如何将system（）的输出重定向到文件？

In this C program

在这个C程序中

#include <stdio.h>
#include <fcntl.h>
int main()
{
    int file = open("Result", O_CREAT|O_WRONLY, S_IRWXU);

    dup2(stdout, file);
    system("ls -l");

    return 0;
}

I'm trying to redirect the output of system() to a file, for that i have used dup2 but it is not working.

我正在尝试将system()的输出重定向到一个文件,因为我已经使用了dup2但它无法正常工作。

What's wrong with this code?
and, please tell me if there is any better way to do this? (without using > at the terminal )

这段代码出了什么问题?并且,请告诉我是否有更好的方法来做到这一点? (不在终端使用>)

4 个解决方案

#1

stdout is a FILE * pointer of the standard output stream. dup2 expects file descriptor, also you've messed up the parameters order. Use

stdout是标准输出流的FILE *指针。 dup2需要文件描述符,你也搞砸了参数顺序。使用

dup2(file, 1);

instead.

On the better-way-to-do-this part. This way is bad because you probably want to restore your standard output after this system call completes. You can do this in a variety of ways. You can dup it somewhere and then dup2 it back (and close the dupped one). I personally don't like writing own cat implementations as suggested in other answers. If the only thing you want is redirecting a single shell command with system to a file in the filesystem, then probably the most direct and simple way is to construct the shell command to do this like

在更好的方式做这部分。这种方式很糟糕,因为您可能希望在此系统调用完成后恢复标准输出。您可以通过多种方式执行此操作。您可以在某处复制它,然后将其复制(并关闭重复的)。我个人不喜欢像其他答案中所建议的那样编写自己的猫实现。如果您唯一想要的是将带有system的单个shell命令重定向到文件系统中的文件,那么可能最直接和最简单的方法是构造shell命令来执行此操作

system("ls -l > Result");

But you have to be careful if filename (Result) comes from user input as user can supply something like 'Result; rm -rf /*' as the filename.

但是如果文件名(结果)来自用户输入,你必须要小心,因为用户可以提供类似'结果; rm -rf / *'作为文件名。

Also, on the topic of security, you should consider specifying the full path to ls as suggested in the comments:

另外,关于安全性主题,您应该考虑在注释中指定ls的完整路径:

system("/bin/ls -l > Result");

#2

The simple thing is to use > indeed:

简单的事情是使用>确实:

#include <stdio.h>
int main()
{
    system("ls -l > /some/file");

    return 0;
}

An alternative is using popen(), something along the lines of

另一种方法是使用popen(),类似于

   #include <stdio.h>
   #include <stdlib.h>
   main()
   {
           char *cmd = "ls -l";
           char buf[BUFSIZ];
           FILE *ptr, *file;

           file = fopen("/some/file", "w");
           if (!file) abort();
           if ((ptr = popen(cmd, "r")) != NULL) {
                   while (fgets(buf, BUFSIZ, ptr) != NULL)
                           fprintf(file, "%s", buf);
                   pclose(ptr);
           }
           fclose(file);
           return 0;
   }

#3

You should use the popen() library function and read chunks of data from the returned FILE * and write them to whatever output file you like.

您应该使用popen()库函数并从返回的FILE *中读取数据块,并将它们写入您喜欢的任何输出文件。

Reference.

#4

use dup instead of dup2. dup creates a alias file descriptor, which value should be always the smallest available file descriptor.

使用dup而不是dup2。 dup创建别名文件描述符,该值应始终是最小的可用文件描述符。

new_fd = dup(file); - In this statement file might be having the value 3 (because stdin is 0, stdout is 1 and stderr is 2). so new_fd will be 4

new_fd = dup(file); - 在此语句中,文件的值可能为3(因为stdin为0,stdout为1,stderr为2)。所以new_fd将是4

If you want to redirect stdout into file. Then do as below.

如果要将stdout重定向到文件中。然后执行以下操作。

close(stdout);
new_fd = dup(file);

Now dup will return 1 as the alias for the file descriptor, because we closed stdout so opened file descriptors are 0,2,3 and 1 is smallest available file descriptor.

现在dup将返回1作为文件描述符的别名,因为我们关闭了stdout所以打开的文件描述符是0,2,3并且1是最小的可用文件描述符。

If you are using dup2 means, dup2(file,1); - do like this

如果你使用dup2手段,dup2(文件,1); - 这样做

#1