Description
In this assignment, you will implement pieces of a UNIX shell, and get some familiarity with
some UNIX library calls and the UNIX process model. By the end of the assignment, you will
have a shell that can run complex pipelines of commands, such as:
The above pipeline takes (a file generally installed on UNIX systems
that contains a list of English words), selects out the words containing the string “cat”, and then
uses sed to replace “cat” with dog, so that, for example, “concatenate” becomes
“condogenate”. The results are output to “doggerel.txt”. (You can find detailed descriptions of
each of the commands in the pipeline by consulting the manual page for the command; e.g.:
“man grep” or “man sed”.)
Start by downloading the shell.c skeleton file attached to this homework. You don’t have to
understand how the parser works in detail, but you should have a general idea of how the flow
of control works in the program. You will also see the “// your code here” comments, which is
where you will implement the functionality to make the shell actually work.
Next, try to compile the source code to the shell:
You can then run and interact with the shell by typing ./shell :
Note that the command prompt for our shell is set to to make it easy to tell the
diff erence between it and the Linux/OS X shell. You can quit your shell by typing Control-C or
Control-D.
cat /usr/share/dict/words | grep cat | sed s/cat/dog/ > doggerel.txt
$ gcc shell.c -o shell
user@cs3224:~$ ./shell
cs3224> ls
exec not implemented
cs3224>
/usr/share/dict/words
cs3224>
runcmd
PATH
redircmd
Problem 1 – CommandExecution (30 points)
Implement basic command execution by filling in the code inside of the block in the
function. You will want to look at the manual page for the exec(3) function by typing
“man 3 exec” (Note: throughout this course, when referring to commands that one can look up
in the man pages, we will typically specify the section number in parentheses — thus, since exec
is found in section 3, we will say exec(3)).
Once this is done, you should be able to use your shell to run single commands, such as
Hint:
1. You will notice that there are many variants on exec(3). You should read through the
diff erences between them, and then choose the one that allows you to run the
commands above — in particular, pay attention to whether the version of exec you’re
using requires you to enter in the full path to the program, or whether it will search the
directories in the environment variable.
Problem 2 – I/O Redirection (30 points)
Now extend the shell to handle input and output redirection. Programs will be expecting their
input on standard input and will write to standard output, so you will have to open the file and
then replace standard input or output with that file. As before, the parser already recognizes the
‘>’ and ‘<‘ characters and builds a structure for you, so you just need to use the
information in that to open a file and replace standard input or output with it.
Hints:
1. Look at the dup2(2) and open(2) calls.
2. The file descriptor the program is currently using for input or output is available in rcmd3. If you’re confused about where is coming from, look at the
function and remember that 0 is standard input, 1 is standard output.
4. Be careful with the open call; in particular, make sure you read about the case when you
pass the O_CREAT flag.
When this is done, you should be able to redirect the input and output of commands:
cs6233> ls
cs6233> grep cat /usr/share/dict/words
>fd .
case ‘ ‘
redircmd
rcmd->fd redircmd
Problem 3 – Pipes (40 points)
The final task is to add the ability to pipe the output of one command into the input of another.
You will fill out the code for the ‘|’ case of the switch statement in runcmd to do this.
Hints:
1. The parser provides the left command in and the right command in pcmd2. Look at the fork(2), pipe(2), close(2), , and wait(2) calls.
3. If your program just hangs, it may help to know that reads to pipes with no data will block
until all file descriptors referencing the pipe are closed.
4. Note that fork(2) creates an exact copy of the current process. The two process share
any file descriptors that were open at the time the fork occurred. You can get a sense for
this behavior by looking at a small test program like this one:
cs6233> ls > a.txt
cs6233> sort -r < a.txt
>right .
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/stat.h>
int main() {
int filedes;
filedes = open(“myfile.txt”, O_RDWR | O_CREAT, S_IRUSR | S_IWUSR);
int rv;
rv = fork();
if (rv == 0) {
char msg[] = “Process 1\n”;
printf(“Hello, I’m in the child, my process ID is %d\n”,
getpid());
write(filedes, msg, sizeof(msg));
}
else {
char msg[] = “Process 2\n”;
printf(“This is the parent process, my process ID is %d and my
child is %d\n”, getpid(), rv);
write(filedes, msg, sizeof(msg));
}
close(filedes);
}
pcmd->left
If you put that code into a file, compile it, and then run the resulting program, you should
see a result like:
You can see that both the parent and child process both got a copy of “filedes”, and that
writes to it from each process went to the same underlying file.
5. You may find it helpful to re-read the first chapter of the xv6 book, which describes in
detail how the xv6 shell works. Note that the code show there will not work as-is — you
will have to adapt it for the Linux/OS X environment.
Once this is done, you should be able to run a full pipeline:
You can now submit your modified shell.c on NYU Classes.
Credits: This assignment is adapted from a homework by Brendan Dolan-Gavitt
user@cs6233:~$ ./a.out
This is the parent process, my process ID is 56968 and my child is 56969
Hello, I’m in the child, my process ID is 56969
user@cs6233:~$ cat myfile.txt
Process 2
Process 1
cs6233> cat /usr/share/dict/words | grep cat | sed s/cat/dog/ > doggerel.txt
cs6233> grep con < doggerel.txt
