in Unix operating system? how can we find system call and command in unixis there any command is available to differenciate it. is there any complete referance for advanced unix is availabel in internet

Commands in UNIX are just programs. Commands like ls, cd, mkdir, and so on are probably executables in your /bin directory.System calls are functions you can call within your C code. They will switch your running level into "privileged mode" in order for you to access system resources like your hard drive. For example, open(2) is a system call, as is chmod(2). You should check the man pages on your system in chapter 2, which is about system calls.