command line arguments

[KBriggs]

Hey all,

I am at a stage learning C where command line arguments are becoming useful, and I understand the basics, but there are a few things that I am missing. For example, why is it comventional to add '-' or '--' flags before most options? How does this aid in processing the arguments?

[MK27]

In a sense, it's just convention. However, if you use getopts() (which I imagine a lot of familiar CLI programs do), then getopts uses the '-' character. There is another very common, slightly more sophisticated function that I have not used which processes long ("--option") and short ("-o") forms, but I cannot remember the name of it...but I am sure it is these two C library functions that are the source of the convention.

Anyway, if you do look up getopt() here's an example you can use at the same time:

#include <stdio.h>
#include <unistd.h>   /* contains getopt() */

int main(int argc, char *argv[]) {
        char options[]="x:y:z:";
        int opt;
        if (argc>1) {
                while ((opt=getopt(argc, argv, options))>0) {
                        if (opt=='?') {puts("Wrong!");return -1;}
                        printf("option: %c argument: %s\n",opt,optarg);
                }        
        }
        return 0;       
}

A normal way to use it is with a "switch (opt) {"

[cmay]

i am not sure i understand the question exactly but i am in the same stage as you.
last night i could not sleep so i wrote this little program to experiment with the options. i never tried use long options before. i try to use the getopt function when ever i can.

but if i was to use getopt function or the argp function i would use it in a slightly larger program and i would think more of the design before writing anything.

as far as i know the options has just a switch '-' or '--' as a UNIX tradition. maybe it is because it was the most easy to parse and became a standard.

however i am not sure of the exact reason. but from, what i understand then in the end it is not needed to use the get opt function for very few options and not all systems has the library but for bigger programs it is the best approach.

/* 
* usage; a simple yes program that print 'y' to stdout unless string giving as second option
* synopsis
* yes [option] 
* yes [string]
* yes  [default , will print'y' to stdout] 
*
* options 
*            --version
*            --help
*            --license 
* 
*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>


void help();
void license();
void version();

int main(int argc, char** argv)
{
   
	int i;
	if( argc == 1)
	{ 
		for(;;)
		   fprintf(stdout,"y\n");
		        
				 
    } 
	     if(argv[1][0] == '-' && argv[1][1] == 'i')
	    {
		       
			 for(;;)
			    fprintf(stdout,"%s\n",argv[2]);
				                  
	    }  
		 
		 for(i=1;i < argc;i++)
		 {
	            
				   
				   if(strcmp(argv[i],"--version") == 0)
				   {
				             version(argv[0]);
				   }
				   
				   else if (strcmp(argv[i],"--license") == 0)
				   {
				            license(argv[0]);
			       }
				   
				   else if (strcmp(argv[i],"--help") == 0)
				  {
					         help(argv[0]);
				
				   } else { 
				    
					          for(;;)
			        	         fprintf(stdout,"%s\n",argv[i]);
				                  
					     }   
	     }
		 
		 
	return 0;
}


void help(const char *progname)
{
	printf("%s: usage no option [option] [string]                        \n"
	       "\t--version                   prints version number to stdout\n"
		   "\t--help                                           this help \n"
		   "\t--license                          prints license to stdout\n"
		   "\tno option prints singel char y to stdout until program killed\n"
		   "\t%s [string]  prints string to stdout until program killed\n",progname,progname);
			exit(0);
}

void license(const char *progname)
{
	fprintf(stdout,"%s\tauthor cmay 2009\n license BSD\n",progname);
	 exit(0);
}

void version(const char *progname)
{
	fprintf(stdout,"%s\tversion 0.0.1\n",progname);
	  exit(0);
}

[MK27]

The GNU "long form" option function is getopt_long()

[cmay]

i found this article which i think is interesting
Command-Line Options

[nonoob]

It's just a matter of convention. A quickie way to differentiate between two types of arguments: file names and flags. Those are the most common items passed.

If you are writing your own program you can decide to have flags preceded by another character and interpret the arguments accordingly, but why change?

[itCbitC]

Hey all,

I am at a stage learning C where command line arguments are becoming useful, and I understand the basics, but there are a few things that I am missing. For example, why is it comventional to add '-' or '--' flags before most options? How does this aid in processing the arguments?

As stated, the reason why options begin with a minus sign is to differentiate them from filenames. There are times when filenames too begin with a minus sign causing much confusion. That's the reason why a double minus "--" was introduced as it signifies the end of options.

some_cmd -o -a -- -inputfile

Here the file name starts with a minus, so to distinguish it from command line options "-o" and "-a" a double minus -- is placed just before its name as it signals the end of options to some_cmd. After seeing the -- some_cmd knows it's done with option processing and whatever comes next is a filename. Ofcourse this is all a convention.

[KBriggs]

That article was interesting, thanks.