Question

I am trying to create a simple shell in Unix. I read a lot and found that everybody uses the strtok function a lot. But I want to do it without any special functions. So I wrote the code but I can’t seem to get it to work. What am I doing wrong here?

void process(char**);
int arg_count;
char **splitcommand(char* input)
{
    char temp[81][81] ,*cmdptr[40];
    int k,done=0,no=0,arg_count=0;
    for(int i=0 ; input[i] != '\0' ; i++)
    {
        k=0;
        while(1)
        {
            if(input[i] == ' ')
            {
                arg_count++;
                break;
            }
            if(input[i] == '\0')
            {
                arg_count++;
                done = 1;
                break;
            }
            temp[arg_count][k++] = input[i++];
        }
        temp[arg_count][k++] = '\0';
        if(done == 1)
        {
            break;
        }
    }
    for(int i=0 ; i<arg_count ; i++)
    {
        cmdptr[i] = temp[i];
        cout<<endl;
    }
    cout<<endl;
}


void process(char* cmd[])
{
    int pid = fork();
    if (pid < 0)
    {
        cout << "Fork Failed" << endl;
        exit(-1);
    }
    else if (pid == 0)
    {
        cout<<endl<<"in pid";
        execvp(cmd[0], cmd);
    }
    else
    {
        wait(NULL);
        cout << "Job's Done" << endl;
    }
}


int main()
{
    cout<<"Welcome to shell !!!!!!!!!!!"<<endl;
    char input[81];
    cin.getline(input,81);
    splitcommand(input);
}

Answer 1

The problem is with the

arg_count++;  

inside the if(input[i] == ' ') and if(input[i] == '\0')

when you are parsing the command line and you find a space or you reach the end of the command line you are increment arg_count before you put a \0 at the end of the command you were reading.

So change it to:

        if(input[i] == ' ')  
        {  
            // arg_count++;  REMOVE THIS.
            break;  
        }  
        if(input[i] == '\0')  
        {  
            // arg_count++;  REMOVE THIS.
            done = 1;  
            break;  
        }  
        temp[arg_count][k++] = input[i++];  
    }  
    temp[arg_count][k++] = '\0'; // add null-char at the end. 
    arg_count++; // increment should happen here.

More bugs:

• You are not returning anything from
  splitcommand
• You cannot just return cmdptr
  because they point to local char
  arrays(temp) which will not persist
  after the function returns. So you’ll
  have to make sure that the array
  temp persists even after function
  call by allocating it dynamically or
  making it global.
• Arguments to execvp look good to
  me. Others please take a look.