.env file syntax

oneenv supports the syntax you expect from a mature .env parser:

# A comment line.
export PATH_STYLE=ok            # "export " prefix is allowed; inline comments too

PLAIN=value
QUOTED="double quoted"          # escapes: \n \r \t \" \\
RAW='single quoted'             # no escapes, no expansion — taken literally
MULTILINE="line one
line two"                       # newlines allowed inside double quotes

GREETING="Hello ${USER}"        # ${VAR} / $VAR expansion — only with WithExpand()
LITERAL='$NOT_EXPANDED'         # single quotes never expand
  • Comments — a # starting a line, or preceded by whitespace on a value line.
  • export prefix — accepted and ignored, so you can source the same file.
  • Quotes — double quotes honour escapes and can span multiple lines; single quotes are literal.
  • Expansion — ${VAR} and $VAR are expanded (when WithExpand() is set) against values already parsed in the file, falling back to the process environment. Write $$ for a literal $.

Syntax errors come back as a *ParseError carrying the file name and line number.

Environment-aware file cascade

WithEnvFiles() layers files by the active environment, the convention used by Rails, Next.js and dotenv-cli. On top of each base file it also reads <base>.local, <base>.<env> and <base>.<env>.local, each optional, later files overriding earlier keys:

cfg, err := oneenv.Parse[Config](oneenv.WithEnvFiles())
// with APP_ENV=production, reads in increasing priority:
//   .env  →  .env.local  →  .env.production  →  .env.production.local

The environment name comes from APP_ENV, then GO_ENV; change the sources with WithEnvVar("MY_ENV"). FilesFor(opts...) returns the resolved list.