在外壳中重定向输入和输出

Question

Hi I've been programming a shell in c and I got stuck while trying to redirect. While redirecting the stdout in my program works the stdin doesn't.

void redirect(node_t* node){
    // mode 0: >$%d mode 1: < mode 2: > mode 3: >>
    int input = 0;
    if(node->redirect.mode == 2){
        input = 1; // >
    } else{
        input = 0; // <
    }
    int pid = 0;
    int *status = 0;
    char * filename = node->redirect.target; // filename
    int fd;
    fd = open(filename, O_WRONLY | O_CREAT | O_TRUNC);
    if((pid = fork()) == 0){
        dup2(fd, input); // STDIN OR STDOUT
        close(fd);
        node_t* node2 = node->redirect.child;
        execvp(node2->command.program, node2->command.argv); // execute program
        printf("failed to execvp\n");
        exit(1);        
    } else {
        wait(status);
    }
}

I'm new to the fork() but my question is what am I doing wrong here that redirecting stdout works but stdin it writes nothing to the given file.

Answer 1

As mentioned in the comments, you need to use different open options depending on whether you're opening the file for input or output redirection. You can put this into your if .

int flags;
if(node->redirect.mode == 2){
    input = 1; // >
    flags = O_WRONLY | O_CREAT | O_TRUNC;
} else{
    input = 0; // <
    flags = O_RDONLY;
}
int pid = 0;
int *status = 0;
char * filename = node->redirect.target; // filename
int fd;
fd = open(filename, flags, 0666);

Also, you need to specify the permission modes for the case where the output file is created. It's OK to specify this argument all the time, it will be ignored when O_CREAT isn't in the flags.