How do you configure Uvicorn with environment variables?

Most Uvicorn CLI flags can be set via UVICORN_* prefixed environment variables, making it easy to configure in containerised environments and CI/CD pipelines without changing command-line arguments.

# Environment variable equivalents of CLI flags
export UVICORN_HOST=0.0.0.0
export UVICORN_PORT=8000
export UVICORN_WORKERS=4
export UVICORN_LOG_LEVEL=warning
export UVICORN_LOOP=uvloop
export UVICORN_HTTP=httptools
export UVICORN_RELOAD=false
export UVICORN_NO_ACCESS_LOG=true
export UVICORN_PROXY_HEADERS=true
export UVICORN_FORWARDED_ALLOW_IPS="10.0.0.1"

# Then run with minimal flags:
uvicorn main:app

# Partial override: env vars + CLI flags (CLI takes precedence)
export UVICORN_PORT=8000
uvicorn main:app --port 9000  # uses 9000 (CLI overrides env var)

# WEB_CONCURRENCY is also respected for workers:
export WEB_CONCURRENCY=4
uvicorn main:app

# NOTE: UVICORN_* env vars cannot be set inside --env-file
# --env-file is for configuring your APPLICATION, not Uvicorn itself

Important distinction - --env-file: the --env-file flag loads environment variables into the application process, but UVICORN_* prefixed settings in that file are ignored. The --env-file is intended for your application's configuration (e.g. DATABASE_URL), not for Uvicorn's own settings.

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