libc: Check if a shell is available if system()'s command argument is NULL

libc/src/stdlib.cpp

@@ -184,11 +184,14 @@ extern "C"
     {
         if (!cmd)
         {
-            // FIXME: Check if there is a shell available in the system.
-            errno = ENOTSUP;
-            return -1;
+            struct stat st;
+            if (stat("/bin/sh", &st) < 0) return 0;
+            return S_ISREG(st.st_mode);
         }
 
+        // FIXME: During the execution of system(), SIGCHLD will be blocked, and SIGINT and SIGQUIT will be ignored, in
+        // the process that calls system().
+
         pid_t child = fork();
         if (child == 0)
         {