Description
Objectives
By completing this programming assignment, you will be familiarized with Unix/POSIX system calls
related to processes, file management, and inter-process communication (Bonus).
Task 1 (Teaming Up)
You can do this programming assignment alone or in a pair. If you decide to do this assignment in a pair,
find a team member and send the names of your team to the TAs:
Mr. Quang Minh Tran (qmtran@memphis.edu)
Ms. Diem-Trang Pham (dpham2@memphis.edu)
You will do the rest of the programming assignments with your team member. Note that only one of you
will hand in the assignment.
Task 2 (Warming Up)
• Install a POSIX-compliant operating system such as Ubuntu (recommended), Linux, MacOS. You
can use a free virtualization software such as VMWare Player or VirtualBox to install a POSIXcompliant operating system on top of that. Note that your submitted program will be graded on
a POSIX-compliant operating system. Thus, it is crucial for you to compile and test your program
in this environment.
• In this programming assignment, you are allowed to use C, C++, or Python. If you decide to learn
about programming in C/C++ in Linux/Unix environment, the following links may be helpful:
https://linuxconfig.org/c-development-on-linux-introduction-i
http://cseweb.ucsd.edu/classes/fa09/cse141/tutorial_gcc_gdb.html
http://teaching.csse.uwa.edu.au/units/CITS2230/resources/gdb-intro.html
Task 3 (Programming)
Write a program that implements a simple shell. The shell takes a user command as input and executes
the command. When the shell is started, it should take a user command as input, execute it and display
the output. The following example shows an execution of the shell. It displays the command prompt
‘uofmsh’ and takes the user command ‘ls’ as input from STDIN. It then executes the command ‘ls’ and
prints the output to STDOUT.
Basically, your shell launches a program (e.g., ‘ls’) and coordinates the input and output of the program.
Note that you should implement your own shell and you are NOT allowed to create a wrapper shell
using a library function system() (for C/C++). In this programming assignment, you should complete 4
subtasks.
Subtask 1 (Implementing Built-in Commands) [10 pts]
• Implement command parsing in your shell. You do not need to write a full parser in yacc/lex to
parse commands. Using plain string functions such as strtok (e.g., for C/C++) should suffice.
Once a command is parsed, the parsed command should be executed.
• Built-in command exit [5 pts]: Your shell should terminate when the exit command is parsed
and executed in your shell. For example,
• Built-in command stat [5 pts]: Your shell should display the history of commands received from
the user along with the system time when it was input by the user. For example,
$./uofmsh
uofmsh> ls
*** output of ls ***
uofmsh> exit
$
$./uofmsh
uofmsh> exit
$
Subtask 2 (Executing Programs) [10 pts]
• Review course slides on the system calls for process creation and replacing the memory space of
a process with a new program. Read the man pages of fork() and exec() for more details on the
system calls.
fork(): http://man7.org/linux/man-pages/man2/fork.2.html
exec(): https://linux.die.net/man/3/exec
Note that there are different versions of the exec() system call. You will have to decide which
version of exec() to use in this programming assignment.
If you decide to use Phyton, here are some useful resources for you.
https://www.python-course.eu/forking.php
• Basically, your shell should execute a program specified by the user input. To execute a program,
a new process should be forked and its memory space should be replaced by the user specified
program. Test your shell with programs with different number of arguments. For example,
$./uofmsh
uofmsh> stat
11:34 exit
11:32 pwd
11:33 cd
$
Subtask 3 (Changing Directory) [10 pts]
• Let’s add support for changing the working directory. Implement the command cd in your shell
using the chdir(2) system call. You can find more information about the system call here:
http://man7.org/linux/man-pages/man2/chdir.2.html
• Make sure that ‘cd -‘ should change the directory to the last directory that the user was in, and
‘cd’ with no arguments should change the directory to the user home directory. Also make sure
to implement ‘cd .’ that leaves you in the current directory and ‘cd ..’ that moves up one
directory.
• Verify that the cd command works correctly by using the ‘pwd’ command which displays the
current working directory.
• Display the current working directory in your shell prompt. For example,
\tmp\homework1> pwd
\tmp\homework1
$./uofmsh
uofmsh> ls
*output of ls
uofmsh> touch f1
*output of touch f1
uofmsh> cp f1 f2
*output of cp f1 f2
uofmsh> rm -i f1 f2
*output of rm -i f1 f2
Bonus: Subtask 4 (Implementing Redirection) [10 pts]
• Redirection is one of the most powerful features of Linux/UNIX systems. Special characters (i.e,.
‘<’, ’>’ ,’|’) are used to accomplish redirection. Read more information about the redirection
here:
https://www.digitalocean.com/community/tutorials/an-introduction-to-linux-i-o-redirection
• Review the following system calls related to file management: open(), close(), read(), write(),
dup()/dup2().
open(): http://man7.org/linux/man-pages/man2/open.2.html
close(): http://man7.org/linux/man-pages/man2/close.2.html
read(): http://man7.org/linux/man-pages/man2/read.2.html
write(): http://man7.org/linux/man-pages/man2/write.2.html
dup/dup2: http://man7.org/linux/man-pages/man2/dup.2.html
• To implement support for redirection, you should modify your parsing code such that these
three special characters are identified and shell-level directives are individually parsed, rather
than passing the entire command to exec(). For example, given a command ‘ls > file’, your shell
should parse this command into ‘ls’, ‘>’, and ‘newfile’. The output of ‘ls’ will be saved in a file
‘newfile’. If the file ‘newfile’ does not exist, your shell will create this file using a system call.
• Test your shell with the following commands:
What to turn in
• Submit all files that are needed to run your shell to the dropbox of eCourseware.
• Leave a short note on how to execute your program.
\tmp\homework1> ls -l > file
*the output of ‘ls-l’ should be saved in a file ‘newfile’.
\tmp\homework1> cat < file
*the contents of the file ‘newfile’ should be displayed.
\tmp\homework1>ls -l | wc -m
*the number of characters in the output of ‘ls -l’ should be displayed
Evaluation criteria
• Your assignment will be evaluated based on the following:
Documentation 10% – your code should be easy to read and well commented. For each
function used in your program, the use of function, its parameters, and return values
should be well described.
Compilation 20% – your program should compile with no errors and/or warnings (base
points)
Correctness 70% – the output of your program should be correct
▪ Check that your program produces correct output at least for the examples
introduced in this document.
References
COMP 530: Operating Systems at UNC Chapel Hill (http://www.cs.unc.edu/~porter/courses/comp530)
