Handling command line flags in C/C++

Question

I am looking for a very simple explanation/tutorial on what flags are. I understand that flags work indicate a command what to do. For example:

rm -Rf test

I know that the rm command will remove the test folder and that the -Rf flags will force the command to erase not just the folder but the files in it.

But, where are the flags read/compiled??? What handles the flags? Can I, for example, write my own C/C++ program and designate different flags so that the program does different things? I hope I am asking the right questions. If not, please let me know.

Answer 1

At the C level, command line arguments to a program appear in the parameters to the main function. For instance, if you compile this program:

#include <stdio.h>
int main(int argc, char **argv)
{
    int i;
    for (i = 0; i < argc; i++)
        printf("argv[%d] = %s\n", i, argv[i]);
    return 0;
 }

and invoke it with the same arguments as your example 'rm' command, you get this:

$ ./a.out -Rf test
argv[0] = ./a.out
argv[1] = -Rf
argv[2] = test

As you can see, the first entry in argv is the name of the program itself, and the rest of the array entries are the command line arguments.

The operating system does not care at all what the arguments are; it is up to your program to interpret them. However, there are conventions for how they work, of which the following are the most important:

• Arguments are divided into options and non-options. Options start with a dash, non-options don't.
• Options, as the name implies, are supposed to be optional. If your program requires some command-line arguments to do anything at all useful, those arguments should be non-options (i.e. they should not start with a dash).
• Options can be further divided into short options, which are a single dash followed by a single letter (-r, -f), and long options, which are two dashes followed by one or more dash-separated words (--recursive, --frobnicate-the-gourds). Short options can be glommed together into one argument (-rf) as long as none of them takes arguments (see below).
• Options may themselves take arguments.
  • The argument to a short option -x is either the remainder of the argv entry, or if there is no further text in that entry, the very next argv entry whether or not it starts with a dash.
  • The argument to a long option is set off with an equals sign: --output=outputfile.txt.
• If at all possible, the relative ordering of distinct options (with their arguments) should have no observable effect.
• The special option -- means "do not treat anything after this point on the command line as an option, even if it looks like one." This is so, for instance, you can remove a file named '-f' by typing rm -- -f.
• The special option - means "read standard input".
• There are a number of short option letters reserved by convention: the most important are
  • -v = be verbose
  • -q = be quiet
  • -h = print some help text
  • -o file = output to file
  • -f = force (don't prompt for confirmation of dangerous actions, just do them)

There are a bunch of libraries for helping you parse command line arguments. The most portable, but also the most limited, of these is getopt, which is built into the C library on most systems nowadays. I recommend you read all of the documentation for GNU argp even if you don't want to use that particular one, because it'll further educate you in the conventions.

It's also worth mentioning that wildcard expansion (rm -rf *) is done before your program is ever invoked. If you ran the above sample program as ./a.out * in a directory containing only the binary and its source code you would get

argv[0] = ./a.out
argv[1] = a.out
argv[2] = test.c

Answer 2

GNU libc, which is very likely available on your system, has a library for this called getopt that can be used to parse the options in a sensible fashion. There are examples to get you started in the documentation linked below.

http://www.gnu.org/software/libc/manual/html_node/Getopt.html#Getopt

Answer 3

Actually you can write your own C++ programm which accepts commandline parameters like this:

int main(int argc, char* argv[]){}

The variable argc will contain the number of parameters, while the char* will contain the parameters itself.

You can dispatch the parameters like this:

for (int i = 1; i < argc; i++)
{  
    if (i + 1 != argc)
    {
        if (strcmp(argv[i], "-filename") == 0) // This is your parameter name
        {                 
            char* filename = argv[i + 1];    // The next value in the array is your value
            i++;    // Move to the next flag
        }
    }
}

Answer 4

This simple program should demonstrate the arguments passed to the program (including the program name itself.)

Parsing, interpreting and using those arguments is up to the programmer (you), although there are libraries available to help.

int main(int argc, char* argv[])
{
    int i;
    for(i=0; i<argc; ++i)
    {   printf("Argument %d : %s\n", i, argv[i]);
    }
    return 0;
}

---

If you compile this program into a.out, and run it as:

prompt$>  ./a.out ParamOne ParamTwo -rf x.c

You should see output:

Argument 0 : a.out
Argument 1 : ParamOne
Argument 2 : ParamTwo
Argument 3 : -rf
Argument 4 : x.c

Answer 5

flags are arguments passed into the main entry point of the program. For example, in a C++ program you can have

int main(int arc, char* argv[]){
return 0;
}

your arc is the # of arguments passed in, and the pointer gives u the list of actual arguments. so for

rm -Rf test

argc would be 3, and the argv array would contain your arguments. Notice argc >= 1 because the program name itself counts (rm). -RF is your 2nd parameter and test is your third.

So whenever you are typing commands in unix, you essentially are executing programs and passing them parameters that they operate on.

If you are really REALLY interested in the unix OS, you should look up forks and how they work. This can get pretty confusing to a newcomer though, so only if you are really interested in OS and how programs are executed.

Answer 6

In your own C program you can process command line options in any way you see fit. Command line parameters in C come in the parameters of the main(int argc, char *argv[]) method as strings.

And if you'd like to process command line parameters in a way similar to most UNIX commands, the function you're probably looking for is getopt()

Good luck!