Command Line Arguments

• Read Appendix B, Kerrisk.
• It is nice to write a program that requires no user interaction when automating tasks.
• But it is also common to write a program which can perform the same task is different ways.
• Or even different tasks.
• A standard way to accomplish this is provide command line arguments.
  • g++ -o executable filename.C
  • ls -alrt
  • dir /w
• C and hence C++, as well as most scripting languages provide for command line arguments.
• These are presented as three optional arguments to main()
  • int main(int argc, char * argv[], char * envp[])
  • int main(int argc, char ** argv, char ** envp)
  • argc is the number of command line arguments.
  • argv is an array of c style strings containing the command line arguments.
    • argc will always be at least 1
    • argv[0] is the name of the executable.
  • envp is a null terminated array of strings containing environment variables.
    • Environment variables are in the form VARIABLE_NAME=VARIABLE_VALUE
    • And the values are always strings.
• Standard practice for processing these by hand

  for(i=0;i<argc;i++) {
      cout << i << argv[i] << endl
  }
  
  for(i=0;envp[i]!=NULL;i++) {
      cout << i << envp[i] << endl
  }
     

• Write a program to print the command line arguments and environment variables.
• Notes:
  • Visual c++ wanted to make a tmain(), not main(), I had to change this.
  • Visual wanted to include some local stdafx.h file to optimize compiling, I went to project options->C/C++ -> Precompiled Headers and turned this off
  • 
  • To pass command line arguments in Visual C++, go to properties->Configuration Properties->Debugging and enter the values into Command Arguments
  • 
  • Or just run it in the command line prompt.
    • 
    • 
• Checking an argument
  • I generally use strcmp, the c string comparison function.
  • This comes from #include &tl;string.h>
  • strcmp(a,b)
    • Returns negative if a is "smaller" than b.
    • Returns 0 if they are the same.
    • Returns positive if a is "larger" than b.
  • Easy: only print the environment variables if the first real command line argument is a -e

  • if(argc > 1 && !strcmp(argv[1],"-e")) {
        // code to print environment variables.
    }
           

  • You could convert this to a c++ string and do a comparison as well.
    • But strcmp is very easy to use
• A better way

  • 	bool showArgs=false, showEnv=false;
    ...
    	for(i=1;i<argc;i++) {
    		if (!strcmp(argv[i],"-e")) {
    			showEnv = true;
    		} else if (!strcmp(argv[i],"-a")) {
    			showArgs=true;
    		}
    	}
    ....
            if(showEnv) {
                 // code to print environment variables.
            }
           

  • You might not want the i++ in the for loop
    • For example -o executable
    • In this case, when you see the -0
      • You would test to see if another argument exists (i > argc)
      • If it does grab this and increment i twice.
      • Otherwise print an error

      • } else if (!strcmp(argv[i],"-o")) {
             i++;
             if (i < argc) {
                  filename = argv[i];
        	  i++;
             } else {
                  cout << "the option -o requires a file name (string), none given" << endl; 
             }
        }
        		

      • This is not the greatest error message, but we will look at this later.
    • You might also want a default else which prints an error if the argument is unknown.
    • In the case of numerical arguments, use atoi or atof.
• An even better way: getopt, appendix B, Kerrisk.
  • But that is unix/linux only default.
  • But there are third party windows code to do this as well.
  • We will look into this later.
• Parsing the environment variables.
  • A 130 string problem.
  • Find the = sign.
  • Everything to the left is the variable name.
  • Everything to the right is a value
  • atoi, atol, atof, if needed.
  • I use c++ strings for this type of function.
  • But there are c string library functions to do this too
    • strchr, index, strtok
    • We may look at these later as well.

  • 	for(i=0;envp[i]!=NULL;i++) {
    	    line = envp[i];
    		pos = line.find('=');
    		var = line.substr(0,pos);
    		value = line.substr(pos+1,line.size());
    		
    		if ((var=="USERNAME")  ||(var=="USER")) {
    			cout << endl;
    			if(value =="dbennett" || value=="bennett") {
    				cout << "Hi Dan!" << endl;
    			} else {
    			    cout << "Forget the world, Hello " << value << endl;
    			}
    		}
    	}
           

• This code works on both linux and windows.