标签：++ about desirable 博士 python std like sid computer
CITS2002 Systems Programming - Project 2 2017
See also: 2 Project 2 clarifications
A shell is a command-interpreter used widely on Unix-based systems. Most shells receive character-based input, typically read from a keyboard or a textfile, parse
their input into a token sequence forming the syntax recognised by the shell (command names, command arguments, and control-flow delimiters), and then attempt to
execute the token sequence. The shell used by most CSSE students under macOS and Linux is named bash (the Bourne Again Shell); a former CSSE graduate is the author of
a very popular book describing the Bash shell.
The goal of this project is to implement a program named myshell supporting a small subset of the features of a standard Unix-based shell. Successful completion of the
project will develop your understanding of some advanced features of the C99 programming language, and your understanding of the role of a number of system-calls
responsible for process invocation and monitoring, and input and output management using file and pipe descriptors.
NOTE - this is a systems-specific project. You will find this project significantly easier to complete on macOS or Linux. You will likely have great difficulty
completing it on Windows, and it is recommended that you don‘t try. As for the 1st project, your submission will be tested and marked on CSSE macOS laboratory
machines.
________________________________________
Suggested steps
A number of C99 source code files are provided as a starting skeleton for the project. The most challenging-to-write part of the project, the command parser, has been
written for you, and should be used without modification (and need not be understood). The parser returns a pointer to a binary tree of shell commands which your code
will traverse and execute.
In summary, the function parse_shellcmd(FILE *fp) accepts a FILE pointer as its only input, and attempts to read and parse its input. If successful, function
parse_shellcmd() returns a pointer to a user-defined datatype named SHELLCMD, a self-referential structure holding all information necessary to execute the requested
command-sequence. Your role is to execute the commands in the structure by (implementing and) calling your own function named execute_shellcmd().
The project only requires implementation of a small subset of a traditional Unix shell. However, that subset will be quite faithful to standard shells. For this
reason, this project description does not specify all details in great detail. If unsure about the role and action of shell features listed in these steps, you should
experiment with the same feature in your standard shell, bash(or obviously ask on help2002).
The approach taken to develop the sample solution has been broken into 9 independent steps, presented below. There‘s no requirement for you to follow these steps, but
they are described here to provide an obvious pathway to a solution, and to explain the project‘s required features.
Step 0. Develop a Makefile
Develop a Makefile, employing variable definitions and automatic variables, to compile and link the project‘s files.
Step 1. Execute simple external commands
1. execute commands without arguments, such as /bin/ls
2. execute commands with arguments, such as /usr/bin/cal -y
Helpful C99 and POSIX functions: fork(), execv(), exit(), wait().
DO NOT use the functions: system(), popen(), execlp(), or execvp() anywhere in your project.
Step 2. Search path
The global variable PATH points to a character string which is interpreted as a colon separated list of directory names.
If the user enters a command name which does not contain a ‘/‘ character, then the members of the search path list are considered as directories from which to attempt
to execute the required command. Once the required command is found and executed, the search terminates.
Helpful C99 and POSIX functions: strchr().
Step 3. Execute internal commands
Internal commands are performed by myshell, itself, rather than by external commands. myshell does not create a new process, with fork(), before running an internal
command. Three internal commands are to be supported:
1. exit - terminate myshell by calling the function exit(). When an additional argument is provided, it should be interpreted as the numeric exit-status. When
requested without any arguments, the exit-status of the recently executed command should be used.
2. cd - change from the current working (default) directory to the directory specified as the first argument to the cd command. If the command cd is given without
arguments, then the variable HOME should be used as the new directory.
As with the role of PATH for command execution, the variable CDPATH points to a character string which is interpreted as a colon separated list of directory names. If
the user enters a directory-name which does not contain a ‘/‘ character, then the members of CDPATH are considered as directories to locate the required directory.
3. time - the ‘following‘ command‘s execution time should be reported in milliseconds (e.g. 84msec) to myshell‘s stderr stream.
