6.3 Command-line Arguments

A program can receive arguments from the command line. argc is the argument count, and argv stores those arguments as strings.

int main(int argc, char **argv) {
    if (argc != 2) {
        fprintf(stderr, "Usage: %s file.txt\n", argv[0]);
        return 1;
    }

    printf("processing %s\n", argv[1]);
    return 0;
}

argv[0] is usually the program name, argv[1] is the first user argument, and so on. Every argument is a string, so convert them with atoi() or strtol() when you need numbers.

int times = atoi(argv[1]);   // "3" -> 3

Write error messages to stderr and normal results to stdout. That way users can redirect normal output to a file while still seeing errors in the terminal.

./greet Ada > out.txt