Section 9.4 - Package Command_Line

There is a special kind of input and output between the operating system and the Ada program. This input is the information from the command line, and the output is what's called an exit status. This was easy to do in Ada 83 but there wasn't a standard way to do it. Ada 95 has added a standard package to perform this input and output called Command_Line. Command_Line is very similar to C's argc, argv, and exit.

Command_Line provides a few subprograms for input from the operating system. This package defines a function named Argument_Count that returns the number of arguments given to it (it will be zero or higher). It also provides a function called Argument that takes an argument number (an index) and returns that argument's value as a String. It also provides function Command_Name, which returns the name of the program as a String; some operating systems don't support this.

Command_Line provides subprograms for returning a Status_Code to the operating system, which is some sort of integer type. There are two predefined Status_Codes: Success and Failure. You can set the Status_Code using the procedure Set_Status; when the program ends, the last value set in Set_Status will be returned to the operating system.

Here's the definition of Command_Line (from the RM section A.15):

  package Ada.Command_Line is
    function Argument_Count return Natural;
    function Argument (Number : in Positive) return String;
    function Command_Name return String;

    type Exit_Status is implementation-defined integer type;
    Success : constant Exit_Status;
    Failure : constant Exit_Status;
    procedure Set_Exit_Status (Code : in Exit_Status);
    -- ...
  end Ada.Command_Line;

Here's an example of Command_Line; program Show takes as arguments a list of files and prints them out, indented with one space:

with Ada.Command_Line, Text_IO, Ada.Strings.Unbounded, Ustrings;
use  Ada.Command_Line, Text_IO, Ada.Strings.Unbounded, Ustrings;

procedure Show is
-- Take each command line argument as a filename and display
-- each file, indented.

  procedure Show_File(Filename : String) is
  -- Open "Filename" and display it, indented.
    File : File_Type;
    Input : Unbounded_String;
      Put("Printing file ");
      Open(File, In_File, Filename);

      while (not End_Of_File(File)) loop
        Get_Line(File, Input);
        Put(' ');          -- indent.
      end loop;
  end Show_File;

begin -- procedure Show
  if Argument_Count = 0 then
    Put_Line(Current_Error, "Error - No file names given.");
    -- Open each file and show it.
    for Arg in 1 .. Argument_Count loop
    end loop;
  end if;
end Show;

Package Command_Line may not be useful if there's no operating system, since in that case there's nothing to communicate with.

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