As I promised in my previous “Bash practices” post, I would discuss a query function in this article. Here you have it:

function create_temporary_directory() {
    directory=$(mktemp -d "$1/XXXXX")
}

create_temporary_directory

echo "$directory"

That’s a bad query function!

This function is supposed to return the path of a temporary directory. It accepts one argument: an existing directory we want to create the temporary directory in. It uses the mktemp utility to do the “heavy” lifting. It accepts as an argument a kind of a template for directory names (XXXXX will be replaced by 5 random characters). After creating the directory, mktemp echos the full path of the directory to stdout, meaning we can copy it into a variable by using the $(...) syntax.

Forgive me for my bad pun. As I mentioned in my previous Bash post I’m going to show you some ways in which you can improve the design of Bash scripts. Again, it’s a weird language, and a lot of what’s below probably won’t feel natural to you. Anyway, there we go.

I started out with a piece of code that looked like this:

BUILD_DIR="build"

function clean_up() {
    rm -r "$BUILD_DIR"
}

clean_up

Function arguments

Inside a function you can use all global and environment variables, which easily leads to smelly code like this: clean_up will behave differently based on what’s in the global variable BUILD_DIR. This makes the function itself quite unpredictable, but also error-prone, as the value of BUILD_DIR may at one point not contain the name of a directory, or even be an empty string. Usually we would fix this by providing the path to the directory we’d like to remove as an argument of the function call, like this:

A bit of reminiscing

When I was a kid, MS Windows was still a program called WIN.COM which you needed to start from the MS-DOS command prompt. You could also add it to AUTOEXEC.BAT which was a so-called batch file. You could write these .BAT files yourself. They were basically just command-line scripts. You could make them execute commands, print things, collect input, and make simple decisions. It wasn’t much, and I remember that you often needed some helper .COM or .EXE programs to accomplish anything useful. The most advanced thing I ever wrote was a nice little ASCII-art menu program, spread across multiple .BAT files (with GOTOs and all), which allowed me to easily start my favorite games, like Super Tetris, known to me as SUPERTET.EXE, or Prince of Persia.