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Command-Line Arguments

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CLA

% c11 -o getAvg getAvg.c
% ./getAvg 98 95 89 90 92 83
The average is 91.2
% ./getAvg 91.2 77 90 68 91
The average is 81.5

• Command-line arguments are data sent to the program when you run it
• Programs execute with different data but you needn’t change the program!
• This is how options to commands work, e.g., ls -l foo

How to handle command-line arguments

#include <stdio.h>
int main(int argc, char *argv[ ]) {
    return 0;
}

• Command-line arguments are passed to main()
• Two arguments are sent to main:
  • the number of command line arguments
  • an array of character strings with the command-line args
• By convention, we name these formal parameters as:
  • argc (number of arguments)
  • argv (array of arguments)
• The name of the program is in argv[0]

Display the command line arguments

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

% ./a.out
The 1 arguments are:
0 a.out
% a.out 98 Cookie Monster rules
The 5 arguments are:
0 a.out
1 98
2 Cookie
3 Monster
4 rules

Example: getAvg.c

#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[]) {
    int sum=0;
    for (int i=1; i<argc; i++)
        sum += atoi(argv[i]);
    printf("The average is: %f\n", sum / (argc - 1.0));
    return 0;
}

% gcc -o getAvg getAvg.c
% ./getAvg 98 95 89 90 92 83
The average is: 91.166667
% ./getAvg 77 90 68 91
The average is: 81.500000

Errors

• If the program should have a fixed number of arguments, then it’s your job to enforce that.
• Do it like the other Unix commands do: emit a usage message.

#include <stdio.h>
int main(int argc, char *argv[]) {
    if (argc != 3) {
        fprintf(stderr, "Usage: %s <minimum> <maximum>\n", argv[0]);
        return 1;
    }
    // your code here
    return 0;
}

Handling arguments

• How to handle command-line arguments in any order
  • Use a loop to walk through list of args
  • Use if statements with strcmp for each expected argument

Handling arguments

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

int main(int argc, char *argv[]) {
    for (int i=1; i<argc; i++) {
        if (strcmp(argv[i], "-h") == 0) // do stuff for -h option
            printf("Help is on its way!\n");
        else if (strcmp(argv[i], "-n") == 0) {
            // now we know the next cmd-line arg is our number
            int num = atoi(argv[++i]);
            printf("Your lucky number is %d\n", num);
        }
        else {
            fprintf(stderr, "Unknown option \"%s\".\n", argv[i]);
            return 2;
        }
    }
    return 0;
}

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