Add a demonstration of the common fork+exec model

exec.c

@@ -0,0 +1,40 @@
+#include <sys/wait.h>
+#include <unistd.h>
+
+int main() {
+  // The program that will get run. It is similar to the command-line:
+  // echo "Hello, world!"
+  char *const cmdline[] = { "echo", "Hello, world!", NULL };
+
+  // Create a new process that's a clone of this one. The original process is
+  // known as the parent, and the new process is known as the child. The child
+  // process activates in the `fork` call, meaning both the parent and the child
+  // will return from fork and assign a value to the variable `pid`.
+  pid_t pid = fork();
+
+  // When the child process returns from `fork`, the value assigned to `pid`
+  // will be 0. In the parent process, `pid` will be the process ID (PID) of the
+  // child process.
+  if(pid == 0) {
+    // This call to `exec` will only be run by the child process. This is
+    // because `exec` replaces the process' instructions with the instructions
+    // found in `cmdline[0]` (in this case, `echo`). The `v` in `execvp` means
+    // vector, which in this case refers to the array `cmdline`. The `p` means
+    // path, indicating that it will search all of the directories found in the
+    // environment vaiable `PATH` for the program (in this case, `execvp` is
+    // searching the directories for `echo`).
+    execvp(cmdline[0], cmdline);
+  } else {
+    // In the parent process, pause until the child process exits. The `wait`
+    // function takes a pointer to a structure that contains information about
+    // how the child process exited. If the exit information isn't necessary,
+    // `NULL` can be passed instead.
+    wait(NULL);
+  }
+
+  // The parent process is now finished, and will return the number `0` to
+  // indicate it was successful. The child process will never reach this point
+  // in this program, since its instructions were replaced with `echo`s
+  // instructions earlier.
+  return 0;
+}