Multiple pipe implementation using system call fork() execvp() wait() pipe() - it is simply not working

Question

I need to implement my shell that handles multiple pipe commands. For example I need to be able to handle this: ls | grep -i cs340 | sort | uniq | cut -c 5. I a

Answer 1

I suggest a new strategy, R2:

function do(commands)
    if commands is of size 1
        exec commands[0] || die
    split commands into c1 (first command) c2 (the rest of them)
    open
    if fork 
        close input end of pipe
        dup output of pipe to stdin
        do (c2) || die
    close output end of pipe
    dup input of pipe to stdout
    exec c1 || die

Using a recursive function, especially if you're maintaining a list, will help you simplify your logic. You don't really have to worry about stack depth here, as your whole address space will be overwritten anyway.

In other news, from the man page:

After a successful return from one of these system calls, the old and new file descriptors may be used interchangeably. They refer to the same open file description (see open(2)) and thus share file offset and file status flags; for example, if the file offset is modified by using lseek(2) on one of the descriptors, the offset is also changed for the other.

Which meanse when you say you're closing both ends of the pipe? You really are closing it - it AND the standard in/out that your program is intending on using.

--> MUCH LATER EDIT <--

As Jonathan Leffler pointed out, the above information is in correct. I have confirmed it with the following program:

#include <unistd.h>

int main(){
    dup2(0, 7);
    write(7, "Hey, 1\n", 7);
    close(0);
    write(7, "Hey, 2\n", 7);
    close(7);
    write(7, "Hey, 3\n", 7);
}

Which results in the following output:

$ gcc dup2Test.c && ./a.out
Hey, 1
Hey, 2

Thanks, Jonathan!