Command Shell
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1. Code requirements for this course Specifically you must include the pledge statement provided on the Programming Standards page in one of your submitted files 9 22 2005 8 46AM2. manner that reflects the logical purpose of the various components of your design You must document your implementation according to the Programming Standards page on the course website You must properly allocate and deallocate memory as needed If your shell does not implement a specified feature it should write an appropriate disclaimer when the user attempts to use that feature something distinguishable from a normal error message resulting from a logically invalid command Any such omissions should also be documented in the User Manual In general you are expected to apply the design and implementation guidelines and skills covered in your previous computer science courses There is no requirement that you base your design on an object oriented analysis of the problem nor are you discouraged from doing so You may implement your solution in pure C or in C Suggestions and assumptions There are some explicit assumptions you may make No command line will be longer than 100 characters and no command will be given more than 10 arguments not counting redirection and amp Each command argument redirection symbol and amp will be preceded by at least one blank space You may find it helpful to consult the UNIX man pages on fork exec getenv access waitpid opendir and related UNIX features cited in those man pages You may also find it helpful to look up the C library function freopen Note that
3. there are many good online C C language references and that if you have installed the Microsoft Visual C documentation that does include a comprehensive language reference Here is an example of a C style internal command handler void cdHandler istringstream User string newDirectory User gt gt newDirectory get target directory if newDirectory length no target directory echo current cout lt lt Environ PWD lt lt endl else if newDirectory newDirectory no change return else string newPWD findNewPWD newDirectory clean up vble Environ PWD newPWD set internal vble setenv PWD newPWD c_str true set system vble 9 22 2005 8 46AM 8 CS 3204 Project 1 Fall 2005 Evaluation Shortly before the due date for the project the TA will post signup sheets inside the McB 124 lab You will schedule a demo with the TA At the demo you will perform a build and run your program on the demo test data which I will provide to the TA The TA will evaluate the correctness of your results In addition the TA will evaluate your project for good internal documentation and software engineering practice Remember that your implementation will be tested in the McB 124 lab environment If you use a different development platform it is entirely your responsibility to make sure your implementation works correctly in the lab Your implementation will be e
4. CS 3204 Project 1 Fall 2005 Command Shell The shell or command line interpreter is the fundamental user interface to an operating system Your first project is to write a simple shell that has the following properties The shell will loop continuously to accept user commands it will terminate when quit is entered The command line prompt must contain the pathname of the current directory Internal commands The shell must support the following internal commands cd lt pathname gt lt directory gt Change the current default directory to lt directory gt Ifthe lt directory gt is not present report the current directory If the directory does not exist an appropriate error message should be reported The command should also change the PWD environment variable clr Clear the screen dir lt pathname gt lt directory gt Llist the contents of lt directory gt Ifthe lt directory gt is not present list the contents of the current directory If the directory does not exist an appropriate error message should be reported environ List all the environment strings echo lt comment gt Write the string lt comment gt on the display followed by a new line help Display the user manual using the more filter SS quit Quit the shell Internals commands are handled by the shell itself This may involve making calls to API functions provided by the underlying operating system You should not simply invoke features o
5. ccount the attributes of relevant files For example if the command usr home me foo is entered and the specified file exists in the specified location but it is not executable the shell will issue an informative error message The shell must support I O redirection on either or both stdin and stdout That is the command line programname argl arg2 lt inputfile gt 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 output file stdout redirection should also be possible for the internal commands dir environ echo and help With output redirection if the redirection token is gt then the output file is created if it does not exist and truncated if it does and its write permissions are set If the redirection token is gt gt then the output file is created if it does not exist and appended to if it does When an output file is created using redirection its access permission must at least include read permission for the owner If redirection targets an existing file whose write permissions are not set the shell will issue an informative error message The shell must support background execution of programs An ampersand amp at the end of a command line indicates that the shell should return to the command prompt immediately after launching that program Changes to shell environment variables should be registered using
6. f an underlying shell from which your shell may have been started Program invocations All other command line input is interpreted as program invocation which should be done by the shell forking and executing the program as its own child processes The programs should be executed with an environment that contains the entry parent lt pathname gt myshel1 as described above Upon finding the executable the shell will echo the full path from the system root to the directory where the executable was found If the executable is not found the shell will issue an informative error message Path specifications When appropriate the user may include path specifications in commands as indicated by lt pathname gt in the internal command specifications above and elsewhere The shell will accept path specifications that start with and Ww oe 9 22 2005 8 46 AM CS 3204 Project 1 Fall 2005 Path specifications are not required In a program invocation when no explicit path is given to an executable the shell will search for the executable according to the values in the environment variable PATH This value must be retrieved using the UNIX system call getenv The shell environment should contain she11 lt pathname gt myshell where lt pathname gt myshe11 is the full path for the shell executable not a hardwired path back to your directory but the one from which it was executed Other considerations The shell must take into a
7. setenv or putenv so those values will be visible when external program invocations are made When your shell exits the environment should be restored to the same state as before the shell was started User documentation You must write a simple user manual describing how to use the shell The manual should contain enough detail for a UNIX beginner 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 no extension and must display properly if opened in vi or emacs For an example of the sort of depth and type of description required you should take a look at the online manuals for one or more of the common UNIX shells csh tcsh bash etc Of course those shells are much more complex than yours will be so your manual will not be so large The user manual should not contain build instructions an included file list or source code This is a User Manual not a developer s guide 9 22 2005 8 46AM 8 CS 3204 Project 1 Fall 2005 Design and implementation requirements There are some explicit requirements in addition to those on the Programming Standards page of the course website Your shell must be implemented in ISO compliant C C code You may make use of any language standard types and or containers you find useful You must decompose your implementation into separate source and header files in some sensible
8. valuated for documentation and design as well as for correctness of results Note that the evaluation of your project may depend substantially on the quality of your code and documentation What to turn in and how This assignment will be collected on the Curator system The testing will be done under Fedora Core 4 using gcc 4 0 1 The Curator will not begin accepting submissions of this assignment before September 10 Submit a single gzip d tar file containing the C C source and header files for your implementation to the Curator System Submit only the source and header files Submit nothing else Be sure that your header files only contain include directives for Standard C C and UNIX header files any other include directives will certainly result in compilation errors It must be possible to unpack the file you submit using the following command tar zxf lt name of your file gt The unpacked files will then be compiled typically using the following command syntax g gcc o lt name we give th xecutable gt cpp c Instructions and the appropriate link for submitting to the Curator are given in the Student Guide at the Curator website http www cs vt edu curator You will be allowed to submit your solution up to three times in order to fix errors you discover before your project is tested Your latest submission will be tested Pledge Each of your program submissions must be pledged to conform to the Honor
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