Reputation: 141

Reading command line arguments for flags and files C

Hello I'm new with C and I'm trying to start this lab where the command syntax is to first list any command line flags then list one or more file names.

I'm having trouble organizing how I want to scan the input arguments and differentiating them between flags and file names.

I thought of doing a loop to see if the argument is a flag or file name. But I'm unsure how to begin implementing it. Since the first 4 arguments can be potential flags in any order then anything after is the file name. But it is also possible for no flags to be given and just start with filenames. I don't know at all how to initalize this loop to go through each argument 1 by 1. Can anyone help please?

Example of possible command line arguments:

wc fcopy.c head.c (no flags given just file names)
wc -l -c -w -L fcopy.c head.c a.exe (flags given and multiple files)
wc -l -w -c -L fcopy.c (flags given and 1 file name)

Upvotes: 1

Answers (4)

Mike

Reputation: 49463

Reading command line args one by one is pretty simple:

int main( int argc, // Number of strings in array argv char *argv[])

The two basic args to a C function are argc (int number of args), and argv (an array of strings for the args)

The first string is always the name of the program that is running, after that is the args that were passed over the command line.

if(argc > 1)
    for( count = 1 count < argc; count++ )
        printf("%s", argv[count]);

Will display to the screen each of the arguments passed in past the name of the calling program. I assume your flags are passed with a "-" as the first character? If so, you can check the contents of the first character in each string for a '-' to tell if its a flag or not.

Upvotes: 2

user1129665

Reputation:

Check out getopt, it allows you to parse command line arguments and their flags:

#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main (int argc, char **argv)
{
  int aflag = 0;
  int bflag = 0;
  char *cvalue = NULL;
  int index;
  int c;

  opterr = 0;

  while ( (c = getopt (argc, argv, "abc:")) != -1 )
    switch (c) {
      case 'a':
        aflag = 1;
        break;
      case 'b':
        bflag = 1;
        break;
      case 'c':
        cvalue = optarg;
        break;
      case '?':
        if (optopt == 'c')
          fprintf (stderr, "Option -%c requires an argument.\n", optopt);
        else if (isprint (optopt))
          fprintf (stderr, "Unknown option `-%c'.\n", optopt);
        else
          fprintf (stderr,
                   "Unknown option character `\\x%x'.\n",
                   optopt);
        return 1;
      default:
        abort ();
      }

  printf ("aflag = %d, bflag = %d, cvalue = %s\n",
          aflag, bflag, cvalue);

  for (index = optind; index < argc; index++)
    printf ("Non-option argument %s\n", argv[index]);
  return 0;
}

Example of Parsing Arguments with getopt

Upvotes: 2