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; begin Put("Printing file "); Put_Line(Filename); Open(File, In_File, Filename); while (not End_Of_File(File)) loop Get_Line(File, Input); Put(' '); -- indent. Put_Line(Input); end loop; Close(File); end Show_File; begin -- procedure Show if Argument_Count = 0 then Put_Line(Current_Error, "Error - No file names given."); Set_Exit_Status(Failure); else -- Open each file and show it. for Arg in 1 .. Argument_Count loop Show_File(Argument(Arg)); 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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