如何使用 scanf 和打印目录（如终端）仅检测新行输入

Question

I wanna reproduce the terminal behavior when the input is just a new line (keeps printing the same string), but don't know how to do it.

Example: When the user just inputs a new line, the terminal keeps printing the directory, until a real command is inserted

int main()
{
    char userInput[1024];
    while (1)
    {
        printf("directory »» ");
        scanf("%[^\n]" , userInput); // This scanf doesn't work
        while (userInput[0] == '\n')  // If the input is only a new line char, keep asking for more inputs and printing the directory
        {
            printf("directory »» ");
            scanf(" %[^\n ]" , userInput); // This scanf doesn't work
        }

        //Input isn't a NewLine, process the input
        process_Input_Function(userInput); //Isn't empty, search for my created commands
    }
}

At the first enter press, it enters the loop, reproduce 1 time, and then the scanf doesn't detect new lines anymore, it just skips and waits to a real string. What can i type inside of the scanf to detect a new line input and keep printing that string till a real command is inserted?

I tried with scanf("%c"...) but the problem with a char, is that i can't process the whole string command, if isn't empty

Answer 1

First of all, your two scanf calls are different. The first one is

scanf("%[^\n]", userInput);

which looks for anything that's not a newline, as you wish to do.

But the second one is

scanf(" %[^\n ]", userInput);

which is also looking for a space before the input, followed by any character that is also not a newline or a space . Thus, scanf is waiting for the space.

---

IMHO, the best way to recreate this behavior is going to be in the parsing step, after you have gotten the command from the command line. Essentially, your command input loop would look like this:

char *userInput = NULL;
size_t n = 0;
while (true) {
    // print the prompt
    printf(">");
    // get the line
    ssize_t userInputLength = getline(&userInput, &n, &stdin); 
    // parse the input, using a function you wrote elsewhere
    parse(userInputLength, userInput);
}

(Note the use of POSIX getline() instead of scanf . This is a more recent standard library function that does exactly the task of getting a line of user input, and also allocates the buffer using malloc and realloc so that you don't have to care about buffer overflows or even sizing the buffer at all.)

The user input function wouldn't care that the userInput portion was blank. The function that would care is the parse function, which will simply interpret a blank userInput string as "do nothing" and continue on its merry way.

Answer 2

Hmm, the code I gave pretty much does that with one exception, it doesn't display a prompt each time...

Is this what you mean:

#include <stdio.h>
#include <stdlib.h>
#include <string.h> // For the memset()

int main() {

    char userInput[1024];

    while (1) {
        printf("»»» ");

        fgets(userInput, 1024, stdin);

        while (userInput[0] == '\n')
        {
            printf(">>> ");
            memset(userInput, '\0', 1024);
            fgets(userInput, 1024, stdin);
        }

        // Your command can be accessed from here //
        printf("Command entered: %s\n", userInput);

        printf("Input isn't a NewLine\n");
    }

}

I changed the scanf() to fgets() to read from stdin so that we don't overwrite the buffer.