dup_example.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>
#include <string.h>
#include <fcntl.h>

// counts files on the current directory
// linux equivalent "ls -l | wc -l"

int main() {
	int pipes[2];
	pipe(pipes); // create pipe

	if (fork() == 0) { // child
		close(pipes[0]); // disable reading to pipe
		int bak = dup(STDOUT_FILENO); // backup stdout
		dup2(pipes[1], STDOUT_FILENO); // set stdout to pipe (write everything printed on pipe)
		execl("/bin/ls", "ls", NULL); // call ls (output will be writtern on pipe)
		dup2(bak, STDOUT_FILENO); // restore stdout from backup
	}

	else { // parent
		close(pipes[1]); // disable writing to pipe from parent

		int bak = dup(STDIN_FILENO); // backup stdin
		wait(NULL); // wait for child to complete
		dup2(pipes[0], STDIN_FILENO); // set stdin to pipe (read ls output)

		execl("/usr/bin/wc", "wc", "-l", NULL); // execute wc with standard input from pipe
		close(pipes[0]); // close pipe
		dup2(bak, STDIN_FILENO); // restore stdin to keyboard
	}
}