To do this in-place, just use std::strncmp(). For example:

bool first_n_equal(const char *lhs, const char *rhs, size_t n) {
    return (std::strncmp(lhs, rhs, n) == 0);
}

Note that strncmp returns 0 if the two strings are equal, and will only compare up to n characters. To pick n you can hardcode 6, or loop over your string and check for where the first word ends. For example,

size_t size_of_first_word(const char *str) {
    size_t i;
    for (i = 0; str[i] != ' ' && str[i] != '\0'; i++) {}
    return i;
}

This function loops over the string until it hits a space or a null terminator (end of string). Then, to actually check your string for a command:

size_t input_len = size_of_first_word(input_string);
size_t command_len = size_of_first_word(command_string);
size_t check_len = std::min(input_len, command_len);
bool is_same = first_n_equal(input_string, command_string, check_len);

I intentionally made this verbose to make it easier to understand, so you can absolutely make this code smaller. You could also use std::string but it's not actually necessary.

strcmp compares the whole string, so your function will only return true if the string you're passing is "/".

You can look at strncmp instead:

if(strncmp(str,"/", 1)) ...

or compare only one character:

(if (str[0] == '/')) ...