return code of system()

First of all, you should be using WEXITSTATUS(res). The standard clearly states:

If command is not a null pointer, system() shall return the termination status of the command language interpreter in the format specified by waitpid().

I suspect the problem is that the command actually succeeds (grep finds itself). Try not to redirect the output for a moment:

[cnicutar@fresh ~]$ ./test  989 sh -c ps ax -o pid -o command | grep sudoku | grep gnomeres = 0

So, since every commands executes successfully, the return code will be 0 :-). You might have better luck with pgrep and the like.

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;}

If you have pgrep, use it instead of your shell pipeline.

system("pgrep -x gnome-sudoku >/dev/null");

When you call

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

the system executes

sh -c 'ps ax -o pid -o command | grep sudoku | grep gnome > /dev/null'

which shows up in ps and passes the grep filters.