Question

Using C Write a program that will serve as a simple shell. This shell will execute an infinite for loop. In each iteration of the loop, the user will be presented with a prompt. When the user enters a command, the shell will tokenize the command, create a child process to execute it and wait for the child process to be over. If the user enters an invalid command, the shell should recognize the situation and show a meaningful message. The shell should also provide a "quit" command that will cause the shell to terminate.

Answer 1

Please look at my code and in case of indentation issues check the screenshots.

-------------shell.c---------------

#include <stdio.h>
#include<stdlib.h>
#include<sys/types.h>
#include<sys/wait.h>
#include<unistd.h>
#include<string.h>

void parseCommand(char *line, char **cmd) //splits the line into words and stores in cmd array
{
   while (*line != '\0') /* if not the end of line ....... */
   {   
       while (*line == ' ' || *line == '\t' || *line == '\n')
           *line++ = '\0'; /* replace white spaces with \0 */
       *cmd++ = line; /* save the argument position */
       while (*line != '\0' && *line != ' ' && *line != '\t' && *line != '\n')
           line++; //skip till end of next argument
   }
   *cmd = '\0'; /* mark the end of argument list */
}

void executeCommand(char **argv) //calls execvp and executes the command
{
   int r = execvp(*argv, argv);
   if(r < 0)
   {  
       printf("*** ERROR: Invalid command, exec failed\n");
       exit(1);
   }
}

int main (int argc, char **argv)
{
   char line[1024]; /* the input line */
   char *cmd[64]; /*to store the command line argument */
   while (1)
   {
       int childPid;
       int status;
       printf("\n$myshell-> "); /* print a prompt*/
       fgets(line, sizeof(line), stdin);
       line[strlen(line)-1] = '\0';
       parseCommand(line, cmd); /* parse the line and store the words in cmd array */

       if (strcmp(cmd[0], "quit") == 0) /* is it a "quit"? */
           break; /* exit if it is */

       childPid = fork(); //creates child

       if (childPid == 0)
       {
           executeCommand(cmd); //calls execvp and executes the command
       }
       else
       {
           waitpid (childPid, &status, 0); //waits for the child
       }
   }  
}

--------------Screenshots--------------

--------------------Output-----------------------

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