In accordance with the present invention, there is provided a computer incorporating a multitasking operating system such as a UNIX-based OS, wherein recursively invokable function calling statements and flow control statements in a procedure-oriented high-level programming language are incorporated as shell external commands stored in a directory of a file system with an active path. With this arrangement, programming can be achieved simply by combining some of the external commands, so that highly sophisticated programming comparable to C programming can be achieved without the use of flow control statements incorporated in a shell. Since the flow control statements and the function calling statements are incorporated as the shell external commands, a loop and a conditional branch are executed by daughter processes individually generated. Even if forceful exit from the loop occurs, only the daughter process controlling the loop is forcefully terminated, but a parent process executing the program is still active so that the programmed process is subsequently continued.