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日期:2020-01-01 10:19

PROGRAMMING PROJECT ONE

DEVELOPING A SHELL

William Stallings

Copyright 2011

Supplement to

Operating Systems, Seventh Edition

Prentice Hall 2011

The Shell or Command Line Interpreter is the fundamental User interface to

an Operating System. Your first project is to write a simple shell - myshell -

that has the following properties:

1. The shell must support the following internal commands:

i. cd <directory> - Change the current default directory to

<directory>. If the <directory> argument is not present, report

the current directory. If the directory does not exist an appropriate

error should be reported. This command should also change the PWD

environment variable.

ii. clr - Clear the screen.

iii. dir <directory> - List the contents of directory <directory>.

iv. environ - List all the environment strings.

v. echo <comment> - Display <comment> on the display followed by a

new line (multiple spaces/tabs may be reduced to a single space).

vi. help - Display the user manual using the more filter.

vii. pause - Pause operation of the shell until 'Enter' is pressed.

viii. quit - Quit the shell.

ix. The shell environment should contain shell=<pathname>/myshell

where <pathname>/myshell is the full path for the shell executable

(not a hardwired path back to your directory, but the one from which

it was executed).

2. All other command line input is interpreted as program invocation,

which should be done by the shell forking and execing the programs as

its own child processes. The programs should be executed with an

environment that contains the entry: parent=<pathname>/myshell

where <pathname>/myshell is as described in 1.ix. above.

–3–

3. The shell must be able to take its command line input from a file. That

is, if the shell is invoked with a command line argument:

myshell batchfile

then batchfile is assumed to contain a set of command lines for the

shell to process. When the end-of-file is reached, the shell should exit.

Obviously, if the shell is invoked without a command line argument, it

solicits input from the user via a prompt on the display.

4. The shell must support i/o-redirection on either or both stdin and/or

stdout. That is, the command line

programname arg1 arg2 < inputfile > outputfile

will execute the program programname with arguments arg1 and arg2,

the stdin FILE stream replaced by inputfile and the stdout FILE

stream replaced by outputfile.

stdout redirection should also be possible for the internal commands

dir, environ, echo, & help.

With output redirection, if the redirection character is > then the

outputfile is created if it does not exist and truncated if it does. If the

redirection token is >> then outputfile is created if it does not exist

and appended to if it does.

5. The shell must support background execution of programs. An

ampersand (&) at the end of the command line indicates that the shell

–4–

should return to the command line prompt immediately after launching

that program.

6. The command line prompt must contain the pathname of the current

directory.

Note: You can assume that all command line arguments (including the

redirection symbols, <, > & >> and the background execution symbol, &) will

be delimited from other command line arguments by white space - one or

more spaces and/or tabs (see the command line in 4. above).

Project Requirements

1. Design a simple command line shell that satisfies the above criteria and

implement it on the specified UNIX platform.

2. Write a simple manual describing how to use the shell. The manual

should contain enough detail for a beginner to UNIX to use it. For

example, you should explain the concepts of I/O redirection, the

program environment, and background program execution. The manual

MUST be named readme and must be a simple text document capable of

being read by a standard Text Editor.

For an example of the sort of depth and type of description required,

you should have a look at the online manuals for csh and tcsh (man

csh, man tcsh). These shells obviously have much more functionality

than yours and thus, your manuals don’t have to be quite so large.

You should NOT include building instructions, included file lists or source

code - we can find that out from the other files you submit. This should

–5–

be an Operator’s manual not a Developer’s manual.

3. The source code MUST be extensively commented and appropriately

structured to allow your peers to understand and easily maintain the

code. Properly commented and laid out code is much easier to interpret,

and it is in your interests to ensure that the person marking your

project is able to understand your coding without having to perform

mental gymnastics!

4. Details of submission procedures will be supplied well before the

deadline.

5. The submission should contain only source code file(s), include file(s), a

makefile (all lower case please), and the readme file (all lowercase,

please). No executable program should be included. The person marking

your project will be automatically rebuilding your shell program from the

source code provided. If the submitted code does not compile it cannot

be marked!

6. The makefile (all lowercase, please) MUST generate the binary file

myshell (all lower case please). A sample makefile would be

# Joe Citizen, s1234567 - Operating Systems Project 1

# CompLab1/01 tutor: Fred Bloggs

myshell: myshell.c utility.c myshell.h

gcc -Wall myshell.c utility.c -o myshell

The program myshell is then generated by just typing make at the

command line prompt.

–6–

Note: The fourth line in the above makefile MUST begin with a tab.

7. In the instance shown above, the files in the submitted directory would

be:

makefile

myshell.c

utility.c

myshell.h

readme

Submission

A makefile is required. All files in your submission will be copied to the

same directory, therefore, do not include any paths in your makefile. The

makefile should include all dependencies that build your program. If a

library is included, your makefile should also build the library.

Do not hand in any binary or object code files. All that is required is

your source code, a makefile and readme file. Test your project by copying

the source code only into an empty directory and then compile it by entering

the command make.

We shall be using a shell script that copies your files to a test directory,

deletes any pre-existing myshell, *.a, and/or *.o files, performs a make,

copies a set of test files to the test directory, and then exercises your shell

with a standard set of test scripts through stdin and command line

arguments. If this sequence fails due to wrong names, wrong case for

names, wrong version of source code that fails to compile, nonexistence of

files, etc. then the marking sequence will also stop. In this instance, the only

–7–

marks that can be awarded will be for the tests completed at that point and

the source code and manual.

Required Documentation

Your source code will be assessed and marked as well as the readme

manual. Commenting is definitely required in your source code. The user

manual can be presented in a format of your choice (within the limitations of

being displayable by a simple Text Editor). Again, the manual should contain

enough detail for a beginner to UNIX to use the shell. For example, you

should explain the concepts of I/O redirection, the program environment and

background execution. The manual MUST be named readme (all lowercase,

please, NO .txt extension).


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