pipe.c

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

#define READ_END  0
#define WRITE_END 1

char buf[256];
char text[] = "hello!!\n";

int main()
{
    pid_t pid;
    int in_pipe[2], out_pipe[2];

    //Create pipe for input and output
    pipe(in_pipe);
    pipe(out_pipe);

    //Fork
    pid = fork();
    if (pid == 0) {
        //Child

        //Close unused ends
        close(in_pipe[WRITE_END]);
        close(out_pipe[READ_END]);


        dup2(in_pipe[READ_END], STDIN_FILENO);
        dup2(out_pipe[WRITE_END], STDOUT_FILENO);

        //Execute command
        execlp("tr", "tr", "[a-z]", "[A-Z]", NULL);
    } else {
        //Parent

        //Write to child input
        write(in_pipe[WRITE_END], text, strlen(text));
        close(in_pipe[WRITE_END]);

        //Wait child process
        wait(NULL);

        //Read from child ouput
        read(out_pipe[READ_END], buf, sizeof(buf));
        close(out_pipe[READ_END]);
        printf("%s", buf);
    }
    return 0;
}