16.c

#include <errno.h>  // Definition for "error handling"
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/wait.h>  // Definition for wait()
#include <unistd.h>    // Definition for fork() and execve()

/* Declarations for getline() */
char * input   = NULL;
size_t capline = 0;  // Capacity

/* Declaration for strtok() */
int   i;
char *token;
char *array[512];

/* Display some info to the user at the start */
void startDisplay()
{
    printf("**************************************************************\n");
    printf("*WELCOME TO THE COMMAND LINE INTERPETER                      *\n");
    printf("*TO RUN A COMMAND SIMPLY TYPE YOUR COMMAND AND PRESS 'ENTER' *\n");
    printf("*EXAMPLE: ls -a, <any linux cmd>                             *\n");
    printf("*Additional: count c/w/l <filename>                          *\n");
    printf("*TO EXIT TYPE 'q' || 'exit'                                  *\n");
    printf("**************************************************************\n");
}

/* Print out "MY_SHELL" */
void displayPrompt()
{
    printf("NewShell$ ");
}

/* Divide input line into tokens */
void makeTokens(char *input)
{
    i     = 0;
    token = strtok(input, "\n ");
    while (token != NULL) {
        array[i++] = token;  // Add tokens into the array
        token      = strtok(NULL, "\n ");
    }
    array[i] = NULL;
}

/* Execute a command */
void execute()
{
    int pid = fork();  // Create a new process
    if (pid != 0) {    // If not successfully completed
        int s;
        waitpid(-1, &s, 0);  // Wait for process termination
    }
    else {
        if (execvp(array[0], array) == -1) {  // If returned -1 => something went wrong! If not then
                                              // command successfully completed */
            perror("Wrong command");          // Display error message
            exit(errno);
        }
    }
}

int main()
{
    startDisplay();
    while (1) {
        displayPrompt();                   // Display a user prompt
        getline(&input, &capline, stdin);  // Read the user input

        if (strcmp(input, "\n") == 0) {
            perror("Please type in a command ");
            continue;
        }

        makeTokens(input);  // Divide line into tokens
        if (strcmp(array[0], "count") == 0) {
            array[0] = "wc";
            if (strcmp(array[1], "c") == 0) {
                array[1] = "-c";
            }
            else if (strcmp(array[1], "l") == 0) {
                array[1] = "-l";
            }
            else if (strcmp(array[1], "w") == 0) {
                array[1] = "-w";
            }
        }

        /* Check if input is "q" | "exit", if yes then exit shell */
        if ((strcmp(array[0], "q") == 0) || (strcmp(array[0], "exit") == 0)) {
            printf("SYSTEM : Shell is exit\n");
            return 0;
        }
        execute();  // Call execvp()
    }
}