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说明：  实现一个简单的shell，实现步骤：（1）在虚拟的shell界面上出现命令提示符（$或＃）； （2）获取用户指令：获取用户在命令提示符后面输入的命令及其参数，并注意命令输入的最大长度； （3）解析指令：对用户输入的命令进行解析，解析出命令名和参数； （4）寻找命令文件：每个命令的执行都必须依靠对应的可执行文件，这些文件的存放路径存放在用户的PATH环境变量里； （5）执行命令：可通过fork( )系统调用创建一个进程来完成执行命令的任务，具体的命令执行用execv( )函数。
( Implement a simple shell, implementation steps: (1) appears on the virtual shell interface command prompt ($ or#) (2) to obtain user instructions: Get the user at the command prompt type the command and its parameters, and Note that the maximum length of command input (3) resolve command: the command to parse user input, parse out the command name and parameters (4) to find the command file: Each command must rely on the implementation of the corresponding executable files storage paths stored in the user s PATH environment variable (5) execute the command: by fork () system call to create a process to execute the command to complete the task, the specific command with execv () function.)