CS作业编程代写This assignment involves writing your own command interpreter or shell using the Python language. The specification for the language of your shell is given below. SpecificationYou are to implement the following features. Invoking the shell Your shell program is to be written in Python3 and called mysh.py. The program will be invoked in the following way.python3 mysh.py [filename]Where [filename] specifies an optional filename. When invoked without the filename argument, mysh is an interactive shell that prints a prompt, then reads a line (and any continuations, described below) and interprets it. When invoked with a file name argument, mysh reads lines from that file and interprets them. No prompts are printed if invoked with a file name. mysh should terminate if it reads an end-of-file (EOF) indicating no more lines to read from a file, or Ctrl-D has been typed at the keyboard.  Reading commands A command will consist of a series of words separated by whitespace (spaces+tabs). Words are defined as being any of the following. ● An identifier which starts with an alphabetic character (upper or lower case) followed by a sequence of either alphabetic or numeric characters or underscore● A filepath consisting of alphabetic or numeric or forward slash characters● A dollar sign followed by an identifier● A number● One of the characters:  | If the input line ends in a backslash and newline, then the next line is read and joined to the previous before interpretation. This may be repeated until a line is read that ends in newline only, without a backslash. In the following example the $ part is the prompt, printed by mysh. $ say cheese cheese$ say   cheese cheese$ say   blue   cheese blue cheese$ exit Goodbye!    Commands The first word of the line is interpreted as either a builtin command or a pathname of a file somewhere in the namespace. For builtin commands, the corresponding function is called with the rest of the words on the line passed in a list. If the command is not a builtin, then treat the command as a path to an executable file. The file is executed with the arguments set by the rest of the words on the line. This file should execute in a separate process. Any executable file should be executable in this manner (i.e. it does not need to be a mysh file). $ /bin/echo hello helloIf the external file does not exist or cannot be interpreted, then an error message is displayed. $ /invalid/fileUnable to execute /invalid/fileBuiltins You are to implement the following built-in commands for mysh.  Exit mysh with message: “Goodbye!”. $ exit Goodbye! say [arg ]Echoes the argument to standard output. If no arguments are specified, a blank line is printed.$ say$ say cheese cheese$ say blue cheese blue cheese changedir [directoryname]Change directory to directoryname, if specified, or to the directory given in the$HOME variable. If the HOME variable does not exist in mysh, do nothing. Before changing directory, save the current directory path to the front of a history list (see historylist). showdirPrint the current working directory. $ showdir/home/alice historylistList the directories previously visited by mysh as a numbered list starting at 0. The entry numbered 0 is the current working directory.$ showdir/home/alice$ changedir uni$ changedir info1112$ changedir$ historylist 0: /home/alice1: /home/alice/uni/info11122: /home/alice/uni3: /home/alicecdn [n]Change directory to the numbered directory as shown in historylist and remove all the directories earlier in the list, leave current directory name at the beginning of the list. If n is not given, change directory to the first directory in historylist. show [filename …]Reads from each filename in turn, writing the data to standard output. If no file name is given, it reads from standard input and writes the data to standard output until an end-of-file is read. set [variable [value ]]If no arguments are given, print all variables which are currently defined, in lexicographical order. If one argument is given, set the variable to an empty string. If more than one argument is given, set the variable to space-separated values. Use a python dictionary to hold variables and their values. unset variableRemove variable from mysh variables. $ set PS=$$ set subject info1112$ set name$ set name=subject=info1112$ unset name$ set PS=$subject=info1112 sleep NSleep for N seconds. $ sleep 2$ exit Goodbye!Shell variables If a word begins with a dollar sign ($), then the word is replaced by a value from a dictionary of variable values maintained by mysh. If the variable does not exist, then it is replaced by an empty string. Some variables may be predefined. For example, the variable PS defines the initial prompt string to be used in interactive mode. Set the value to $ initially.$ set PS=$$ set foo bar$ say $foo bar$ set PS mysh  mysh  exit Goodbye! Redirection The characters  and  are used to indicate standard input and output, respectively. A filename must follow. For example: justdoit arg1 arg 2 myinput myoutput This will be involve executing the file justdoit with arguments arg1 and arg2 with standard input coming from the file myinput and standard output going to the file myoutput .$ say hello   sample$ show sample hello$ exit Goodbye! Piping The pipe character (|) is used to redirect standard output of a command to the standard input of another command. justdoit arg1 arg2 | show This will be involve executing the file justdoit with arguments arg1 and arg2 . The standard output of this command is then used as standard input into the program “show”.$ say hello | show hello$ exit Goodbye!Restrictions You are limited to using the following Python modules. ● os (excluding the os.system function)● sys● time 代写python编程You must not use Python features such as the os.system() function or the subprocess module to directly execute commands within the system shell. The purpose of this assignment is to emulate how the shell works, rather than write a Python interface for the command line (i.e. builtin commands should be implemented in Python; not by launching a bash subprocess). If you have any questions regarding the scope of these restrictions, please ask on Ed.Submitting your code An Ed assessment workspace will be available soon for you to submit your code. Public and hidden test cases will be rolled out over the next few weeks leading to the deadline, up until the 8th October (one week before the due date). Additionally, there will be a set of unreleased test cases which will be run against your code after the due date. Any attempt to deceive the automatic marking system will be subject to academic dishonesty proceedings.最先出自Essayghost cs代写 python代写合作：315代写

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