# Testing command-line tools and web apps How command-line tools and web apps behave at runtime and how to test them, from exit codes and output streams to headless browsers. Start here ## [How to test a command-line application](https://specstory.com/learning/cli-and-web/testing-command-line-applications) Testing a command-line application means running it the way users and scripts do and checking its output, errors, exit codes, and behavior in pipes. 11 min read ## Concepts ### [What are exit codes?](https://specstory.com/learning/cli-and-web/exit-codes) An exit code is the number a program returns when it finishes, where 0 usually means success and any other value signals an error that scripts can check. ### [What is a headless browser?](https://specstory.com/learning/cli-and-web/headless-browser) A headless browser is a web browser that runs without a visible window, controlled by code, so tests and agents can load pages and act on them. ### [What is a browser test framework?](https://specstory.com/learning/cli-and-web/browser-test-frameworks) A browser test framework is an open-source library that drives a real browser from test code, so a test can load pages, act on them, and check results. ### [What is Playwright MCP?](https://specstory.com/learning/cli-and-web/playwright-mcp) Playwright MCP is a server that lets an AI agent drive a real browser through the Model Context Protocol, using page snapshots instead of screenshots. ## How-to guides ### [How to verify a CLI that a coding agent built](https://specstory.com/learning/cli-and-web/verifying-agent-built-cli) Verifying a CLI that a coding agent built means installing it fresh, running the commands a user would, and checking exit codes, output, and files. ### [How to test a terminal UI (TUI)](https://specstory.com/learning/cli-and-web/testing-terminal-uis) Testing a terminal UI (TUI) means running it in a pseudoterminal, sending keystrokes, and comparing the screen it draws with a reviewed snapshot. ### [How to test shell scripts](https://specstory.com/learning/cli-and-web/testing-shell-scripts) Testing a shell script means running it with controlled input and environment, then checking its output, exit code, and the files it changes. ## Comparisons ### [What is the difference between stdout and stderr?](https://specstory.com/learning/cli-and-web/stdout-vs-stderr) stdout is a program's standard output for results, and stderr is a separate stream for errors and diagnostics, so each can be redirected on its own. ## Common questions ### [Why does CLI output change when piped?](https://specstory.com/learning/cli-and-web/tty-vs-pipe) CLI output changes when piped because programs and runtimes check whether stdout is a terminal (a TTY) and switch color, buffering, and layout if not. ## Failures and bugs ### [What is a broken pipe error (SIGPIPE)?](https://specstory.com/learning/cli-and-web/broken-pipe) A broken pipe error happens when a program writes to a pipe or socket whose reader has closed, which sends it SIGPIPE and often ends it with exit code 141. ### [What is a hydration error?](https://specstory.com/learning/cli-and-web/hydration-error) A hydration error happens when the HTML a server rendered does not match what the browser's JavaScript renders, so the framework warns or re-renders. ## Terms in this topic **[Browser test framework](https://specstory.com/learning/glossary#browser-test-framework)** A browser test framework is an open-source library that drives a real browser from test code, so a test can load pages, act on them, and check the results. **[Command-line interface (CLI)](https://specstory.com/learning/glossary#command-line-interface)** A command-line interface is a way of using a program that takes typed commands, flags, and input in a terminal or script, and returns output, errors, and an exit code. **[Exit code](https://specstory.com/learning/glossary#exit-code)** An exit code is the number that a program returns when it finishes, where 0 usually means success and any other value signals an error that scripts can check. **[Headless browser](https://specstory.com/learning/glossary#headless-browser)** A headless browser is a web browser that runs without a visible window and is controlled by code, so tests and agents can load pages and act on them. **[Hydration error](https://specstory.com/learning/glossary#hydration-error)** A hydration error is a web app failure that happens when the HTML a server rendered does not match what the browser's JavaScript renders on load. **[Playwright MCP](https://specstory.com/learning/glossary#playwright-mcp)** Playwright MCP is an open-source server that lets an AI agent control a real browser through the Model Context Protocol, using structured page snapshots instead of screenshots. **[SIGPIPE](https://specstory.com/learning/glossary#sigpipe)** SIGPIPE is the signal that a program receives when it writes to a pipe whose reader has closed. By default it ends the program, and shells report exit code 141. **[Standard streams](https://specstory.com/learning/glossary#standard-streams)** The standard streams are the three channels that every process gets by default, which are standard input for data in, standard output for results, and standard error for diagnostics. **[stderr](https://specstory.com/learning/glossary#stderr)** stderr is the standard error stream that a program writes errors and diagnostics to, so they stay separate from the results on standard output. **[stdout](https://specstory.com/learning/glossary#stdout)** stdout is the standard output stream that a program writes its normal results to, so another program, a file, or a terminal can read them. **[TTY](https://specstory.com/learning/glossary#tty)** A TTY is a terminal device that a program can detect, and many programs change their output, color, or prompts when they write to a pipe instead of a TTY. 11 terms from the [Learning Center glossary](https://specstory.com/learning/glossary). --- Source: [Testing command-line tools and web apps | SpecStory](https://specstory.com/learning/cli-and-web)