tailieunhanh - UNIX UNLEASHED PHẦN 2

Các Giới thiệu Rick Rummel hạt nhân và Shell chức năng của Shell một Command Line Giải thích Chương trình Khởi đầu vào-đầu ra Redirection đường ống kết nối thay thế Tên tập tin Duy trì của biến Môi trường kiểm soát Lập trình Shell Tóm tắt thông tin | Part II Hunt for Shells What Is a Shell Bourne Shell Korn Shell C Shell Which Shell Is Right for You Shell Comparison TOC - TOC - . 10 o What Is a Shell By Rick Rummel Introduction The Kernel and the Shell The Functions of a Shell Command Line Interpretation Program Initiation Input-output Redirection Pipeline Connection Substitution of Filenames Maintenance of Variables Environment Control Shell Programming Summary 10 What Is a Shell By Rick Rummel Introduction You can do many things without having an extensive knowledge of how they actually work. For example you can drive a car without understanding the physics of the internal combustion engine. A lack of knowledge of electronics doesn t prevent you from enjoying music from a CD player. You can use a UNIX computer without knowing what the shell is and how it works. However you will get a lot more out of UNIX if you do. Three shells are typically available on a UNIX system Bourne Korn and C shells. They are discussed in Chapters 11 12 and 13. In this chapter you ll learn What a shell is What a shell does for you How a shell relates to the overall system The Kernel and the Shell As the shell of a nut provides a protective covering for the kernel inside a UNIX shell provides a protective outer covering. When you turn on or boot up a UNIX-based computer the program unix is loaded into the computer s main memory where it remains until you shut down the computer. This program called the kernel performs many low-level and system-level functions. The kernel is responsible for interpreting and sending basic instructions to the computer s processor. The kernel is also responsible for running and scheduling processes and for carrying out all input and output. The kernel is the heart of a UNIX system. There is one and only one kernel. As you might suspect from the critical nature of the kernel s responsibilities the instructions to the kernel are complex and highly technical. To protect the user from the complexity of the .