Creating background process for my shell

Shell Programming and Scripting
1

I actually posted this problem on a different forum, but figured this would be a more appropriate place to post it.

OK so I've created my own shell, but I can't get the background process function to run properly! What I want to do is to run a process in the background, and also print when the job is finished. The printing part is where I've been having some problems.

So if I type sleep 5 &, it works and prints out when finished. However, if I type sleep 5 &, and then type in another command like ls or ps after it, the shell hangs and it prints out that it's finished when it clearly has not finished.

void handler(int sig)
{
        int pid;
        int status;
        pid = wait(NULL);
        printf("Pid %d exited.\n", pid);
}



int runInBackground(Command* command, int givenPath)
{
        int pid;
        int status;
        char *commandName = command->name;
        if(givenPath == 0)
        {
                commandName = getCommandPath(command->name);
        }

        if(commandName == NULL)
        {
                printf("Command not found.\n");
                return 1;
        }

        pid = fork();

        if(pid < 0) //Checks for error
        {
        }

        if(commandName == NULL)
        {
                printf("Command not found.\n");
                return 1;
        }

        pid = fork();

        if(pid < 0) //Checks for error
        {
                printf("Error in the fork process.\n");
                return 1;
        }
        else if (pid == 0) //Child
        {
                execv(commandName, command->argv);
                return 1;
        }
        else
        {
                signal(SIGCHLD, handler);
        }
}

OUTPUT:

<project3> sleep 5 &
<project3>
<project3>
<project3> Pid 399 exited.
<project3>
<project3> sleep 5 &
<project3> ls
bob                 project1.c  hey                 proj1               proj1.c
Pid 402 exited.

I pressed enter a few times the first time I ran sleep 5 in the background, then after 5 seconds, it printed out the pid perfectly! But notice what happens when I ran sleep 5 in the background and then ran ls following it right afterwards. System printed out my files/folders but posted the pid and got hung.

2

Did you make a mistake while posting your code? The following seems to have been duplicated:

        if(commandName == NULL)
        {
                printf("Command not found.\n");
                return 1;
        }

        pid = fork();

        if(pid < 0) //Checks for error
        {

If that's not a mistake, then that's one bizarre shell you're cooking up. For each async command it will create 4 instances of itself, two of which will exec the command and two which will register a signal handler and return from runInBackground. Depending on what happens after returning from runInBackground, this could pollute your process table with idle shells (which isn't as bad as each async command executing twice ;)).

Regards,
Alister

3

Ha! Actually yeah that's a typo, not sure how I got that from copying and pasting my code.

Anyway, no need for this thread anymore, I've finished this assignment.