c - 如何在 C 语言中的 shell 中追加到文件末尾？

Question

我正在尝试用 C 语言编写用于 I/O 重定向的 shell 程序。输入和输出工作正常，但我还必须使用“>>”将输出 append 到文件末尾，但它不起作用。每当我在命令行中使用 >> 时，它只会覆盖文件的输出。而且运行程序时我还收到警告:警告:多字符字符常量。如果有人能在正确的方向上帮助我解决这个问题，我将非常感激。

        // Check for redirected input
        input = redirect_input(args, &input_filename);

        switch(input) {
        case -1:
          printf("Syntax error!\n");
          continue;
          break;
        case 0:
          break;
        case 1:
          printf("Redirecting input from: %s\n", input_filename);
          break;
        }

        // Check for redirected output
        output = redirect_output(args, &output_filename);

        switch(output) {
        case -1:
          printf("Syntax error!\n");
          continue;
          break;
        case 0:
          break;
        case 1:
          printf("Redirecting output to: %s\n", output_filename);
          break;
        }

        // Check for redirected append output
        append = redirect_append(args, &append_filename);

        switch(append) {
        case -1:
          printf("Syntax error!\n");
          continue;
          break;
        case 0:
          break;
        case 1:
          printf("Redirecting output to: %s\n", output_filename);
          break;
        }



        // Do the command
        do_command(args, block, 
               input, input_filename, 
               output, output_filename,
               append, append_filename);
      }
    }


    /* 
     * Do the command
     */
    int do_command(char **args, int block,
               int input, char *input_filename,
               int output, char *output_filename,
               int append, char *append_filename) {

      int result;
      pid_t child_id;
      int status;

      // Fork the child process
      child_id = fork();

      // Check for errors in fork()
      switch(child_id) {
      case EAGAIN:
        perror("Error EAGAIN: ");
        return;
      case ENOMEM:
        perror("Error ENOMEM: ");
        return;
      }

      if(child_id == 0) {

        // Set up redirection in the child process
        if(input)
          freopen(input_filename, "r", stdin);

        if(output)
          freopen(output_filename, "w+", stdout);

        if (append)
          freopen(append_filename, "a", stdout);

        // Execute the command
        result = execvp(args[0], args);

        exit(-1);
      }

      // Wait for the child process to complete, if necessary
      if(block) {
        printf("Waiting for child, pid = %d\n", child_id);
        result = waitpid(child_id, &status, 0);
      }
    }

    /*
     * Check for input redirection
     */
    int redirect_input(char **args, char **input_filename) {
      int i;
      int j;

      for(i = 0; args[i] != NULL; i++) {

        // Look for the <
        if(args[i][0] == '<') {
          //free(args[i]);

          // Read the filename
          if(args[i+1] != NULL) {
        *input_filename = args[i+1];
          } else {
        return -1;
          }

          // Adjust the rest of the arguments in the array
          for(j = i; args[j-1] != NULL; j++) {
        args[j] = args[j+2];
          }

          return 1;
        }
      }

      return 0;
    }

    /*
     * Check for output redirection
     */
    int redirect_output(char **args, char **output_filename) {
      int i;
      int j;

      for(i = 0; args[i] != NULL; i++) {

        // Look for the >
        if(args[i][0] == '>') {
          //free(args[i]);

          // Get the filename 
          if(args[i+1] != NULL) {
        *output_filename = args[i+1];
          } else {
        return -1;
          }

          // Adjust the rest of the arguments in the array
          for(j = i; args[j-1] != NULL; j++) {
        args[j] = args[j+2];
          }

          return 1;
        }
      }

      return 0;
    }

    /*
     * Check for append redirection
     */
    int redirect_append(char **args, char **append_filename) {
      int i;
      int j;

      for(i = 0; args[i] != NULL; i++) {

        // Look for the >>
        if(args[i][0] == '>' && args[i][1] == '>') {
          //free(args[i]);

          // Read the filename
          if(args[i+2] != NULL) {
        *append_filename = args[i+2];
          } else {
        return -1;
          }

          // Adjust the rest of the arguments in the array
          for(j = i; args[j-1] != NULL; j++) {
        args[j] = args[j+2];
          }

          return 1;
        }
      }

      return 0;
    }

Answer 1

if(args[i][0] == '>>') {

应该是:

if(args[i][0] == '>' && args[i][1] == '>') {

这是“多字符常量”警告以及输出重定向问题的根源。