dotenv

f, err := dotenv.Open(".env")     // a missing file is not an error
if err != nil {
	return err
}

v, ok := f.Get("DATABASE_URL")
f.Has("DATABASE_URL")
f.Keys()      // active keys, in file order, deduplicated
f.Pairs()     // active pairs, in file order, duplicates included
f.Map()       // effective view: last occurrence wins

created := f.Set("API_KEY", "secret")   // updates in place, or appends
f.Unset("OLD_KEY", false)               // comments it out — reversible
f.Unset("OLD_KEY", true)                // removes it outright
err := f.Save()                         // atomic

f := dotenv.Parse("")
f.Append(
    dotenv.NewComment("------------------", "DATABASE", "------------------"),
    dotenv.NewPair("DB_HOST", "localhost"),
    dotenv.NewPair("DB_PORT", "5432"),
    dotenv.NewBlank(),
    dotenv.NewComment("------------------", "AUTH", "------------------"),
    dotenv.NewPair("AUTH_SECRET", "change me"),
)
f.Path = ".env.example"
f.Save()

for _, e := range f.Entries() {
    switch e.Kind {
    case dotenv.KindPair:         // active
    case dotenv.KindDisabledPair: // turned off, but Key and Value are readable
    case dotenv.KindComment:      // prose
    case dotenv.KindBlank:
    case dotenv.KindOther:        // unrecognised, preserved verbatim
    }
}

f.Disabled()   // []Pair of every commented-out setting, in file order