Question

#include <stdlib.h>
#include <string.h>
#include <stdio.h>

int main() {

int res = system("ps ax -o pid -o command | grep sudoku | grep gnome > /dev/null");

printf("res = %d \n", res);

return 0;
}

I want to see if sudoku is running or not by just examining the return code of system() (or any other call for that matter). I do not want any output to be printed anywhere.

I do not quite understand the return code of system() even after looking at the man page

Whether sudoku is running or not, I get res = 0.

Answer 1

The way you are trying to capture the output of grep may not work.

Based on the post:
C: Run a System Command and Get Output?

You can try the following. This program uses popen()

#include <stdio.h>
#include <stdlib.h>


int main( int argc, char *argv[] )
{

    FILE *fp;
    int status;
    char path[1035];

    /* Open the command for reading. */
    fp = popen("/bin/ps -x | /usr/bin/grep gnome-sudoku", "r"); 
    if (fp == NULL) {
        printf("Failed to run command\n" );
        exit;
    }
    /* Read the output a line at a time - output it. */
    while (fgets(path, sizeof(path)-1, fp) != NULL) {
      printf("%s", path);
    }
    pclose(fp);
return 0;
}

For reference to popen() see:

http://linux.die.net/man/3/popen

And if you try to use grep then you can probably redirect the output of grep and read the file in the following way:

#include <stdlib.h>
#include <string.h>
#include <stdio.h>
int main() {

    int res = system("ps -x | grep SCREEN > file.txt");
    char path[1024];
    FILE* fp = fopen("file.txt","r");
    if (fp == NULL) {
      printf("Failed to run command\n" );
      exit;
    }
    // Read the output a line at a time - output it.
    while (fgets(path, sizeof(path)-1, fp) != NULL) {
      printf("%s", path);
    }
    fclose(fp);
    //delete the file
    remove ("file.txt");
    return 0;
}