Scientific computing

(for the rest of us)

The Path

One of the main obstacle to reproductible projects is issues with describing where files are. In this module, we will talk about the path, and how to refer to locations in a way that will work on any computer.

The first question we have to answer is, where are we? The answer to this question is pwd, or print working directory. This is telling us where Julia is actually executing things. In our case, the script for this lesson is running in:

pwd()
"/home/runner/work/ScientificComputingForTheRestOfUs/ScientificComputingForTheRestOfUs/dist/content/07_files"

The working directory is a very important concept: we can look for things within it, but we cannot look for things outside of it.

This is not quite entirely true. In some very specific cases, we may not want to store a few Gb of data in our working directory, and we will refer to those using their absolute path; the best practice in this case is probably to setup an environment variable to point to the data.

Why is that? The simple answer is that when you distribute your project, you will not distribute the rest of your machine. When working on a laptop (at home), a desktop (in the lab), and a cluster (for larger simulations), the only certainty is that the working directory is the same; it is unlikely to be in the same place, or to have folders outside of it organized in the same way.

For this reason, the folder containing our material is going to be our main unit of organization.

Julia has an interesting macro to refer to the place where the file being run is located:

@__DIR__
"/home/runner/work/ScientificComputingForTheRestOfUs/ScientificComputingForTheRestOfUs/dist/content/07_files"

Note that this is an absolute path - it starts with a /, which is the root of your filesystem. But the absolute path is not hard-coded! Working on a different system, you would see a different path leading up to your @__DIR__.

Julia can also print the actual name of the file:

@__FILE__
"/home/runner/work/ScientificComputingForTheRestOfUs/ScientificComputingForTheRestOfUs/dist/content/07_files/01_path.md"

Another important concept is the home directory, which is where the operating system will put your user files:

homedir()
"/home/runner"

Paths are made of different parts, so we can splith @__FILE__ into its components:

splitpath(@__FILE__)
10-element Vector{String}:
 "/"
 "home"
 "runner"
 "work"
 "ScientificComputingForTheRestOfUs"
 "ScientificComputingForTheRestOfUs"
 "dist"
 "content"
 "07_files"
 "01_path.md"

This is quite nice, because it turned our path into an array of strings. Notice that it’s making a difference between / meaning the root, and / meaning the filesystem separator.

Can we create a path in a safe way? Absolutely! Let us create a data folder:

data_path = joinpath(pwd(), "data")
"/home/runner/work/ScientificComputingForTheRestOfUs/ScientificComputingForTheRestOfUs/dist/content/07_files/data"

Now, this folder does not exist. It is a string of text describing where it is. Can we create it? Yes! But first, let’s try a few functions:

isfile(@__DIR__)
false
isdir(@__DIR__)
true
ispath(@__DIR__)
true

These three functions are very useful when working on path issues. isfile will take a string, and let you know if there is a file at this location. isdir will do the same for a directory (folder; we will stick to directory as it is the more correct term). ispath will do the same for either a folder or a file.

In our case, we want data_path to be a directory, so we will first check that it does not exists:

isdir(data_path)
false

If it does not exist, we will create it:

if ~isdir(data_path)
    mkdir(data_path)
end
"/home/runner/work/ScientificComputingForTheRestOfUs/ScientificComputingForTheRestOfUs/dist/content/07_files/data"

This line (we will go into the details of if, and booleans more broadly, in the next few modules) will create the directory if it does not exist. We can now read the content of our working directory:

readdir(pwd())
1-element Vector{String}:
 "data"

There seems to be a data directory. Note that readdir has a number of options, and that Julia offers additional ways to walk through a series of nested directories if neede.

To finish up, let’s remove this directory. We will use isdir again because we do not want to remove a directory that doesn’t exist. It is worth looking at the documentation for rm, as it has a number of important options and keyword arguments.

if isdir(data_path)
    rm(data_path)
end

readdir(pwd())
String[]

As a final bit of information, Julia can create temporary files, i.e. files that will not be stored in the working directory, and will not persist after you restart your computer. Your temporary files are always stored in the tempdir of your computer:

tempdir()
"/tmp"

You can generate a temporary path with:

tempname()
"/tmp/jl_wNQx65Opkr"

Note that this string describes just this: a path. You can turn it into a file, or a directory. Working with temporary files is very useful when you, for example, need to download data in bulk, but do not want to save the raw download.