How to use PIPE & FORK system calls to run three commands together.

pipedemo.c

#include        <stdio.h>
#include        <stdlib.h>
#include        <unistd.h>
 
#define oops(m,x)       { perror(m); exit(x); }
 
main(int ac, char **av)
{
        int     thepipe[2],                     /* first pipe for the first and second command  */
                thesecondpipe[2],               /* second pipe for the second and third commands*/
                pid;                            /* and the pid          */
 
        if ( ac != 4 ){
                fprintf(stderr, "usage: pipe cmd1 cmd2 cmd3\n");
                exit(1);
        }
 
        if ( pipe( thepipe ) == -1 )            /* get the first pipe*/
                oops("Cannot get a pipe", 1);
 
        if ( pipe(thesecondpipe) == -1 )        /* get the second pipe*/
                oops("Cannot get a pipe", 1);
 
        if ( (pid = fork()) == -1 )             /* get the first child for executing first command*/
                oops("Cannot fork", 2);
 
        if(pid == 0){                           /*First child reads from the stdin, executes the first command and writes the output to the first pipe*/
                close(thesecondpipe[0]);        /*First child doesn't read or write the second pipe*/
                close(thesecondpipe[1]);
                close(thepipe[0]);              /* First child doesn't read from the first pipe*/
                if ( dup2(thepipe[1], 1) == -1 )
                        oops("could not redirect stdout", 4);
                close(thepipe[1]);              /* stdout is duped, close the first pipe write end*/
                execlp( av[1], av[1], NULL);
                oops(av[1], 5);
        }
 
        if ( pid > 0 ){
 
                if((pid = fork()) == -1 )       /*create the second child to execute the second command*/
                        oops("Cannot fork", 2);
 
                if(pid == 0){                   /*second child reads from the first pipe, executes the second command and writes the output to the second pipe*/
                        close(thepipe[1]);      /*Second child doesn't write to the first pipe*/
                        if( dup2(thepipe[0], 0) == -1)
                                oops("Could not redirect stdin", 3);
                        close(thepipe[0]);      /*stdin is duped, close the first pipe read end*/
                        close(thesecondpipe[0]);/*Second child doesn't read from the second pipe*/
                        if( dup2(thesecondpipe[1], 1) == -1)
                                oops("Could not redirect stdout", 3);
                        close(thesecondpipe[1]);/*stdout is duped, close the second pipe write end*/
                        execlp( av[2], av[2], NULL);
                        oops(av[2], 5);
                }
 
                if(pid > 0){                    /*parent process reads from the second pipe, executes the third command and writes the output to the stdout*/
                        close(thepipe[0]);      /*Parent doesn't read or write the first pipe*/
                        close(thepipe[1]);
                        close(thesecondpipe[1]);        /* parent doesn't write to the second pipe*/
                        if ( dup2(thesecondpipe[0], 0) == -1 )
                                oops("could not redirect stdin",3);
                        close(thesecondpipe[0]);        /* stdin is duped, close the second  pipe read end*/
                        execlp( av[3], av[3], NULL);
                        oops(av[2], 4);
                }
        }
}