Assignment 2 Writing a Unix Shell


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CSCE 451/851 Programming Assignment 2
Writing a Unix Shell
Interprocess Communication via Pipes
– This means no while(1), or equivalent ANYWHERE in your code.
– use a bounded for loop instead, for example
for (int should_run = 0; should_run < 25; should_run++)
– You may safely assume that files are not infinite, and therefore, you can read lines of a file until
the file ends. For example
while(getLine(file, inputString) != false)
– If we find an infinite loop this in submitted code you’ll get a 0.
• The program should exit on exit command.
• Finally, you may not use the system() function.
1 Overview of Project (PA1 and PA2)
This project consists of writing a C (or C++) program to serve as a shell interface that accepts user commands and executes each command in a separate process. A shell interface gives the user a prompt, after
which the next command is entered. The example below illustrates the prompt osh> and the user’s next
command: cat prog.c. (This command displays the file prog.c on the terminal using the Unix cat command.)
osh> cat prog.c
The above is an example of a simple command (i.e., it does not contain any operators). We will extend
this program to execute more complex commands which contains one or more simple command connected
together by an operator such as:
osh> cat < code.c
osh> cat < code.c > out
osh> ps | cat > out
Your shell will support the following operators (much like the standard csh or bash shells in Unix).
• > – Redirect stdout to a file
• < – Redirect stdin from a file
• >> – Append stdout to a file
• && – Execute next command only on success.
• || – Execute next command only on failure.
• ; – Execute next command regardless of success or failure.
• | – Pipe stdout of one command into stdin of next.
The functionality defined here for the above operators are basic, and may slightly deviate from the functionality defined by the standard shells available on Unix. But it is not the purpose of this project to write
a fully functional Unix shell. Rather, it is to introduce and familiarize ourselves with various concepts useful in understanding operating systems and systems programming such file I/O, processes control, and
inter-process communication. As such, we will only code parts of the shell that help introduce these topics.
PA1 This project is broken up into two programming assignments. In PA1 you will add support for:
• Input parsing
• fork and exec to create new processes
• All logical operators listed above except | (pipes)
PA2 In PA2, you will build upon PA1 and add support for:
• Interprocess communication via pipes
2 Submission
Use web handin to hand in your assignment. Submit a single zip file, <UNL username>_<pa#>.zip (e.g., containing only:
• source files
• makefile with (at least) targets:
– all: this must compile your osh binary and must be the first target in your makefile.
Executing make in your project directory should produce an executable named osh. Remember to verify
that your code compiles and runs on the CSE servers.
3 PA2: Evaluation and Points Distribution
The PA2 zip contains 4 test scripts, alongside the corresponding expected output files. The table describes
the test case, and the points awarded for each test case.
Script Test Case Description Points
6.singlePipe.txt ea6.txt Two command connected by a single pipe 25
7.malformed.txt ea7.txt Identify malformed commands 15
8.morePipes.txt ea8.txt Multiple commands connected by multiple pipes 25
9.simplePipeAndLogical.txt ea9.txt Multiple commands connected by pipe and logical
makefile The program compiles successfully on command
Total 100
The test scripts can be used to test your program and are the test scripts we use for grading. A fully
functional executable, osh, is included in the distribution zip file for this project and is executable on the
CSE servers. You can use it to see what your outputs should look like.
3.1 Critical “Gotchas” for grading
• Your code MUST compile and run on the CSE server. If it doesn’t, you’ll get a 0
• Ensure that the output binary produced by makefile is osh.
• Points will be awarded only if your output matches the expected output exactly (except for blank
lines). Please conform to the error strings as specified in the expected output for Malformed commands.
• You MUST implement the ‘-t’ option. This was described in the PA1 handout. If you don’t, you’ll get
a 0.
• Although you need to implement the ‘-t’ option, you still must have interactive shell capability. We
will test that it functions as an interactive shell.
• The parser has debug information and extra blanks in the output, so you should adjust that accordingly:
– Your shell needs to work interactively as a shell without extraneous debug information. . . that is
only printing command outputs and osh>
• Note that even common programs behave differently on different OSs. That is, the behavior of ls on
the CSE servers may not exactly match the behavior of ls on a Mac. So if you do your development
on your local machine be sure you test it regularly on the CSE servers since some output may differ.
