What is this?

A simple shell, written to learn the trio of system calls necessary to control processes: fork(), exec(), and wait().

Currently, there are no built-ins (i.e., no cd, pwd, help, etc.) no redirection (i.e., no |, >, <, etc.), and no quoting (i.e., echo 'Hello, world!' will actually print 'Hello, world!' not Hello, world!.) But, at least it works!

How does it work?

The fork() system call is used to create a new process. It does this by creating a copy of the calling process, with a few differences. The child process has its own address space, registers, and instruction pointer.

The child process's call to fork() returns 0 as it's process identifier (PID). The parent process's call to fork() returns the child's PID.

int main(void) {
	pid_t pid = fork();

	if (pid < 0) {
		printf("failed to fork\n");
	} else if (pid == 0) {
		printf("(%d) I'm a child.\n", getpid());
	} else {
		printf("(%d) I'm a parent.\n", getpid());
		printf("(%d) My child is %d\n", getpid(), pid);
	}
}

The wait() system call pauses execution of the parent process until the child process completes execution.

In the following program, the child process will run before the parent process, rather than the order being determined by the scheduler.

int main(void) {
	pid_t pid = fork();

	if (pid < 0) {
		printf("failed to fork\n");
	} else if (pid == 0) {
		printf("(%d) I'm a child.\n", getpid());
	} else {
		wait(NULL);
		printf("(%d) I'm a parent.\n", getpid());
		printf("(%d) My child is %d\n", getpid(), pid);
	}
}

The exec() system call transforms the current process into one running the given program -- therefore nothing after the call to exec() should be expected to execute under normal conditions, which is why a fork() is necessary.

How do I use it?

1. Compile the shell using make.
2. Enter the shell with ./shell.
3. Profit.