command line and the other from the cloud.