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.

Solution 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);

Solution 2

The system function takes a string. In your case it's using the text *curr_val_str* rather than the contents of that variable. Rather than using sprintf to just generate the number, use it to generate the entire system command that you require, i.e.

sprintf(command, "echo -n %d > /file.txt", curr_val);

first ensuring that command is large enough.

Solution 3

The command that is actually (erroneously) executed in your case is:

 "echo -n curr_val_str  > /file.txt"

Instead, you should do:

char full_command[256];
sprintf(full_command,"echo -n  %d  > /file.txt",curr_val);
system(full_command);

Solution 4

#define MAX_CALL_SIZE 256
char system_call[MAX_CALL_SIZE];
snprintf( system_call, MAX_CALL_SIZE, "echo -n %d > /file.txt", curr_val );
system( system_call );

man snprintf

Solution 5

Have you considered using C++'s iostreams facility instead of shelling out to echo? For example (not compiled):

std::ostream str("/file.txt");
str << curr_val << std::flush;

Alternately, the command you pass to system must be fully formatted. Something like this:

curr_val=60;
std::ostringstream curr_val_str;
curr_val_str << "echo -n " << curr_val << " /file.txt";
system(curr_val_str.str().c_str());

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Comments

[Mark B]

This is worth +1 just for using snprintf instead of sprintf.