Start by downloading the shell.c skeleton file attached. 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: $ gcc shell.c -o shell
You can then run and interact with the shell by typing ./shell : user@cs6233:~$ ./shell cs6233> ls exec not implemented cs6233> Note: The command prompt for our shell is set to cs6233> to make it easy to tell the difference between it and the Linux/macOS shell. You can quit your shell by typing Control-C or Control-D.
Problem 1: Command Execution (30 points) Implement basic command execution by filling in the code inside of the case ' ' blockin the runcmd 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 cs6233> ls cs6233> grep cat /usr/share/dict/words Hint:
- You will notice that there are many variants on exec(3). You should read through the differences 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 PATH 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 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 redircmd structure for you, so you just need to use the information in that redircmd to open a file and replace standard input or output with it. Hints:
- Look at the dup2(2) and open(2) calls.
- The file descriptor the program is currently using for input or output is available in rcmd->fd .
- If you're confused about where rcmd->fd is coming from, look at the redircmd function and remember that 0 is standard input, 1 is standard output.
- 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 > a.txt cs6233> sort -r < a.txt 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:
- The parser provides the left command in pcmd->left and the right command in pcmd- >right.
- Look at the fork(2), pipe(2), close(2) and wait(2) calls.
- 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.
- 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:
#include #include #include #include <sys/stat.h>
<stdio.h> <unistd.h> <fcntl.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); }
If you put that code into a file, compile it, and then run the resulting program, you should see a result like:
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 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/macOS environment. Once this is done, you should be able to run a full pipeline: cs6233> cat /usr/share/dict/words | grep cat | sed s/cat/dog/ > doggerel.txt cs6233> grep con < doggerel.txt