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Environment Variable Template Expander

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Environment Variable Template Expander

Paste a template containing variable placeholders along with your environment variables, and instantly see the fully expanded result. Supports ${VAR}, {{VAR}}, %VAR%y $VAR syntaxes, with auto-detection to handle all four in the same template. Unresolved placeholders are highlighted in a sidebar so you can see at a glance which keys are missing.

The whole thing runs in your browser. Nothing is uploaded to a server, which matters when you are pasting actual secrets while previewing a config render. The substitution is deterministic, so the output you see is exactly what a build pipeline using the same placeholders would produce.

Cómo Usar

Paste your template into the Plantilla box. It can be YAML, JSON, an Nginx config, a docker-compose file — anything that contains placeholders.
Pega tu .env contents (or any list of KEY=value pairs) into the Variables de entorno box. Comments, quoted values, and the export prefix are all parsed correctly.
Elige un Variable Syntax. Leave it on auto-detect if your template mixes styles, or lock it to a single syntax if you want strict matching.
Choose how to handle unresolved variables: leave the original placeholder in place, or strip it to an empty string.
Copy or download the expanded output. The summary bar shows the resolved/unresolved counts and the unresolved list shows each missing name with its occurrence count.

Características

Four placeholder syntaxes – ${VAR} (shell, Docker Compose), {{VAR}} (Handlebars, Mustache), %VAR% (Windows batch), and bare $VAR (POSIX shell).
Auto-detect mode – mix all four syntaxes in the same template and expand them in one pass.
Standards-compliant .env parser , separadores de espacios en blanco y continuaciones de línea con barra de la misma manera # comments, single/double-quoted values, the export KEY=value form, and inline trailing comments.
Unresolved variable report – each missing placeholder is listed with the number of times it appears in the template.
Case-sensitivity toggle – match keys exactly or fall back to case-insensitive lookups for cross-platform configs.
Two unresolved-handling modes – leave placeholders intact for debugging, or substitute empty strings for a clean preview.
Previsualización en tiempo real – the output regenerates as you type, with a short debounce so it stays smooth on large templates.
Primero en privacidad – substitution runs entirely client-side, so your variables and secrets never leave the browser.

Casos de uso común

CI/CD template preview – see exactly what a GitHub Actions or GitLab CI pipeline will produce after variable substitution.
Docker Compose interpolation – verify how ${VAR} placeholders resolve before docker compose up.
Kubernetes manifest preview – preview a Helm-like substitution without spinning up a templating tool.
Config file generation – render Nginx, Apache, or HAProxy configs from a single template per environment.
Secrets injection sanity check – preview the file a deployment will receive without leaking secrets to a log.
Documentation rendering – fill in versioned {{VAR}} placeholders in README or markdown snippets.

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 Preguntas frecuentes

What is variable interpolation in configuration files?

Variable interpolation is the process of replacing named placeholders in a text template with values from an external source — typically environment variables or a key-value file. It lets one template serve multiple deployment environments by keeping environment-specific values out of the file itself. Tools like Docker Compose, Kubernetes Helm, Ansible, and shell script substitution all rely on this technique.
Why are there so many different placeholder syntaxes?

Each syntax grew out of a different ecosystem. The dollar-brace form ${VAR} comes from POSIX shell and was adopted by Docker Compose and most Unix tools. Double-brace {{VAR}} is the Mustache/Handlebars convention used by Helm, Jekyll, and many templating engines. Percent-wrapped %VAR% is the Windows batch and CMD convention. Bare $VAR is the original POSIX shell form for simple substitution. There was never a single standard, so cross-platform tools usually have to support several.
What does a .env file actually contain?

A .env file is a flat text file of KEY=VALUE pairs that conventionally lives at the root of a project and holds environment-specific configuration: database credentials, API keys, feature flags, hostnames. It is read by tools like dotenv libraries, Docker Compose, and many process supervisors at startup. The format supports comments starting with #, quoted values to preserve whitespace, and an optional export prefix for shell compatibility.
What is the difference between $VAR and ${VAR}?

In POSIX shell both forms expand the same variable, but they have different parsing rules. The braced form ${VAR} is unambiguous because the braces explicitly mark where the variable name ends, which matters when the variable is immediately followed by alphanumeric characters — for example ${PREFIX}_value versus $PREFIX_value, where the latter would look for a variable named PREFIX_value. The braced form also supports parameter expansion features like default values and substring operations in shells.
Why is deterministic substitution important for builds?

Deterministic means the same inputs always produce the same output, with no randomness or hidden state. Build pipelines and infrastructure-as-code tooling depend on this property because it lets teams verify outputs in code review, reproduce a build from any historical commit, and trust that a config rendered in CI matches what runs in production. Probabilistic tools — including large language models — cannot guarantee this and can quietly introduce inconsistencies between renders.