c++ linux system command

Question

I have the following problem:

I use in my program this function:

  system("echo -n 60  > /file.txt"); 

it works fine.

But I don't want to have constant value. I do so:

   curr_val=60;
   char curr_val_str[4];
   sprintf(curr_val_str,"%d",curr_val);
   system("echo -n  curr_val_str > /file.txt");

I check my string:

   printf("\n%s\n",curr_val_str);

Yes,it is right. but system in this case doesn't work and doesn't return -1. I just print string!

How can I transfer variable like integer that will be printed in file like integer, but don't string?

So I want to have variable int a and I want to print value of a with system function in file. A real path to my file.txt is /proc/acpi/video/NVID/LCD/brightness. I can't write with fprintf. I don't know why.

Answer 1

you cannot concatenate strings like you are trying to do. Try this:

curr_val=60;
char command[256];
snprintf(command, 256, "echo -n %d > /file.txt", curr_val);
system(command);