how to check if a string is a number with a built-in Library?

I tried using the isdigit function but it only works for chars. I know I can write a function that'll check every char in a string but is there an already built-in way in c

#include <cs50.h>
#include <ctype.h>
#include <stdio.h>

int main(int argc, string argv[])
{
    if (argc != 2)
    {
        printf("Usage: ./caesar key");
        return 1;
    }
    else if (isdigit(argv[1]) != 1)
    {
        printf("Usage: ./caesar key");
        return 1;
    }
}

You cannot use isdigit() to way you do in the question: its argument must be a single character (all values of type unsigned char) or the special negative value EOF. Passing a string has undefined behavior.

Your question has many possible answers, depending on what you mean by if a string is a number:

• To check if a string pointed to by a pointer s is composed of digits only using strspn defined in <string.h>:

  if (*s && s[strspn(s, "0123456789")] == '\0') {
      // s is not empty and contains only digits
  }
  

• same test using sscanf defined in `<stdio.h>:

  int n = 0;
  sscanf(s, "%*[0-9]%n, &n);
  if (n > 0 && s[n] == '\0') {
      // s is not empty and contains only digits
  }
  

• same test using isdigit defined in `<ctype.h> in a loop:

  const char *p = s;
  while (isdigit((unsigned char)*p))
      p++;
  if (p > s && *p == '\0') {
      // s is not empty and contains only digits
  }
  

• same test, using strtol defined in <stdlib.h>. Performs the conversion and accepts optional leading whitespace and an optional sign, may set errno in case of overflow (errno is defined in <errno.h>):

  char *p;
  long value;
  errno = 0;
  value = strtol(s, &p, 10);
  if (p != s && *p == '\0') {
      // s contains the decimal representation of an integer
      if (errno) {
          // value is too large for type `long`
      }
  }
  

Here is an alternative using no library function correctly:

#include <cs50.h>
#include <limits.h>
#include <stdio.h>

enum key_result {
   KEY_OK,          // string is a valid integer
   KEY_TOO_LARGE,   // value is outside the range of type `int`
   KEY_EMPTY,       // string is empty
   KEY_NOT_NUMBER,  // string does not contain only digits
};

/* parse the argument as a key, store value into `*dest` */
enum key_result get_key(const char *s, int *dest) {
    enum key_result res;
    int value;

    // string must not be empty
    if (*s == '\0')
        return KEY_EMPTY;

    value = 0;
    res = KEY_OK;
    // consume digits
    while (*s >= '0' && *s <= '9') {
        int digit = *s++ - '0';
        if ((value > INT_MAX / 10)
        ||  (value == INT_MAX / 10 && digit > INT_MAX % 10)) {
            value = INT_MAX;
            res = KEY_TOO_LARGE;
        } else {
            value = value * 10 + digit;
        }
    }
    if (*s != '\0')
        return KEY_NOT_NUMBER;

    *dest = value;
    return res;
}

int main(int argc, char *argv[]) {
    int key = 0;

    if (argc != 2) {
        fprintf(stderr, "Usage: ./caesar key\n");
        return 2;
    }
    switch (get_key(argv[1], &key)) {
      case KEY_OK:
        printf("caesar: key is %d\n", key);
        break;
      case KEY_TOO_LARGE:
        fprintf(stderr, "caesar: key value is too large: '%s'\n", argv[1]);
        return 1;
      default:
        fprintf(stderr, "caesar: key must be a number\n");
        return 1;
    }
    //...
}