# Junk Drawer

For all those little papers scattered across your desk

# Day 8, Context and Computers

27 Jun 2018 in Blog

I will keep this alliteration up until it kills me. Just sayin’.

# Today I Learned

2. Context is king in shell scripting

## Does not compute

Remember how I still didn’t have my work computer? I was able to put Ubuntu on it this evening, and tomorrow should be using primarily it. Huzzah.

Let me tell you something about it though. The Ubuntu download is an ISO. No, not like Tron: Legacy, it’s an image file, a snapshot of an entire file system. And everywhere online will tell you that, in order to make a bootable USB from one, you need special software.

You don’t.

At least, not anymore. Ubuntu decided to put out some kind of hybrid ISO, so you can copy bit-by-bit to your USB and it will be bootable. I know; I did it. There’s a small bug that prevents macOS Disk Utility from working on it, which is unfortunate, but dd does the job just fine. And you can use raw write mode to speed things up.

## Context

Here’s the thing about a lot of shell scripts. Many establish, much like a makefile recipe, that the way to accomplish a task is a sequential set of steps. A recipe. To foo the bar, we follow this list, frobbing along the way.

Now, we have some basic control flow: we can loop, doing the same step multiple times; we can make decisions about which steps to do; and we can even have someone else do a step for us in the background.

Recipes are fine, you’re thinking (I can hear your thoughts)–They work in the kitchen. Well, bub, my shell is not your kitchen. For one, my shell won’t taste bad if I leave it on too long. More importantly, if I screw up, I brick my laptop and not my whole house. Which of those things is more important, I’m still working on. The reason recipes require attention is context. In a shell script, any one step could be affected by what’s gone before. Current directory matters, whether or not files exist matters, &c. This is not unlike cooking, where having the dough mixed properly is essential to baking a cake. But you can go back and fix the dough. A script can’t do that on its own.

Here comes the tricky part–shell functions are a really nifty feature for refactoring, enhancing the code’s ability to read like English (or your language of choice). They encapsulate components of the recipe, much like make_the_dough might be a multi-step process necessary for make_me_a_cake. But functions inherit their callers context. Current directory, environment variables, &c. And this is a problem when we try to think of functions single, isolated, perfect units.

In a perfect world, each function could be run entirely independently, and so cd would never fail because we’re not in the directory we thought we were (thanks, caller function). This happened to me today and it borked my build.

Functions are not independent.

Unless you write pure functions, like sed and grep and other UNIX-style filters that take input (or parameters) and output things. These transformations are perfect. They are independent, and often idempotent. But eventually we need I/O to Get Things Done™.

So, no, bash isn’t purely functional (although we can do a good job building reliable and composable units in it). Knowing this, we have to be defensive in our functions. When we do something, we have no guarantees about our context, and the burden is on us to handle that the right way. Sometimes, that means fail spectacularly. Others, in the case of my cd problems, meant using absolute paths so it couldn’t fail unless things were really screwed up. But at the end of the day, we have to recognize that things might not be the way we expect. How do we react to that?