8.2.4. Command Line Arguments

Command-line arguments were a staple of the old command-line interfaces (CLI) of DOS, Unix, and similar operating systems. Although most users now interact with computers through a graphical user interface (GUI), most modern operating systems still provide a CLI. Windows has two CLIs: the "Command Prompt" and the "Power Shell," and Linux has a "Terminal." A full command line includes a command's name (often a program a user wishes to run) and some optional command arguments. Suppose we rewrite the name_box program from the last demonstration as a CLI program. The command line would give the name of the program and could provide the full name around which the user wants to draw the box:

Windows cmd.exe	Linux MATE Terminal

Today, many will argue that the widespread use of GUIs leaves the old CLIs obsolete and a waste of time. If you are an end-user who only runs application programs, you will likely never need to use and, therefore, never need to know about the command line. If, on the other hand, you are a software developer, then you should consider the following:

1. Launching a program by clicking on an icon does use the command line. Each icon contains command-line information, but it's hidden in the icon's details. Hiding CLI information in an icon makes it easier for everyday users to run programs, but it implies that application developers must understand the CLI even in a predominantly GUI environment.
2. Whenever the operating system spawns a new process (better known as running a program), it locates the executable file (the program) and passes information into it through the CLI. You must know about command-line arguments if you create a program that launches another program, which is not uncommon in "real world" programming.
3. The operating system automatically starts some programs (e.g., file and web servers) when the computer boots up, and they run without communicating directly with a user. These programs do not have or need a GUI.
4. Although the list of computer maintenance and administration programs is continuously shrinking, there are still important there are still some that do not have a GUI.

In summary, if you only use a computer to surf the web, interact with friends through social media, run office automation software, post pictures of your grandkids, etc., you probably don't need to know about the command line. However, if you are a computer professional, knowing how to use the CLI and command-line arguments is a necessary, basic skill.

Processing Command Line Arguments

In C++, accessing the command line arguments begins with adding two arguments to function main. Remember, it is the operating system that "calls" your program, and so it is the operating system that will pass data into main through the arguments. Note that the order in which the arguments appear is important. Like any function, you may choose the names for your function arguments; however, argc and argv are traditional names, and I recommend that you use them unless there is a compelling reason to choose different names.

C:\Users\dab\>name_box Cranston Q. Snort	C:\Users\dab\>.\name_box Cranston Q. Snort
(a)	(b)
main(int argc, char* argv[])
(c)
(d)

Command Line Arguments: Example Code

#include <iostream>
using namespace std;

int main(int argc, char* argv[])
{
	cout << "argc is " << argc << endl;
	for (int i = 0; i < argc; i++)
		cout << "argv[" << i << "] is " << argv[i] << endl;

	return 0;
}

public class CommandLine
{
	public static void main(String[] args)
	{
		System.out.println("args.length is " + args.length);
		for (int i = 0; i < args.length; i++)
			System.out.printf("args[%d] is %s%n", i, args[i]);
	}
}