Thread: Append to end of file for shell in C

I am trying to write the shell program in C for I/O redirection. The input and output is working fine, but I also have to append output to the end of a file with the '>>', but it's not working. Whenever, I use >> in the command line, it just overwrites the output to the file. And I also get a warning when I run the program: warning: multi-character character constant. I would really appreciate it if someone can please assist me in the right direction to fix this problem.

Code:

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

        switch(input){
        case-1:
          printf("Syntax error!\n");
          continue;
          break;
        case0:
          break;
        case1:
          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;
        case0:
          break;
        case1:
          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;
        case0:
          break;
        case1:
          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];
          }

          return1;
        }
      }

      return0;
    }

    /*
     * 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];
          }

          return1;
        }
      }

      return0;
    }

    /*
     * 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];
          }

          return1;
        }}