Question

I am writing a shell program in C++, to run this program I would run it in terminal like the following: ./a.out "command1" "command2"

using the execv function how to execute command 1 and 2 if they are stored in argv[1] and argv[2] of the main function?

Answer 1

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

---------main.cpp---------------

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

using namespace std;

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 = NULL; /* mark the end of argument list */
}

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

int main(int argc, char *argv[])
{
   if(argc !=3)
   {
       cout << "Usage ./a.out command1 command2 " << endl;
       return -1;
   }
   char *cmd1[10]; //to store command words
   char *cmd2[10]; //to store command words
   int status;

   parseCommand(argv[1], cmd1);           //parse the argv[1] command and store the words in cmd1 array
   parseCommand(argv[2], cmd2);           //parse the argv[2] command and store the words in cmd2 array

   int pid1 = fork(); //forks to create a child
   if(pid1 == 0) //CHILD1
   {
       cout << "\nChild Process, pid = " << getpid() << " executes " << argv[1] << endl;
       executeCommand(cmd1); //calls execvp and executes the command
   }
   else //PARENT
   {
       cout << "\nParent Process, pid = " << getpid() << endl;
       waitpid (pid1, &status, 0); //waits for the child process to terminate

       int pid2 = fork(); //forks to create another child
       if(pid2 == 0) //CHILD2
       {
           cout << "\nChild Process, pid = " << getpid() << " executes " << argv[2] << endl;
           executeCommand(cmd2); //calls execvp and executes the command
       }
       else //PARENT
       {
           cout << "\nParent Process, pid = " << getpid() << endl;
           waitpid (pid2, &status, 0); //waits for the child process to terminate
       }
   }

   return 0;
}

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

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

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