INFO1112辅导、讲解Python程序、讲解shell program、辅导Python

INFO1112 Assignment 2018

Due Week 11, Monday 15th October at 12PM (Noon)

This assignment is worth 20% of your overall grade for the course.

Assessment

The assignment will be marked with a testing system that will run your program

against a number of scripts written in the language. An automatic mark will be

given based on percentage of tests passed (16%) and a manual mark will be given

for overall style, quality, readability, etc (4%).

Marks will allocated for:

● basic command loop

● reading line continuations

● builtins

● using variables

● i/o redirection

● exec-ing external commands

● exec-ing with correct i/o redirection

● pipes

● code layout, readability, overall quality, etc

Other behaviour will not be assessed.

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.

INFO1112 Assignment 2018 1

Specification

You 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 file path 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.

INFO1112 Assignment 2018 2

$ 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

hello

If the external file does not exist or cannot be interpreted, then an error message is

displayed.

$ /invalid/file

Unable to execute /invalid/file

INFO1112 Assignment 2018 3

Builtins

You are to implement the following built-in commands for mysh.

exit

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).

showdir

Print the current working directory.

$ showdir

/home/alice

INFO1112 Assignment 2018 4

historylist

List 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

$ historylist

0: /home/alice/uni/info1112

1: /home/alice/uni

2: /home/alice

cdn [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.

INFO1112 Assignment 2018 5

unset variable

Remove variable from mysh variables.

$ set

PS=$

$ set subject info1112

$ set name

$ set

name=

PS=$

subject=info1112

$ unset name

$ set

PS=$

subject=info1112

sleep N

Sleep for N seconds.

$ sleep 2

$ exit

Goodbye!

INFO1112 Assignment 2018 6

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!

INFO1112 Assignment 2018 7

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 and os.popen functions)

● sys

● time

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.

Please note, further clarifications to this specification will be posted on Ed.

INFO1112 Assignment 2018 8

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.