使用execvp在输出重定向期间接收错误代码 [英] Receiving error code during output redirection using execvp

问题描述

我正在尝试将输出从ls重定向到文件，该文件在C语言中创建的外壳中.我键入:

I am trying to redirect the output from ls to a file, in a shell I created in C. I type in:

ls > junk 

我得到的是:

ls: cannot access >: No such file or directory

然后，如果我使用CTRL-D退出外壳程序，则会在退出之前将ls命令的结果打印到屏幕上.我试图使用打印语句找出发生的地方，并且在以下情况下没有打印打印语句:

Then if I use CTRL-D to exit the shell it prints the results of the ls command to the screen before exiting. I tried to use print statements to figure out where it is happening and no print statements get printed after:

dup2(f, STDOUT_FILENO); Also tried  dup2(f, 1);

代码:

            pid = fork();

            if(pid == 0)
            {
              // Get the arguments for execvp into a null terminated array  
                    for(i = 0; i <= count; i++)
                    {   if(i == count)
                        {
                            args[i] = (char *)malloc(2 * sizeof(char));
                            args[i] = '\0';
                        }
                        else
                        {
                            str = strlen(string[i]);
                            args[i] = malloc(str);
                            strcpy(args[i], string[i]);                     
                        }
                    }                       

                if(count == 1)
                {

                }
                else if(strcmp(string[(numargs + 1)], ">") == 0) //numargs is the number of arguments typed in by the user
                {
// printed out string[numargs+2] previously, and it says junk
                    int f = open(string[(numargs + 2)], O_WRONLY | O_CREAT | O_TRUNC, S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH);

                    if(f < 0)
                    {
                        printf("Unable to open output file\n");
                        status = 1;
                    }
                    else
                    {
                        fflush(stdout);
                        dup2(f, STDOUT_FILENO);

                        close(f);

                    }
                }  

                j = execvp(string[0], args); // The first element of the string array is the first thing the user enters which is the command ls in this case

称为垃圾文件的文件已创建，但放置在其中的所有文件均为垃圾文件.我已经为此苦苦挣扎了一段时间，因此对弄清为什么重定向不起作用的任何帮助将不胜感激.谢谢.

The file called junk gets created, but all that gets placed in it is junk. I have been struggling with this for a while so any help figuring out why the redirection won't work would be greatly appreciated. Thanks.

推荐答案

您不能使用execvp来解析shell命令.

You cannot use execvp to parse shell commands.

shell可以识别重定向(>)字符(例如bash，sh，ksh)，并且execvp执行直接传递该命令的命令.它不会尝试解释参数并创建文件重定向等.

The redirection (>) character is understood by the shell (e.g., bash, sh, ksh) and execvp executes the command you pass it directly. It does not try and interpret the arguments and create file redirections, etc.

如果要执行此操作，则需要使用system调用.参见系统(3)

If you want to do that you need to use the system call. See System(3)

类似地，任何其他特殊的shell字符(竖线，* 、?和&等)都将无效.

Similarly, any other special shell characters (pipe, *, ?, &, etc) won't work.

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