Here's an example of how to pipe three commands together using C. This one uses `ps aux | grep root | grep sbin`. This topic is horribly documented online so hopefully this'll help someone else out.

threePipeDemo.c

// This program is an example of how to run a command such as
// ps aux | grep root | grep sbin
// using C and Unix.

#include <stdlib.h>
#include <unistd.h>

int pid;
int pipe1[2];
int pipe2[2];

void main() {

  // create pipe1
  if (pipe(pipe1) == -1) {
    perror("bad pipe1");
    exit(1);
  }

  // fork (ps aux)
  if ((pid = fork()) == -1) {
    perror("bad fork1");
    exit(1);
  } else if (pid == 0) {
    // stdin --> ps --> pipe1
    exec1();
  }
  // parent

  // create pipe2
  if (pipe(pipe2) == -1) {
    perror("bad pipe2");
    exit(1);
  }

  // fork (grep root)
  if ((pid = fork()) == -1) {
    perror("bad fork2");
    exit(1);
  } else if (pid == 0) {
    // pipe1 --> grep --> pipe2
    exec2();
  }
  // parent

  // close unused fds
  close(pipe1[0]);
  close(pipe1[1]);

  // fork (grep sbin)
  if ((pid = fork()) == -1) {
    perror("bad fork3");
    exit(1);
  } else if (pid == 0) {
    // pipe2 --> grep --> stdout
    exec3();
  }
  // parent


}


void exec1() {
  // input from stdin (already done)
  // output to pipe1
  dup2(pipe1[1], 1);
  // close fds
  close(pipe1[0]);
  close(pipe1[1]);
  // exec
  execlp("ps", "ps", "aux", NULL);
  // exec didn't work, exit
  perror("bad exec ps");
  _exit(1);
}

void exec2() {
  // input from pipe1
  dup2(pipe1[0], 0);
  // output to pipe2
  dup2(pipe2[1], 1);
  // close fds
  close(pipe1[0]);
  close(pipe1[1]);
  close(pipe2[0]);
  close(pipe2[1]);
  // exec
  execlp("grep", "grep", "root", NULL);
  // exec didn't work, exit
  perror("bad exec grep root");
  _exit(1);
}

void exec3() {
  // input from pipe2
  dup2(pipe2[0], 0);
  // output to stdout (already done)
  // close fds
  close(pipe2[0]);
  close(pipe2[1]);
  // exec
  execlp("grep", "grep", "sbin", NULL);
  // exec didn't work, exit
  perror("bad exec grep sbin");
  _exit(1);
}

For additional help, an excellent example of pipes in C in Linux is here under the "EXAMPLES" section: https://man7.org/linux/man-pages/man2/pipe.2.html

---

Also, I can confirm that threePipeDemo.c does compile, contrary to what @nskoro says. Just make sure you do NOT use -Werror in your compile command, as there are a variety of warnings.

Here is my compile command. I tested this on Linux Ubuntu 18.04.

# compile
mkdir -p bin && gcc -Wall -Wextra -O3 -std=c17 threePipeDemo.c -o bin/a

# then run the program with:
bin/a 

Here's my compile command and the output with a bunch of warnings:

eRCaGuy_hello_world/c/todo$ mkdir -p bin && gcc -Wall -Wextra -O3 -std=c17 threePipeDemo.c -o bin/a
threePipeDemo.c:52:6: warning: return type of ‘main’ is not ‘int’ [-Wmain]
 void main() {
      ^~~~
threePipeDemo.c: In function ‘main’:
threePipeDemo.c:56:5: warning: implicit declaration of function ‘perror’ [-Wimplicit-function-declaration]
     perror("bad pipe1");
     ^~~~~~
threePipeDemo.c:66:5: warning: implicit declaration of function ‘exec1’; did you mean ‘execl’? [-Wimplicit-function-declaration]
     exec1();
     ^~~~~
     execl
threePipeDemo.c:82:5: warning: implicit declaration of function ‘exec2’; did you mean ‘execl’? [-Wimplicit-function-declaration]
     exec2();
     ^~~~~
     execl
threePipeDemo.c:96:5: warning: implicit declaration of function ‘exec3’; did you mean ‘execl’? [-Wimplicit-function-declaration]
     exec3();
     ^~~~~
     execl
threePipeDemo.c: At top level:
threePipeDemo.c:104:6: warning: conflicting types for ‘exec1’
 void exec1() {
      ^~~~~
threePipeDemo.c:66:5: note: previous implicit declaration of ‘exec1’ was here
     exec1();
     ^~~~~
threePipeDemo.c:118:6: warning: conflicting types for ‘exec2’
 void exec2() {
      ^~~~~
threePipeDemo.c:82:5: note: previous implicit declaration of ‘exec2’ was here
     exec2();
     ^~~~~
threePipeDemo.c:135:6: warning: conflicting types for ‘exec3’
 void exec3() {
      ^~~~~
threePipeDemo.c:96:5: note: previous implicit declaration of ‘exec3’ was here
     exec3();
     ^~~~~

---

This is incredibly helpful for anyone trying to understand the super confusing execlp() cmd: I do not understand how execlp() works in Linux