• Additionally, some commands have different behavior when outputting to an interactive shell vs. a
file. For example, the ls command, in its documentation reads:
“If standard output is a terminal, the output is in columns (sorted vertically) and control
characters are output as question marks; otherwise, the output is listed one per line and
control characters are output as-is.”
So, running the commands from the testscripts interactively may produce outputs different than
what’s in the “ea*.txt” files. But your output should match exactly when run with the -t option.
3.2 Grading Procedure
On our end, after running make to compile your program, we will run:
./osh -t < testscripts/testscript.txt > & tmp ; diff -B tmp testscripts/ea.txt ;
for each test script in the testscripts directory. Note that the “> &” redirects stderr to stdout in csh. This
redirection looks different if you’re using a bash (i.e., 2>&1) shell, but for grading we will use csh. If there
are no differences you get all the points, otherwise we take points off according to the table above.
We will dock points for the following:
• Having an infinite loop in your code
• If your program doesn’t run as an interactive shell
• If, when running as an interactive shell, you print extraneous output (outside osh> and the command
4 PA2: Detailed Discussion and Description
This is the most challenging part of the shell project. We’ll handle pipes in two steps. First we implement
the logic to handle a single pipe, then extend it to handle any number of pipes in a command.
Pipes work similar to redirectors. But unlike a redirector, where the input/output is a file, pipe connects
two commands. Consider the following command:
osh> ls | cat
This command will pass stdout from ls to stdin of cat. We could equivalently write this as,
osh> ls > tempfile; cat < tempfile
which consists of two steps:
• we overwrite stdout of ls to point to tempfile
• we overwrite stdin of cat to point to tempfile
But creating a file is expensive if its purpose is just to act as a temporary buffer. If we can hold this buffer in
memory, it would be much more efficient. Interprocess communication (IPC) can help us do this. We will
use pipes in this PA to accomplish this IPC. There is a system call, pipe(), that can be used:
pipe() –
“pipes” are either bidirectional or unidirectional data channels, used for IPC. For this assignment we assume pipes are unidirectional and only pass information in one direction. For this project, using pipes
boils down to 3 steps:
• create the pipe
• connect stdout of the first command (e.g., ls) to one end of the pipe
• connect stdin of the next command (e.g., cat) to the other end of the pipe
The challenge in this assignment is to get the timing of the connections correct. The process image is copied
upon a call to fork(), so to pass the file descriptors of the pipe along to the child, the pipe needs to be
created before the fork. Once this is done, connect the pipes at the beginning of your loop (after fork()
and before calling exec()). We can generalize this as follows:
• If the current command is connected to the next command by a pipe:
1. create a pipe
2. store the pipe handles at a temp location
3. connect stdout of the current process to the pipe
• If the current command is connected to the previous command by a pipe:
1. connect the stdin of the current command to the pipe created earlier
Once these connections are made, the rest of the procedure for exec(), error handling, etc. remains unchanged. At a high level, your procedure should look like this:
prevpipe = null;
while(processing commands) {
get command()
new process = fork()
if (current command is connected to previous command by pipe) {
// connect stdin of new process to prevpipe
if (current command is connected to next command by pipe) {
prevpipe = pipe();
// connect stdout of new process to prevpipe
Remember that you still need to implement this in the context of forking the child processes and managing
them. So think carefully about how to connect the pipes amongst the child processes when they’re either
the left command, right command, or middle command in left_cmd | middle_cmd | right_cmd.
You should be able to handle commands with more than one pipe. It could be two pipes, or many more:
osh> ls | cat | grep foo
NOTE: In the case of a pipe, we do not wait for the current process to exit before starting the next one.
