C编程fork（）多个管道（）

Question

Im simulating a shell using pipes() forks() exec() dup(). Ive seen a few posts on stackoverflow to guide be along the way. But my prog seems to have similar issues as others have encountered here.

I use a LinkedList struct contains: char* cmd and char** args (ex. cmd='ls' args = '-l -d')

Here are some output results: ls -l ls -l -a (as many args I want) ls -l | sort

ls -l | wc -w (runs but spits out wrong value)
ls | wc -w (runs, but spits out the wrong value)
ls (alone - no args, spits out A NULL argv[0] was passed through an exec system call.)
ls -l | sort | wc -w (causes system to hang)

It now at least pretend like it is taking the multiple args but invalid results or system hanging.

void runCommand(Node* head)
{
    int old_fd[2], new_fd[2];
    int isValid, cpid, pid;

    //Loop through all commands
    for(int cmd = 0; cmd < cmdCount; cmd++)
    {
        //if(curr cmd has next cmd)
        if(cmd+1 < cmdCount)
        {
            //Create pipe new_fd
            if( pipe(new_fd) == -1) //pipe error
            {
            perror("Pipe Error.");
            exit(-1);
            }
        }

        //Parent
        if( (pid=fork()) != 0 ) //parent
        {
            //Wait for child process
            //wait(0); //moved below

            //Curr cmd has next command
            if(cmd+1 < cmdCount)
            {
                old_fd[0] = new_fd[0];
                old_fd[1] = new_fd[1];
            }

            //Curr cmd has previous command
            if(cmd-1 > -1)
            {
            close(old_fd[0]);
            close(old_fd[1]);
            }
        }
        //Child process
        else //if fork() == 0
        {
            //Curr cmd has previous command
            if(cmd-1 > -1)
            {
                dup2(old_fd[0], 0); // setting up old_pipe to input into the child
                close(old_fd[0]);
                close(old_fd[1]);
            }

            //Curr cmd has next command
            if(cmd+1 < cmdCount)
            {
                close(new_fd[0]); // setting up new_pipe to get output from child
                dup2(new_fd[1], 1);
                close(new_fd[1]);
            }

            printf("Running command '%s': \n",getCmd(cmd,head));
            printf("Arguments: "); printArgs(cmd,head);
            //Below works for 1 cmd 1+ args, but not 1 cmd 0 arg or mult pipes
            isValid = execvp(getCmd(cmd,head), getArgs(cmd,head));

            if(isValid == -1)
            printf("%s: Command not found.\n", getCmd(cmd,head));
        }
     wait();

    }
}

Note this thread: Another_Stack_Overflow_Example I used the example there and replaced my function with their "Working" sample. Their working sample seems to work for most everything except a single command with no arguments like "ls" it simply just does nothing. and asks for input

Here is the requested getCmd() and getArgs() functions, just calls "getNode()" which returns a Node* struct (containing char* and a char** and two ints) the getCmd extracts the char* cmd and the getArgs extracts the char** args.

char* getCmd(int index, Node* head)
{
  Node* curr = getNode(index,head);
  return curr->cmd;
}
char** getArgs(int index, Node* head)
{
  Node* curr = getNode(index,head);
  return curr->args;
}

Answer 1

Two things:

1. You should pass a NULL terminated array to execvp() :

execvp("ls", NULL); // invalid!!!

This should be:

const char *args[1] = { NULL };
execvp("ls", args);

2. "Pipe operator" | is a shell extension so you cannot use it in exec... functions. You can specify only a single executable there.