Helpful C99 and POSIX functions: exit(), chdir(), gettimeofday().
Step 4. Sequential execution
There are three forms of sequential execution, and thus three subparts to this step. The three forms differ in the action they take when a command fails. By
convention, a command which fails will return a non-zero exit status.
1. A sequence of commands separated by the ";" token requires myshell to execute each command sequentially, waiting until each command has finished before
beginning the next. In this form, myshell should continue to the next command regardless of the result of earlier commands. The sequence ls ; date monday ; ps
would execute ls, then on its completion execute date monday (which will fail), then execute ps (even though the previous command failed). The exit value of sequential
execution is the exit status of the command sequence to the right of the ";" token.
2. If the commands are separated by the token "&&" then myshell should continue with further commands only if the command sequence to the left of the token
returns an exit value indicating success. For the command sequence a && b, myshell should wait for command a to terminate, test its return value and execute b if and
only if a‘s return value indicates success. The exit value of an "&&" command sequence is the exit status of the last command executed.
3. If the token "||" is used to separate commands then myshell should continue to the next command only if the first command sequence returns an exit value
indicating failure. This is the converse of part 2. For the command sequence a || b, myshell should wait for command a to terminate, test its return value and execute
b if and only if a‘s return value indicates failure. The exit value of a "||" command sequence is the exit status of the last command executed.
Step 5. Subshell execution
The sequence ( commands ) should cause commands to be executed in a subshell. A subshell is created by forking a copy of myshell to execute the commands. The exit
value for a subshell is the exit value of the command sequence inside the parentheses.
Step 6. stdin and stdout file redirection
Commands may redirect their standard input and standard output from and to files, respectively, before the command is executed.
1. The sequence command < infile requests that the command use infile as its standard input.
2. The sequence command > outfile requests that the command use outfile as its standard output. If the file outfile does not exist then it is created. If it
does exist it is truncated to zero then rewritten.
3. The sequence command >> outfile requests that the command appends its standard output to the file outfile. If outfile does not exist it is created.
NOTE: in all of the above cases command could be a subshell. It is valid to have both input and output redirection for the same command. Thus the sequence ( sort ;
ps ) < junk1 >> junk2 would take input from the file junk1 and append output to the file junk2.
Helpful C99 and POSIX functions: open(), close(), dup2().
Step 7. Pipelines
The sequence command1 | command2 requests that the stdout of command1 be presented as stdin to command2. By default, the stderr output of command1 is not
redirected and appears at its default location (typically the terminal). With reference to this example, the myshell parser will not permit the stdout of command1 to
be redirected to a file (with >), nor the stdin of command2 to be received from a file (with <). Note that the sequence command1 | command2 | command3 requests that
a pipeline of two different pipes be established.
Helpful C99 and POSIX functions: pipe(), dup2().
Step 8. Shell scripts
Once an executable file name has been found (possibly using the search path) the standard function execv() can be called to try to execute it. If execv() fails, you
should assume that the file is a shell script (a text file) containing more myshell commands. In this case you should execute another copy of myshell to read its input
from the shell script. No specific filename extension is required.
Helpful C99 and POSIX functions: access(), fopen(), fclose().
Step 9. Background execution
The token "&" causes the preceding command to be executed without myshell waiting for it to finish (asynchronously). Thus the sequence ls ; ps & date should start
the command sequence ls, once this has completed start ps and immediately proceed to the command date. The exit value of background execution is success unless the
fork call fails, in which case the exit value should indicate failure.
Commands placed into the background may take a long time to execute and, thus, we wish to know when they have completed. The parent is informed of the termination of
child processes using asynchronous signal-passing. When so informed, myshell should report which background process has terminated.
Similarly, when myshell exits, it should first terminate all of its still-running background processes, and wait for them to finish.
Helpful C99 and POSIX functions: signal() and kill().