Instead we wait for the last process in the chain of commands connected by pipes. When we create a pipe,
a buffer of size PIPE_BUF will be allocated. When this buffer is full, it blocks the write (i.e., the current
process writing to the pipe also blocks). So we will need another process at the other end reading from the
5 Tips
5.1 Tip #1
Processes reading from pipes will hang (i.e., not receive an EOF) until all write file descriptors to the pipe
have been closed. Creating the pipes in the parent and forking two children will require careful closing of
the file descriptors for pipes to work. To minimize open write pipe descriptors, follow the recommendations
• The pipes need to be kept open until the fork() in order to replicate the pipe file descriptors to the
• After the first child is launched (writes to pipe) the parent should close the write end of the pipe before
the second fork
• The first child should close the write end of the pipe after it’s dup()-ed to stdout.
5.2 Tip #2
The best way I’ve found to approach this assignment is to start by thinking about it sequentially, with all
the loops unrolled, outside of the parser and logical operator support in PA1. In other words, we want to
simplify the task as much as possible and focus only on the pipes. Don’t worry about reading commands
from the command line, just hard code some commands for now. Once we have that working we can
incorporate it into what we wrote for PA1.
Start by assuming we have the command sequence:
osh> ls | cat
To process these commands this I need 2 child processes, one for ls and one for cat. So write up some
code to do this outside of PA1 and the parser:
// psuedocode for command “ls | cat”
int main() {
int fdp[2] // file descriptor array for pipe
pipe(fdp) // create pipe
pid = fork() // fork the first child process for “ls”
if (child) {
close(fdp[READ_END]); // don’t need the read end
dup2(fdp[WRITE_END], STDOUT_FILENO) // use the write end
close(fdp[WRITE_END]) // can close the write end now since it’s STDOUT
execvp(“ls”, args) // now exec the first command
} else { // now the parent
// close the write end of the pipe (remember it’s a copy) cuz here we need
// the read end
// at this point only the read end of the pipe is open in the parent
pid = fork() // fork another child now for “cat”
if (child) {
// use the read end of the pipe for cat
close(fdp[READ_END]) // now can close read end since it’s STDIN
execvp(“cat”, args) // exec cat
// finish work in parent by waiting
wait(&status) // wait for a child
wait(&status) // wait for other child
Once you have this working try putting things into a loop in this manner:
// psuedocode for command “ls | cat”
int main() {
int fdp[2] // file descriptor array for pipe
pipe(fdp) // create pipe
for(int i=0; i<2; i++) {
pid = fork() // fork the first child process for “ls”
if (child) {
if (i==0) { // if first command “ls”
// do all the stuff from above for “ls”
} else { // if second command “cat”
// do all the stuff from above for “cat”
} else { // this is the parent
// do all the stuff for the parent
You should start seeing a lot of duplicated code that could be simplified and placed more intelligently.
You should also start seeing there is a pattern for working with the pipes depending on where the current
command is. The above pseudocode can work for commands of the form osh> command1 | command2.
But what if I have osh> command1 | command2 | command3? You now need to start generalizing how
to work with the pipes if it’s a first command, middle command, or last command.
Let’s continue with the example above but add a third command, and rethink the conditions for using the
// psuedocode for command “ls | cat | grep foo”
int main() {
int fdp[2] // file descriptor array for pipe
int initial_pipe_in_chain // have to save the first pipe if we have multiple pipes
pipe(fdp) // create pipe
for(int i=0; i<3; i++) { // now need 3 child processes
pid = fork() // fork the first child process for “ls”
if (child) {
if (i==0) { // if first command
// do all the stuff from above for a first command
} else if (i==1) { // if middle command
// This is the new case. Here, we have to work with two pipes.
// Think carefully about how to connect the pipes to the command
// and which ends to close.
execvp(“cat”, args) // execute the middle command
} else { // if last command
// do all the stuff from above for a last command
} else { // this is the parent
// if there are multiple pipes in the chain we need to save the first one.
// but really we only need the read end
initial_pipe_in_chain = fdp[READ_END]
// do all the remaining stuff for the parent
At this point, depending on you implementation, hopefully it should be relatively straightforward to incorporate the above code into your existing PA1. There’s lots of repeated code you can simplify, after which
you can add a few conditions to determine whether you’re working with the first command, middle command, or last command.
Incidentally, compilable versions of the first two set of pseudocode presented are in the “helpful_code”
folder in the OneDrive class folder.

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