Check if string is empty, then continue - c

Question

I'm creating a very basic shell. When the user hits enter without typing any commands, the program should go to next line and continue.
If the user type any command, the program should print a text message.
I'm having issues with the empty string condition.
Tried strcmp() with another char array.
#include <stdio.h>
#include <string.h>
int main(void){
char input[256];
char str[4] = {"exit"};
do
{
printf("CSC327> ");
scanf("%[^\n]", &input);
if(getchar() == '\n')
continue;
if(strcmpi(input, str))
printf("Command not found!\n");
}while(strcmpi(input,str));
printf("\nGoodbye.");
return 0;
}

Answer 1

If you want to check if a string is empty, just use
int lenght = strlen(your_string)
The length of the string is returned. If it is 0, the string is empty.

Answer 2

First the working version of code here!.
Code
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int my_strcmp(char *a, char *b)
{
while(*a == *b){
a++;
b++;
}
///check line ending
if (*a == '\n' && *b=='\0')
return 0;//are equal
return 1;
}
int main(void)
{
char input[256]; //Array of characters for user input
int flag = 1;
do
{
printf("CSC327> ");
fgets(input, 256, stdin); //Read user input
if(*input == '\n')
continue;
if(my_strcmp(input, "exit")){ //Compare temp with 'exit' case insensitive
printf("Command not found!\n");
}
else{
printf("Ok! Exiting\n");
flag=0;
}
}while(flag);
printf("\nGoodbye.");
return 0;
}
Main problem is in strcmp() function. I prefer to write a simple version under the name of my_strcmp() to work exactly in this question.
Consider last character of string source and destination.
source ending is '\n' and destination is '\0'.
An ugly version to do this comparison used here. my_strcmp() can be very better but now just enough.
You call strcmp() two times, by using a flag one of them can be remove.
As Steve Summit say at comment we can use if(*input == '\n') continue; instead of your way.
Related link about writing a shell
Tutorial Write a Shell in C by Stephen Brennan, GitHub
Making your own Linux Shell in C
Want to build a simple shell? Here’s how you do it.
In brenns shell Consider line 187 if (position >= bufsize) as Steve Summit say in comment you must handle a situation if user input a larger buffer input.
Take a look at ASCII table that guide me to find out why builtin strcmp() not work properly because of different line ending 10 vs. 0.
Edit
Use '\n' instead of 10 as suggested in comment
my_strcmp() not work as case intensive compare I will improve it soon.
fgets() add \n line feed character at the end of inputted string. i.e: input[last]= '\n'. But char s[] = "test" adds '\0'
at the end of string s. For this we cant compare simply these string with library functions for example strcmp and others.
change this character and use library function strcasecmp()
By this do while loop can be rewrite
do
{
printf("CSC327> ");
fgets(input, 256, stdin); //Read user input
//fgets add \n at the end of string
/// "salam" add \0 at end of string
///change last char
input[strlen(input)-1]='\0';
//printf("len: %ld", strlen(input));
if(*input == '\n')
continue;
if(strcasecmp(input, "exit")){ //Compare temp with 'exit' case insensitive
printf("Command not found!\n");
}
else{
printf("Ok! Exiting\n");
flag=0;
}
}while(flag);
printf("\nGoodbye.");
return 0;
}

Answer 3

int main()
{
char input[256]; //Array of characters for user input
char temp[4]; //Array of 4 characters for getting the first 4 characters of input
do
{
printf("CSC327>");
fgets(input, 256, stdin); //Read user input
strncpy(temp, input, 4); //Get first 4 characters of input
if(strlen(input) == 1) //Determine if input is empty
continue;
if(strcmpi(temp, "exit")) //Compare temp with 'exit' case insensitive
printf("Command not found!\n");
}
while(strcmpi(temp, "exit"));
printf("\nGoodbye.");
return 0;
}//main ends