command_line_options.md

##Command Line Options

Command line options are flags or configurations that are listed after the name of the program when you call it on the command line. The -a option can be used with the ls program to show even hidden files:

ls -a

##Command Line Arguments

When your program only needs one or more strings of the same type, (a list of filenames to read from, or a name to use for saving output), then use command line arguments. An example of this would be when you specify the name when calling the mkdir program from the command line:

mkdir my_file

Reading Command Line Arguments

To access the command line arguments, your [main function][] must take parameters:

int main(int argc, char *argv[])
{
 //...
 // Access elements of argv as follows:
 argv[0] // Will be the name of your program as called on the command line
 argv[1] // Will be the first command line argument
 argc // Gives the length of the argv array
}

Reading Command Line Options

int main(int argc, char *argv[])
{
  // You can loop through the options using getopt
  // "ae:" here are the allowed command line options
  // the ":" means that it takes an additional argument
  while ((ch = getopt(argc, argv, "ae:")) != EOF) {
    switch(ch) {
      case 'a':
        // do something
        break;
      case 'e':
        // do something with optarg
        break;
      default:
        fprintf(stderr, "unknown option: '%s'\n", optarg);
        return 1;
    }

    // The following incrementations allow you to cycle through the options:
    argc -= optind;
    argv += optind;
  }
}

Command Line Options	Command Line Arguments
Configurations, settings	Provides variable data to the program (often filenames)
Listed first	Listed second, after the command line options (if any)
Prefaced with a '-'	No special format (Optionally preface with '---' to separate from options)

Separating the Command Line Options and Command Line Arguments

The program identifies the command line options by their leading -, but what if you need to pass a commend line argument (not an option) that begins with a -?

In that case, use -- to separate the command line options from the command line arguments.

> ./set_temperature -c -- -4