Processes are the fundamental abstraction in any operating system- it can be argued that the main purpose of an OS is to simplify execution of computer programs. A central part of this process is the program that allows a user to select and execute other programs: on Unix-based OSes this is the terminal or shell. The shell is not part of the operating system kernel, but it is a natural extension of it and in many ways represents the operating system itself!
We never want to take programs like
for granted, so we'll go through the dirty work of building or own shell
In this lab, you will:
Please complete this assignment in teams of two or three.
slush - SLU shell
slush is a very simple command-line interpreter. It uses a different syntax than
bash, and has much less functionality.
slush executes in a loop in
which it displays a prompt, reads in a command line from standard input, and
executes the command.
There are two types of commands: built in commands which are executed by
slush itself, and program execution commands which are carried out by separate
A built in command must appear on a line by itself. The only built in command is:
cd dir - change current directory to
Program execution commands have the form:
prog_n [args] ( ... prog_3 [args] ( prog_2 [args] ( prog_1 [args]
This command runs the programs
prog_n, ... ,
prog_1 (each of which may
have zero or more arguments) as separate processes in a "pipeline". This means
the standard output of each process is connected to the standard input of the
The syntax of
slush is backwards from shells you're used to, and is intended
to emphasize the functional nature of pipeline commands. As an example, the
more ( sort ( ps aux
should produce a paginated, sorted list of processes.
slush should catch ^C typed from the keyboard. If a command is running, this
should interrupt the command. If the user is entering a line of input,
should respond with a new prompt and a clean input line.
slush will exit when it reads an end-of-file on input.
If a program name does not exist or is not executable,
slush should print an
error message of the form:
prog1: Not found
(To do this automatically, you can use
perror() after the
Syntax errors such as:
more ( ( ps au
more ( ps au (
should be handled with an appropriate message, such as:
Invalid null command
slush. You can create a EOF by typing ^D at the keyboard, or by running your program with noninteractive input. For example,
echo ls | slushshould run
slush, execute the
lscommand, and quit
slush. I suggest you read input using
fgets()- the return value is false (0) on EOF.
strtok()function. For simplicity, you may restrict each command in a pipeline to at most 15 arguments. You may also limit the length of an input line to 256 characters. Violations of these limits may result in an error or simply be ignored, so long as
slushdoes not crash.
fork()off right to left as you return from the series of recursive calls.
close()the file descriptor for the end of the pipe it's not going to use (or else it will never terminate). Do this after each
fork(). Both parent and child may need to close something. Count your children... you'll need to
wait()for them later.
SIGINTsignal. You need to set up a signal handler to catch the signal. What happens if a signal is caught while reading a line of input? How about if a signal is caught while
wait()is waiting? One more hint for ^C handling: typing ^| will send a
SIGQUITand stop your process if you've screwed up ^C.
The following man pages will be useful:
slushprompt to print the current directory. Don't print the entire directory path- just print everything past the user's home directory. For example, if I login and issue the command '
cd dir1/dir2', then the whole directory path is actually:
That's much too long- I really just want to know the directory path within my home directory. That is, I want my shell prompt to look something like:
The manual pages
be helpful here, as well as the knowledge that there is an environment variable
HOME that stores the current user's home directory.
Submit your program and related files via the Git repository.
Your submission must include a makefile
that will compile your program by simply issuing the command
make. You must also include a text file with your
answers to the required exercises. Please include your name and the names of
any partners in the body of your email.