The command line

Core Module

Contrary to popular belief, the command line (also commonly known as the terminal) is not a mythical being that has existed since the dawn of time. Instead, it was created at a time when it was not given that your computer had a graphical interface that you could interact with. Think of it as a text interface for your computer.

The terminal is a well-known concept to users of Linux; however, Mac and (especially) Windows users often do not need it or therefore encounter it. Having a basic understanding of how to use a command line can help improve your workflow. The reason that the command line is an important tool to get familiar with is that doing any kind of MLOps will require us to be able to interact with many different tools, many of which do not have a graphical interface. Additionally, when we get to working in the cloud later in the course, you will be forced to interact with the command line.

Note that if you're already a terminal wizard then feel free to skip the exercises below. They are very elementary.

The anatomy of the command line

Regardless of the operating system, all command lines look more or less the same:

As already stated, it is essentially just a big text interface for interacting with your computer. As the image illustrates, when trying to execute a command, there are several parts to it:

The prompt is the part where you type your commands. It usually contains the name of the current directory you are in, followed by some kind of symbol: $, >, : are the usual ones. It can also contain other information, such as in the case of the above image, which also shows the current conda environment.
The command is the actual command you want to execute. For example, ls or cd.
The options are additional arguments that you can pass to the command. For example, ls -l or cd ...
The arguments are the actual arguments that you pass to the command. For example, ls -l figures or cd ...

The core difference between options and arguments is that options are optional, while arguments are not.

❔ Exercises

We have put a cheat sheet in the exercise files folder belonging to this session that gives a quick overview of the different commands that can be executed in the command line.

Windows users

We highly recommend that you install Windows Subsystem for Linux (WSL). This will install a full Linux system on your Windows machine. Please follow this guide. Remember to run commands from an elevated (as administrator) Windows Command Prompt. You can in general complete all exercises in the course from a normal Windows Command Prompt, but some are easier to do if you run from WSL.

If you decide to run in WSL, you need to remember that you now have two different systems, and installing a package on one system does not mean that it is installed on the other. For example, if you install pip in WSL, you need to install it again in Windows if you want to use it there.

If you decide to not run in WSL, please always work in a Windows Command Prompt and not Powershell.

Start by opening a terminal.
To navigate inside a terminal, we rely on the cd command and pwd command. Make sure you know how to go back and forth in your file system. (1)
1. Your terminal should support tab completion which can help finish commands for you!
The ls command is important when we want to inspect the content of a folder. Try to use the command, and also try it with the additional option -l. What does it show?
Make sure to familiarize yourself with the which, echo, cat, wget, less, and top commands. Also, familiarize yourself with the > operator. You are probably going to use some of them throughout the course or in your future career. For Windows users, these commands may be named something else, e.g., where command on Windows corresponds to which.
It is also important that you know how to edit a file through the terminal. Most systems should have the nano editor installed; otherwise, try to figure out which one is installed on your system.
1. Type nano in the terminal.
2. Write the following text in the script
```
if __name__ == "__main__":
    print("Hello world!")
```
3. Save the script as a .py file and try to execute it using python <filename>.py.
4. Afterwards, try to edit the file through the terminal (change Hello world to something else).
All terminals come with a programming language. The most common system is called bash, which can come in handy when you're able to write simple programs in bash. For example, if you want to execute multiple Python programs sequentially, this can be done through a bash script.

Windows users

Bash is not part of Windows, so you need to run this part through WSL. If you did not install WSL, you can skip this part or as an alternative do the exercises in Powershell, which is the native Windows scripting language (not recommended).
1. Write a bash script (in nano) and try executing it:
```
#!/bin/bash
# A sample Bash script, by Ryan
echo Hello World!
```
2. Change the bash script to call the Python program you just wrote.
3. Try to Google how to write a simple for-loop that executes the Python script 10 times in a row.
A trick you may need throughout this course is setting environment variables. An environment variable is just a dynamically named value that may alter the way running processes behave on a computer. The syntax for setting an environment variable depends on your operating system:
WindowsLinux/Mac
set MY_VAR=hello echo %MY_VAR%
export MY_VAR=hello echo $MY_VAR
1. Try to set an environment variable and print it out.
2. To use an environment variable in a Python program, you can use the os.environ function from the os module. Write a Python program that prints out the environment variable you just set.
3. If you have a collection of environment variables, these can be stored in a file called .env. The file is formatted as follows:
```
MY_VAR=hello
MY_OTHER_VAR=world
```
  To load the environment variables from the file, you can use the python-dotenv package. Install it with pip install python-dotenv and then try to load the environment variables from the file and print them out.
```
from dotenv import load_dotenv
load_dotenv()
import os
print(os.environ["MY_VAR"])
```

🧠 Knowledge check

Here is one command from later in the course when we will be working in the cloud
```
gcloud compute instances create-with-container instance-1 \
    --container-image=gcr.io/<project-id>/gcp_vm_tester
    --zone=europe-west1-b
```
Identify the command, options, and arguments.
Solution
- The command is gcloud compute instances create-with-container.
- The options are --container-image=gcr.io/<project-id>/gcp_vm_tester and --zone=europe-west1-b.
- The arguments are instance-1.
The tricky part of this example is that commands can have subcommands, which are also commands. In this case, compute is a subcommand to gcloud, instances is a subcommand to compute, and create-with-container is a subcommand to instances.
Two common arguments that nearly all commands have are the -h and -V options. What does each of them do?

Solution

The -h (or --help) option prints the help message for the command, including subcommands and arguments. Try it out by executing python -h.

The -V (or --version) option prints the version of the installed program. Try it out by executing python --version.

This ends the module on the command line. If you are still not comfortable working with the command line, fear not as we are going to use it extensively throughout the course. If you want to spend additional time on this topic, we highly recommend that you watch this video on how to use the command line.

If you are interested in personalizing your command line, you can check out the starship project, which allows you to customize your command line with a lot of different options.