________________________________________
Starting files
The amount of code to be written for this project is similar to that of the 1st project, although less needs to be designed "from scratch" because you‘re extending an
(incomplete) code skeleton.
An executable sample solution will be available soon.
Start your project by downloading, reading, and understanding the files in the archive myshell.zip.
? myshell.h - provides the definition of myshell‘s user-defined datatypes and the declaration of global variables.
? myshell.c - provides the main() function, and calls the parse_shellcmd() function.
? globals.c - defines global variables, and the helpful print_shellcmd0() function.
? execute.c - where you define your execute_shellcmd() function.
? parser.c - defines the parse_shellcmd() and free_shellcmd() functions, which should be used without modification (and need not be understood).
You may modify any file or add any additional files to your project.
________________________________________
Program requirements
1. Your project, and its executable program, must be named myshell.
2. Your project must be developed using multiple C99 source files and must employ a Makefile, employing variable definitions and automatic variables, to compile
and link the project‘s files.
3. If any error is detected during its execution, your project must use fprintf(stderr, ....) or perror() (as appropriate) to print an error message.
4. Your project must employ sound programming practices, including the use of meaningful comments, well chosen identifier names, appropriate choice of basic
data-structures and data-types, and appropriate choice of control-flow constructs.
________________________________________
Assessment
This project is worth 15% of your final mark for CITS2002. It will be marked out of 40. The project may be completed individually or in teams of two. You are strongly
encouraged to work with someone else - this will enable you to discuss your initial design, and to assist each other to develop and debug your joint solution.
During the marking, attention will obviously be given to the correctness of your solution. However, a correct and efficient solution should not be considered as the
perfect, nor necessarily desirable, form of solution. Preference will be given to well presented, well documented solutions that use the appropriate features of the
language to complete tasks in an easy to understand and easy to follow manner. That is, do not expect to receive full marks for your project simply because it works
correctly. Remember, a computer program should not only convey a message to the computer, but also to other human programmers.
Up to half of the possible marks will come from the correctness of your solution. The remaining marks will come from your programming style, including your use of
meaningful comments, well chosen identifier names, appropriate choice of basic data-structures and data-types, and appropriate choice of control-flow constructs.
Your project will be marked on the computers in CSSE Lab 2.03, using the macOS environment. No allowance will be made for a program that "works at home" but not on
CSSE Lab 2.03 computers, so be sure that your code compiles and executes correctly on these machines before you submit it.
________________________________________
Submission requirements
1. The deadline for the project is 12noon Friday 3rd November (end of week 13).
2. Your submission will be compiled, run, and examined using the macOS platform on computers in CSSE Lab 2.03. Your submission must work as expected on this
platform. While you may develop your project on other computers, excuses such as "it worked at home, just not in the lab!" will not be accepted.
3. Your submission‘s C99 source files should each begin with the lines:
4.
5. /*
6. CITS2002 Project 2 2017
7. Name(s): student-name1 (, student-name2)
8. Student number(s): student-number-1 (, student-number-2)
9. Date: date-of-submission
10. */
11. You must submit your project electronically using cssubmit. No other method of submission is allowed. You should submit all C source-code (*.c) and header
(*.h) files and a Makefile that you wish to be assessed. You do not need to submit any additional testing scripts or files that you used while developing your project.
If working as a team, only one team member should make the team‘s submission. The cssubmit facility will give you a receipt of your submission. You should print and
retain this receipt in case of any dispute. Note also that the cssubmit facility does not archive submissions and will simply overwrite any previous submission with
your latest submission.
12. You are expected to have read and understood the University‘s guidelines on academic conduct. In accordance with this policy, you may discuss with other
students the general principles required to understand this project, but the work you submit must be the result of your own efforts. All projects will be compared
using software that detects significant similarities between source code files. Students suspected of plagiarism will be interviewed and will be required to
demonstrate their full understanding of their project submission.
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标签：++ about desirable 博士 python std like sid computer
原文地址：https://www.cnblogs.com/codinghelp/p/8877334.html
