Bạn có thể sử dụng một máy tính UNIX mà không cần biết những gì vỏ và làm thế nào nó hoạt động. Tuy nhiên, bạn sẽ nhận được rất nhiều trong của UNIX nếu bạn làm. Ba vỏ thường có sẵn trên một hệ thống UNIX: | Part IV Process Control What Is a Process Administering Processes Scheduling Processes TOC 4 TOC -4 18 - What Is a Process By Rachel and Robert Sartin What Happens When You Execute a Command Forking a Process Running a Command Looking at Process Visiting the Shell Again Processing a Command Checking the Aliases and Built-Ins Make a New Process with fork Start a New Command with exec An Example Executing in the Background An Example Kinks in Your Pipeline A Special Process Called Daemon init inetd cron Summary 18 - What Is a Process By Rachel and Robert Sartin This chapter introduces the concept of processes and how you use UNIX to interact with them. What Happens When You Execute a Command When you execute a program on your UNIX system the system creates a special environment for that program. This environment contains everything needed for the system to run the program as if no other program were running on the system. Forking a Process Each process has process context which is everything that is unique about the state of the program you are currently running. The process context includes then following The text program instructions being run The memory used by the program being run The current working directory The files that are open and positions in the files Resource limits Access control information Others various low-level information Every time you execute a program the UNIX system does a fork which performs a series of operations to create a process context and then execute your program in that context. The steps include the following 1. Allocate a slot in the process table a list of currently running programs kept by UNIX. UNIX creates the illusion of multiple programs running simultaneously by switching quickly between active processes in the process table. This allocation can fail for a number of reasons including these o You have exceeded your per user process limit the maximum number of processes your UNIX system will allow you to run. o The system .
