Hey Heather, it's me again.

Building a shell: pipe down!

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Hey HEATHER! It’s me again.

Unfortunately I didn’t get to accomplish as much as I would’ve liked to last week. It doesn’t help that my sleep schedule has been messed up for some time. Woe is me. Anyway. On Friday, I went to the shell workshop and started Stage 2 which is on Files and Pipes.

I went over the notes, the man pages, the code, I asked some questions. But when I got home, I realized that I seldom use pipes! Besides dmesg | tail, journalctl | tail, and the thankfully rare plumbing problems, I can’t think of another instance where I would pull out a pipe. Which just means I don’t know the true power of pipes. Let’s see if I can change that and conjure up some powerful pipes.

Pages, man.

Let’s read man pages again. In the Linux Programmer’s manual, the definition reads “pipe() creates a pipe”, so far so good. The said pipe is “a unidirectional data channel that can be used for interprocess communication”. Simple enough. Hmm, interesting. The tutorial has a link to the FreeBSD manual as well, so I checked it out. It says “the pipe() function creates a pipe, which is an object allowing bidirectional data flow (…)”. Ohno.

Now that I’ve emerged from the rabbit hole (thanks pomodoro timer!), I acquiesce that although it’s possible to use pipes in a bidirectional manner on some systems (not Linux tho), it’s best to use them in the conventional way. I would’ve liked to know in which circumstances pipes could be used bidirectionally but I think this Stackoverflow answer is enough to quell my curiosity. It just states that the POSIX standard for pipes doesn’t specify their directional behavior. Moving on.

Wrong model

When I started looking into pipes, I thought I had a good idea about how they worked. It turns out I was wrong. To illustrate, this is the model of how I thought of pipes:

dmesg     |  less
↳read end    ↳write end

But it didn’t make sense when I thought about it. When you ask the question “what about when there’s more than one pipe?” For instance:

dmesg     | grep 'usb' |  less
↳read end   ↳read end     ↳write end

You can’t just read something and pass it on without writing. And here is where I finally understood this part of the definition:

The array pipefd is used to return two file descriptors[fd] referring to the ends of the pipe. pipefd[0] refers to the read end of the pipe. pipefd[1] refers to the write end of the pipe.

I was aware that the output of one became the input of the other command but clearly I didn’t grasp what that meant. That is until I saw the first two figures on this website. The gist of it is that pipes transfer data from a write endpoint to a read endpoint. Here’s the command again.

dmesg | grep 'usb' | less

// flow
write fd -> stdout -> stdin -> read fd -> write fd -> stdout -> stdin -> read fd

The flow of the data is from left to right and goes something like this:

  1. We make a system call to dmesg that writes to the stdout end of the pipe;
  2. the data flows to the stdin end of said pipe into the read fd of grep;
  3. grep writes to the stdout end of the second pipe;
  4. the data again flows to the stdin end of the pipe;
  5. where it is then read by less.

Woah. I mean, I’m literally repeating what I’ve read, but it just seems to make so much more sense now. Like a well constructed aqueduct each end ties into the other. I don’t need to conjure anything up. Pipes are powerful in themselves.

This turned out to be a great bedtime story somehow. Which reminds me where I ought to be at this time. Dang it.

Last note: I really like the YouTube channel Computerphile and it turns out they have a video on Unix pipeline presented by Brian Kernighan. I like how he explains the benefits. Enjoy!

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