.env Validator

What this tool checks

Paste a .env file (or open it from disk) and it reports, line by line: invalid assignments, invalid key names, duplicate keys, empty values, quoting problems, undefined ${VAR} references and a few things that are valid but surprising.

It is not a secret scanner. It does not look for API keys, and it does not send the file anywhere: the content is parsed in your browser and never stored.

The .env format is not one format

This matters more than it sounds. .env has no single specification, and different tools disagree:

• dotenv (Node) accepts multi-line values inside quotes; Docker Compose does not.
• Compose only expands ${VAR}; a bare $VAR is left alone.
• Some parsers treat # as a comment start anywhere; in others only after whitespace.
• KEY = value with spaces works in some and fails in others.

This validator is explicit about the dialect it implements: the common subset shared by dotenv and Compose env_file. When something falls outside it — multi-line values above all — it says so instead of guessing. A validator that pretends to accept everything is worse than one that tells you where its confidence ends.

Why duplicate keys are the important one

DB_HOST=postgres
DB_HOST=redis

Nothing fails. No warning, no error at runtime: the last value simply wins, and the first line becomes a lie that stays in the file forever. It is the most common cause of “but I changed the variable and it still connects to the old host”. This validator reports it as an error.

Quoting changes the value

A=hello world      # value is: hello world
A="hello world"    # value is: hello world
A='hello world'    # value is: hello world
A="line1\nline2"   # double quotes: \n becomes a real newline
A='line1\nline2'   # single quotes: the backslash-n stays literal

Double quotes process escapes; single quotes are literal. Neither is wrong, but they are different, and mixing them up is how a password with a $ in it ends up expanded and shortened.

References

${VAR} is expanded from the environment. If you reference a variable that is not defined in the file, Compose and dotenv will substitute an empty string — which usually fails later, far from the cause. The validator flags those references. References with a fallback (${VAR:-default}) are accepted without complaint, because they define their own behaviour.

Once the file is clean, the Docker Compose builder can wire it into a service with env_file.