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 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.
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
$? changedir
$? historylist
0: /home/alice
1: /home/alice/uni/info1112
2: /home/alice/uni
3: /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.
unset variable
Remove variable from mysh variables.
$? set
PS=$
$? set subject info1112
$? set name
INFO1112 Assignment 2018 5
$? set
name=
PS=$
subject=info1112
$? unset name
$? set
PS=$
subject=info1112
sleep N
Sleep 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!
INFO1112 Assignment 2018 6
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!
INFO1112 Assignment 2018 7
Restrictions
You are limited to using the following Python modules.
● os (excluding the os.system function)
● 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.
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.
INFO1112 Assignment 2018
http://www.daixie0.com/contents/3/1824.html
因为专业，所以值得信赖。如有需要，请加QQ：99515681 或邮箱：[email protected]
微信：codinghelp