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chainable shell functions

07 Feb 2017

I learned a neat shell script refactoring strategy yesterday that I’d like to share. First some background:

I used to use a rubygem to help me write this blog. When I wanted to create a new post called “chainable shell functions” I would have run:

bin/poole draft "chainable shell functions"

And it would create a file called _drafts/chainable-shell-functions.md with some metadata in the first few lines.

Yesterday I got the urge to try replacing that rubygem with a custom shell script which does exactly the same thing.

I am an enthusiastic novice shell scripter.

I’m vaguely aware there are different dialects of shell scripting and that I’m probably using the wrong one.

Really I’m not expert in this stuff.

But while writing this one I learned something interesting that I’m going to share with you now.

Here is the first draft (annotated with comments for your convenience):

#!/usr/bin/env bash

# fail fast if any expression fails
set -e

# read all of the arguments into a string
title="$*"

# OK don't worry about this gnarly line, I'm going to break it down
slug=$(
  echo "$title" | sed "s/ /-/g" | tr -dc '[:alnum:]-' | tr '[:upper:]' '[:lower:]'
)

# the file we're going to create
filename="./_drafts/$slug.md"

# create the folder if it doesn't already exist
mkdir -p _drafts

# stop if the file already exists -- I don't want to overwrite an in-progress draft
if [[ -e "$filename" ]]; then
  echo "$filename already exists"
  exit 1
fi

# create the draft by piping a string into a file
echo "---
title: $title
date: $(date '+%Y-%m-%d')
---

Alright, this is where your post goes." > $filename

# Print a successful message
echo "Created $filename"

OK did you read that? Great.

So you saw that line I promised I would break down? The idea with that line is that I want to take the input, which is the title of the post, and figure out what is an appropriate filename for the post. I’m figuring that out by applying a series of transformations to the title:

OK so that’s a lot going on in one line, and because of the compact nature of these commands, it’s not super readable.

In other languages, when I have a lot going on in one function, I want to split out smaller, well-named functions. Can I do the same thing here?

At first I wasn’t sure. I knew it was possible to write functions that received arguments by checking $1, $2, etc in the function, but I wasn’t sure how to make them “return” values…

After a little googling I learned: you can just write a shell function that calls commands that read from a pipe, and pipe things to that function.

Let me show you what I mean.

Here’s the second (and, frankly, final) draft:

#!/usr/bin/env bash

set -e

function dashify() {
  sed "s/ /-/g"
}

function removeSpecialChars() {
  tr -dc '[:alnum:]-'
}

function downcase() {
  tr '[:upper:]' '[:lower:]'
}

title="$*"
slug=$(
  echo "$title" | dashify | removeSpecialChars | downcase
)
filename="./_drafts/$slug.md"
mkdir -p _drafts

if [[ -e "$filename" ]]; then
  echo "$filename already exists"
  exit 1
fi

echo "---
title: $title
date: $(date '+%Y-%m-%d')
---

Alright, this is where your post goes." > $filename

echo "Created $filename"

Look at that!