how to read output of system('ls')?

Question

I am doing some file IO with c code. I have a particular pattern in my file. I can verify this by a shell command cat abc.txt | grep abc | wc -l. When I execute

Answer 1

You can try:

...
pid_t pid = fork();
if(pid == 0){
    int fd = open("output", O_RDWR | O_CREATE);
    close(1);
    (void) dup(fd);  // this will return 1
    system(your_cmd);
    // read output from fd
    // close fd
    // exit
}
else{
...

This will redirect output from command executed by system() to your "output" file.

Answer 2

What grep and wc are doing are reading the STDIN file handle as part of the pipe | operator.

Answer 3

You don't want system(3) for that. Try popen(3) and friends.