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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 - Change the current default directory to
. If the 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 - List the contents of directory .
iv. environ - List all the environment strings.
v. echo - Display 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=/myshell
where /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=/myshell
where /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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