使用標準函式庫讀取環境變數

任務

未使用任何套件

程式碼
(* Both `Sys.getenv` and `Sys.getenv_opt` are functions that take the name of an environment and read its value. `Sys.getenv` returns the value directly, but raises a `Not_found` exception if the variable doesn't exist. ) let () = try let path = Sys.getenv "PATH" in Printf.printf "The path is %s\n" path with Not_found -> print_string "The path is not set.\n" ( In contrast, `Sys.getenv_opt` returns a value of type `string option`: `Some value` if the variable exists and `None` if it doesn't. *) let () = match Sys.getenv_opt "API_KEY" with | Some p -> Printf.printf "Api key is %s\n" p | None -> print_string "Api key is not set.\n"

Sys.getenv 和 Sys.getenv_opt 都是函式，它們會接收環境變數的名稱並讀取其值。

Sys.getenv 會直接回傳值，但如果變數不存在，則會引發 Not_found 例外。

let () =
  try
    let path = Sys.getenv "PATH" in
    Printf.printf "The path is %s\n" path
  with Not_found ->
    print_string "The path is not set.\n"

相反地，Sys.getenv_opt 會回傳 string option 型態的值：如果變數存在則回傳 Some value，如果不存在則回傳 None。

let () =
  match Sys.getenv_opt "API_KEY" with
  | Some p ->
      Printf.printf "Api key is %s\n" p
  | None ->
      print_string "Api key is not set.\n"

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