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I'm working on a simple shell, but right now I am just trying to understand redirection. I'm just hard coding an ls command and trying to write it to a file for now. Currently, the ls runs, and the output file is created, but the output still goes to stdout and the file is blank. I'm confused as to why. Thanks in advance.

Here is my code:

int main()
{
    int ls_pid; /* The new process id for ls*/
    char *const ls_params[] = {"/bin/ls", NULL}; /* for ls */
    int file; /* file for writing */

    /* Open file check user permissions */
    if (file = open("outfile", O_WRONLY|O_CREAT) == -1) /* , S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP */
    {
        perror("Failed to open file");
        _exit(EXIT_FAILURE);
    } 

    ls_pid = fork(); /* Create new process for ls */

    if (ls_pid == -1) /* error check */
    {
        perror("Error forking ls (pid == -1)");
        _exit(EXIT_FAILURE);
    }
    else if (ls_pid == 0) /* Child of ls */
    {
        /* Redirect output to file */
        if (dup2(file, STDOUT_FILENO) == -1) /* STDOUT_FILENO = 1 */
        {
            perror("Error duping to file");
            _exit(EXIT_FAILURE);
        }
        close(file);

        execvp("ls", ls_params); /* create the sort process */
        /* execlp("ls", "ls", NULL); */

        /* if this does not end the program, something is wrong */
        perror("Exec failed at sort");
        _exit(EXIT_FAILURE);
    }
    else /* ls parent (1) */
    {
        /* wait for child */
        if (wait(NULL) == -1)
        {
            perror("fork failed on parent");
            _exit(EXIT_FAILURE);
        }
    }

}
4

1 回答 1

0

我看不出有什么问题。我自己试过你的代码。它有效！我的简化代码如下所示：

int main(void) {
    int fd;

    char* const param[] = {"/bin/ls", NULL};

    fd = open("outfile", O_WRONLY | O_CREAT, S_IRUSR | S_IWUSR);

    if (fd < 0) {
        perror("open failed\n");
        exit(0);
    }

    pid_t pid = fork();

    if (pid < 0) {
        perror("fork failed\n");
        exit(0);
    }
    else if (pid == 0) {
        dup2(fd, STDOUT_FILENO);

        close(fd);

        execvp("ls", param);
    }
    else {
        wait(NULL);
    }
}

我编译并执行它，并在 outfile 中找到预期的结果。

唯一的区别是我用 S_IRUSER | 喂食。S_IWUSER 权限选项，否则打开失败。我在你的代码中看到了类似的东西，但不知何故你评论了它们......

于 2012-03-10T13:41:09.903 回答