Uma brincadeira que eu fiz para resolver uma equação do segundo grau utilizando comandos.

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/* This program solves a quadratic equation */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>

typedef struct s_cmd_line {

    char cmd[4];
    float a;
    float b;
    float c;
          
} t_cmd_line;

t_cmd_line cmd_line;
char line[100]; /* command line */

void stread () {

    int i = 0;
    
    fgets(line,sizeof(line),stdin);

    /* cmd */
    for (i = 0; i < 4; ++i) {
        cmd_line.cmd[i] = line[i];
        line[i] = ' '; /* erase the cmd */
    }

    /* a, b, c */
    sscanf(line, "%f %f %f", &cmd_line.a, &cmd_line.b, &cmd_line.c);
    
}

int main () {

    float a = 0, b = 0, c = 0;  /* coeficients */
    float delta = 0;            /* discriminant */
    float r1 = 0, r2 = 0;       /* roots */

    /* Display */
 
    system("clear");
    printf("\n==================\n");
    printf("Quadratic Equation");
    printf("\n==================\n\n");
    printf("Welcome. Type 'help' for help.\n\n");
   
    /* Commands */
    
    /*
    
        - zero : clear all the variables
        - read %f %f %f : enter new coeficients
        - show : show the equation informed
        - root : find the roots, the parameter
                 shows the result in fractions
        - exit : kill the program
    
    */
    
    while (strcmp(cmd_line.cmd,"exit")) {
        
        printf("> ");
        stread(); /* read the command line */
        
        /* zero */
        if (!strcmp(cmd_line.cmd,"zero")) {
            a = 0;
            b = 0;
            c = 0;
        }
        
        /* read */
        if (!strcmp(cmd_line.cmd,"read")) {
            a = cmd_line.a;
            b = cmd_line.b;
            c = cmd_line.c;
        }
        
        /* show */
        if (!strcmp(cmd_line.cmd,"show"))
            printf("(%f) x^2 + (%f) x + (%f) = 0\n", a, b, c);        
        
        /* root */
        if (!strcmp(cmd_line.cmd,"root")) {
            
            if (a != 0) {

                /* discriminant */
                delta = b * b - 4 * a * c;
                
                r1 = - b / (2*a);
                r2 = r1;
            
                if (delta >= 0) {
                    /* real roots */            
                    r1 = r1 + (sqrt(delta)/(2*a));
                    r2 = r2 - (sqrt(delta)/(2*a));                    
                    printf(" r1 = %f \n r2 = %f\n", r1, r2);
                } else {
                    /* complex roots */
                    delta = - delta;
                    printf(" r1 = %f + i %f \n r2 = %f - i %f\n", r1, (sqrt(delta)/(2*a)), r2, (sqrt(delta)/(2*a)));
                }

            } else { /* a == 0 */
            
                printf("'a' mustn't be zero \n");
            
            }
            
            
        }
        
        /* help */
        if (!strcmp(cmd_line.cmd,"help")) {
            printf("\nCommands\n");
            printf(" - zero : clear all the variables\n");
            printf(" - read %%f %%f %%f : enter new coeficients \n");
            printf(" - show : show the equation informed\n");
            printf(" - root : find the roots\n - exit : kill the program\n\n");
        }
      
    }
    
    printf("\nbye\n\n");
    
    return 0;

}

Utilizando ponteiros na manipulação de strings

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