COP 4600 – Homework 3
Total points: 6 + 1 bonus points
Create a shell program (6 pt)
Use the Unix environment you installed in the previous homework to write a C or C++
program called mysh replaces the command shell in Unix. After started, it prints a
prompt “#” and reads a command line terminated by newline. This line should be
parsed out into a command and all its arguments. In other words, tokenize it.
• You may assume that the only supported delimiter is the whitespace character
(ASCII character number 32).
• You do not need to handle “special” characters. Do not worry about handling
quotation marks, backslashes, and tab characters. This means your shell will be
unable support arguments with spaces in them. For example, your shell will not
support file paths with spaces in them.
• You may set a reasonable maximum on the number of command line arguments,
but your shell should handle input lines of any length.
Implement built-in commands
Your shell should implement the following commands
# movetodir directory
Your program should have an internal variable called “currentdir”. This command checks
whether the specified directory exists and it is a directory. If it exists, the currentdir
variable should be assigned this value. If the specified directory does not exist, the
command should write an error message and the currentdir variable should not be
Note: do not use the chdir() function call here!!!
Prints the value of the currentdir variable.
# history [-c]
Without the parameter, prints out the recently typed commands (with their arguments),
in reverse order, with numbers. For instance, it can print:
1: movetodir /etc
2: movetodir /eetc
If “-c” is passed, it clears the list of recently typed commands. You will need to
implement an internal data structure to keep the list of commands. The data structure
should allow you to grow the data structure to an arbitrary length.
You will need to save the history data structure to an external file (when exiting), and
load it (when starting the shell).
Terminates the mysh shell (and it should save the history file).
# replay number
Re-executes the command labeled with number in the history
# start program [parameters]
The argument “program” is the program to execute. If the argument starts with a “/”
(such as /usr/bin/xterm, the shell should interpret it as a full path. Otherwise, the
program will be interpreted as a relative path starting from the current directory.
The program will be executed with the optional “parameters”. It uses fork() + exec() to
start the program with the corresponding parameters, and waits until the program
terminates (use the waitpid() call).
run /usr/bin/xterm –bg green
would bring up a terminal with a green background. The prompt would not return until
the terminal is closed.
Display an error message if the specified program cannot be found or cannot be
# background program [parameters]
It is similar to the run command, but it immediately prints the PID of the program it
started, and returns the prompt.
# dalek PID
Immediately terminate the program with the specific PID (presumably started from this
command line interpreter). Use the kill() function call to send a SIGKILL signal to the
program. Display success or failure.
Extra credit (1 point)
Implement a repeat command as follows:
# repeat n command …
Interprets n as the number of times the command must be run, command as the full
path to the program to execute, and the others as parameters. The command should
start the specified number of program instances, print the PIDs of the created processes
and then return to the prompt, without waiting for the processes to terminate. For
repeat 4 /usr/bin/xterm
would bring up 4 terminals and print out something like:
PIDs: 31012, 31013, 31014, 31015.
More extra credit (1 point)
Implement the following command:
Which immediately terminates all the programs previously started by the mysh shell
which had not been previously terminated by it, or by exterminate. It should output
something like this:
Exterminating 3 processes: 16000 31012 31013
What to submit:
• The code as a single .c or .cpp file.
• If you implemented the extra credit part: a text file describing the syntax of the
implementation, and example of use.
Note 1: If you had not used a command line shell before, I would recommend that
before you start the homework familiarize yourself with some shell(s). Bash – the
default shell in most Linux installations and recent MacOS is a good choice. It is also
available in Windows.
Note 2: Many of the individual components of this project are standard code snippets.
Don’t be afraid to google for samples. For instance you might want to Google these and
o Tokenize string c++
o Fork process c++
Note 3: This is a relatively long, but easily modularizable program. This is an excellent
opportunity to improve and show off your programming skills by organizing the program
in a clever way. For instance, clearly, the individual commands should be implemented
in their own function. Parameters should be passed to them. They should return an
error / success code. The parsing should be done in one location, and the command
functions called from there.
You should be able to debug these functions individually.
/usr/bin/xterm will work on Linux, or on Windows with VcXsrv or other X environment
enabled. If you are on MacOS you will need to find a command line to bring up the Mac