Tag Archives: plumbers

A program calling command-line tools is the moral equivalent of web scraping.

I gave this talk at LPC 2012. It promotes the idea that programs layered on top of human-centric interfaces is a bad idea.


The timing of this post with the announcement of the most recent bash vulnerability is not entirely coincidental.

Plumbing needs an API: “libification”

Here are my slides and video from my talk “All Plumbing needs an API” from Linux Plumbers Conference 2012.

My thesis is we would improve the quality of our platform if we recognize the flaws in excessive use of other commandline tools by other tools, and worked towards more APIs and libraries for low-level parts of the Linux platform. There are many reasons given in the talk I don’t want to rehash; here are some more thoughts since I gave the talk:

First, this is not a push for every single instance of one program being called by another to be replaced by an API. But on the spectrum of cmdline-parsing vs libraries, I think we are too far towards the former, and should make an effort to shift dramatically towards the latter.

Second, we can take a “pave the cowpaths” approach toward this — we should first libify those tools most commonly parsed by other tools. I’ve been helping out on liblvm, a library for LVM (whose tools are unquestionably over-parsed.) There are also many other network, storage, and system configuration tools that are candidates for libification.

The move towards virtualization & cloud computing has led to many tools formerly configured directly by the sysadmin now being configured by other tools. Broadening the coverage of our system-level management APIs will improve Linux’s flexibility and reliability as a virtualized OS.

A hundred other languages want to call your code

The users of a hundred programming languages would like to call your low-level code, but they can’t.

Things have changed in the last 20 years. More people are using languages like Python, Ruby, and a hundred more, that are further from the bare metal. People are building service stacks that tie together many lower-level functions.

Libraries and APIs that make low-level features available to convenient high-level languages (HLLs) are a good thing. As a HL coder, it’s pretty handy to install python-foo, type “import foo” and then have access to that functionality.

What if python-foo isn’t there? HLL users are out of luck, unless they are so determined they make their own python-foo that calls system(), and then parses the output using their language’s fancy text parsing features.

But system() is the devil. We hate system(), folks. If your code calls system() it’s bad, for four reasons:

  1. Overhead. It creates a new process and subshell.
  2. Security. If your code has elevated privileges and is including text input by an untrusted user, watch out. Remember little Bobby Tables, a semicolon is a dangerous thing.
  3. Ease. Parsing command-line programs’ output can be a pain, even if your language helps lessen it. Parsing of errors is even harder and prone to be overlooked.
  4. Portability. A different platform may (or may not) have the program you’re relying on, or its output may be different, and you won’t know.

Early on when I was learning Python, I tried to write a gui for OProfile by parsing its output. OProfile did nice (for the user) things like adding headers on its output, and changing the format of output depending on what it found. Great for users, but it doomed my project. I couldn’t parse the output reliably.

You want to make it easy for the people who are language gurus for each of the hundred languages out there to wrap your functionality without having to become an expert in your code, or even change it. Then the hordes using all the hundred languages can use your library without being an expert in your code or being enough of a guru in their language to write a wrapper. They can just happily use it.

Here’s a positive development, kmod. kmod is a new implementation of the utilities in module-init-tools: modprobe, lsmod, lsmod, etc. Not only does kmod include a libkmod C library, but the commandline programs use it, so we know it works. Yeah! This made it super easy for someone (me) to come along and write a language wrapper (python-kmod) without having to know about module internals. python-kmod makes it easy for Python users to manipulate modules using the friendly language features they’re used to, like exceptions for errors, and lists. If I had been forced to use system(), it probably would have mostly worked, but it would have failed when output parsing failed for some edge case.

I encourage all low-level program writers, my fellow Linux Plumbers, to consider how to make native language bindings possible for your code. You don’t have to write them, just make them possible and you will find all sorts of people calling your code, safely, who couldn’t before.