C in 100 Seconds: Command Line Arguments

Command Line Arguments

Every C program can accept input from the command line through two parameters in main: argc (argument count) and argv (argument values).

argc and argv

int main(int argc, char *argv[]) {
  printf("argc: %d\n", argc);
  for (int i = 0; i < argc; i++) {
    printf("argv[%d] = %s\n", i, argv[i]);
  }
}

argc is always at least 1 because argv[0] is the program name itself. Every argument after that is a string in the argv array.

Parsing Flags

for (int i = 1; i < argc; i++) {
  if (strcmp(argv[i], "-v") == 0) {
    verbose = 1;
  } else if (strcmp(argv[i], "-n") == 0 && i + 1 < argc) {
    count = atoi(argv[++i]);
  } else {
    name = argv[i];
  }
}

Loop from index 1 to skip the program name. Use strcmp to match flags and atoi to convert string arguments to integers.

Two Runs

With no arguments: argc is 1, defaults apply. With ./args Alice -n 3 -v: argc is 5, the name is Alice, count is 3, and verbose is on.

Student Code

Try it yourself: episode38/args.c