How I learned to love the Bash

My tale of woe

When I started to write my first ebook I decided to use pandoc to transform markdown into pdf.
It basically looked like this

pandoc metadata_de.yaml -o ./level_up_de.pdf --from markdown -V lang=de-DE level_up_de.md

This simple command can be pasted into the command line everytime I want to build a new version of the ebook.
But: I have to keep in mind the command which is tedious. I could write it into a text file and copy it everytime I use it.
There is a more elegant way: let’s make a script!

Make a script

A shell script is basically a file with the extension “.sh”
When You want to specify which shell shall be used, you put

#!/bin/bash

in the first line of your script.

Echo

To put some text on the command line for e.g. displaying the start of your script, you can use the command echo like this

echo "Start generating PDF"

The script now looks like that

#!/bin/bash
echo "Start generating PDF"
pandoc metadata_de.yaml -o ./level_up_de.pdf --from markdown -V lang=de-DE level_up_de.md

Variables

After finishing the German version I also wanted to create an English version,
so the “de” strings should be replaced by a variable to switch between languages.

To declare a variable, you write down the name and assign it a value with the equal sign.
You use a variable by using thew $ sign to access its content.

language="en"
echo "Language: $language"

Caveat: whitespaces are not allowed around the equal sign!

language = "en" # this won't work

Let’s have a look at our script now:

#!/bin/bash
echo "Start generating PDF"
lang="en"
echo "Language: $lang"
pandoc metadata_$lang.yaml -o ./level_up_$lang.pdf --from markdown -V lang=de-DE level_up_$lang.md

Pretty neat!

But we need to get rid of the “de-DE” as well

Conditions

When we get “en” as language we want to use en-US as language subtag. When “de” is used we want de-DE
Let’s write some conditions.

if [[ $lang = "en" ]]
then
  lang_subtag="$lang-US"
elif [[ $lang = "de" ]]
then
  lang_subtag="$lang-DE"
else
  echo "Wrong language"
fi

Now we can replace the de-DE with $lang_subtag

pandoc metadata_$lang.yaml -o ./level_up_$lang.pdf --from markdown -V lang=$lang_subtag level_up_$lang.md

Command Line Arguments

Now that we can generate the ebook for english and german we could introduce a command line parameter.
In bash every argument provided to the script is stored in variables $1 .. $n

lang="$1"

Arrays and loops

I wanted to break down the big markdown file into individual chapters.
To make an array variable in bash you can do the following:

chapters=(
./de/chapter_01.txt
./de/chapter_02.txt
./de/chapter_03.txt
)

Looping over an array looks like this:

for chapter in ${chapters[*]}
do
  cat "$chapter"
  printf "\n"
done

So now we can pass the chapters array to pandoc

pandoc metadata_$lang.yaml -o ./level_up_$lang.pdf --from markdown -V lang=$lang_subtag ${chapters[*]}

To be continued …

You can find the source code here: https://github.com/jboegeholz/shell_tutorial