Goのflagパッケージで最高のusageを出力する方法

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デフォルト値

ゼロ値でなければデフォルト値を出力できる。

flag.Var(&overwriteFields, "replace", "field-value expression")
flag.Usage = func() {
  w := flag.CommandLine.Output()
  fmt.Fprintln(w, "usage:", progname, "[options] [file ...]")
  fmt.Fprintln(w, "\noptions:")
  flag.PrintDefaults()
}
flag.Parse()