Reading 1: Introduction to the Command Line

Objectives

This is the first of a series of readings AppDev exec will publish to members to help them develop skills. Each readings is intended to be a short concrete expansion on the content covered in slides. The talk is intended to come first; the readings will reference content on slides.

This reading presents:

• a concrete tutorial on how to use the command line
• how to navigate the filesystem
• how to install packages
• how to manage access control
• links to more resources to learn about the command line

The single most important command in Linux

There's a famous saying: give a man a fish, and he eats for a day. Teach a man how to fish, and he eats for a lifetime.

The man command in Linux is how you fish. It is short for manual, and its usage is

man <name-of-command>

For example, man curl tells me everything I want to know about curl. It documents its usage, option flags, behavior, etc. These are known as the man pages.

If you're learning a new language and encounter unfamiliar words, you consult a dictionary. You should always consult the man pages.

Practice: run man man to learn more about the man command

Structure of a command

Recall that a command is just an executable file. But what you actually type into the terminal to run this file takes a very specific structure:

<arg-0> <arg-1> <arg-2> <arg-3> <...>

The shell reads your command as a series of space-separated arguments (essentially words). The first one, labelled arg-0 above, is used to look up an executable file to run. The remaining arguments are passed into the program to work with. For example, this is how man knows which command you want man pages for.

There are some very strong conventions for what constitutes an argument. Depending on the command, 99% of the time it will fall into one of three categories: a file path, a string, or an option flag. The option flag always begins with a - and usually consists of a single letter, e.g. -a or -v. Most commands also accept combinations, so -av would be equivalent to -a -v. Sometimes, there's long versions of flags that start with --: one example could be -v having the alias --verbose.

Navigating the filesystem

Let's look at the first few lines of the man pages for ls:

NAME
       ls - list directory contents

SYNOPSIS
       ls [OPTION]... [FILE]...

DESCRIPTION
       List information about the FILEs (the current directory by default).  Sort entries alphabetically if none of -cftuvSUX nor --sort is specified.

The first part, NAME, tells us that ls lists the files in a directory. SYNOPSIS tells us how to use the command. Finally, DESCRIPTION gives us some more information about what ls actually does. Most man pages may not follow the same format, but the important information will almost always be given right at the top. If it isn't, the developer is pretty bad at their job.

Check: try running ls. Can you get the command to show hidden files as well? Can you get a more detailed description of each file (in the format shown on the slides)? Can you make the file sizes human readable (e.g. 234M)?

There are a few other relevant commands:

• cd: changes directory (i.e. navigates somewhere else). Usage: cd <path>
• rm: removes a file
• mv: moves a file
• cp: copies a file
• mkdir: makes a directory
• rmdir: removes a directory

pwn.college

We won't waste any more time rehashing linux fundamentals that others have created excellent resources for. One excellent resource we strongly recommend is https://pwn.college/linux-luminarium/. This platform is primarily a cybersecurity teaching platform; you will find a lot of content about various types of security and hacking. But as part of one of its intro modules, they go over the basics of linux in a very "from scratch" approach. (We'll also come back here when we inevitably talk about web security).

The structure of pwn.college is capture the flag: lessons are divided into challenges. Each challenge contains a secret file at the /flag path, and the goal is to read this file and paste it into the prompt box. (In a cybersecurity setting, accessing /flag usually means that you've compromised the server pretty heavily. Of course, in these fundamental exercises, many examples are contrived to give you a flag as a reward.) This approach carries two major advantages:

1. It's really really fun.
2. You get to work on an actual Linux machine while doing the challenges. Most other platforms have you type the command in a small subset / simulator, or expect to test things on your own machine.

pwn.college allows you to access its virtual machines in a variety of ways, but we encourage you to not use the GUI or Code interfaces. You should use either terminal or SSH.

Practice: create an account on pwn.college, and complete the following modules in the Linux Luminarium:

• Hello Hackers
• Pondering Paths
• Comprehending Commands
• Shell Variables
• Untangling Users
• Perceiving Permissions

As an aside, when I was initially drafting this reading, I planned a whole tutorial on creating an ssh key and connecting to their machines. Then, when revisiting the site to check the accuracy of my steps, I realized that a built in web terminal was now available, with a much better UI than the previous options had! I still recommend you to use SSH if you're comfortable, though. If you're not familiar, don't stress about it.

Understanding .bashrc

There is some content I want to cover that doesn't seem to show up anywhere in pwn.college. Part of the power of Linux is the ability to customize your own setup and make your life more convenient in the future. We do this with rc (run command) files, the most notable of which is your .bashrc.

The .bashrc file lives in your home directory. First, answer the a few questions so you're confident you know what that means:

Check: Which of the following paths should lead to your .bashrc file, if you are the user linus and your current working directory is /home/linus? Options: .bashrc, ./bashrc, /.bashrc, ./.bashrc, /./.bashrc, ././.bashrc, ../.bashrc, ~bashrc, ~.bashrc, ~/.bashrc, ./~bashrc, ./~/.bashrc, /home/.bashrc, /linus/.bashrc, /home/linus/.bashrc, /home/eric/.bashrc, /root/.bashrc.

Check: Repeat the same question, but now your current directory is /home/linus/homework.

Check: Repeat the same question, but now your current directory is /root and you are the root user.

Okay, now we're sure we know where it goes. As for the contents of the file, it's a series of commands in bash (the language that your shell runs in). Essentially, you can think of it as plugging each line in the file and running it as a command, in order. For example, you could consider copy and pasting

alias l='ls -la'

Check: Before you run it, what does this command do? Note that alias unfortunately does not have a man page. You should consult the internet.

The idea with .bashrc is that it runs upon startup. That is, without having to lift a finger, in future terminal sessions, you will get the convenience of using l.

To use it in this terminal session, look into the source command.

Installing packages

In a modern operating system, you extend what your computer can do by installing software. This is no different on a Linux machine.

You can install software via a package manager. On a machine running the Ubuntu distribution, this will be apt, but it'll be different on others (for example, Arch uses pacman).

Check: try to install the nginx package, then uninstall it.

Check: you may see online tutorials require you to use sudo. Is this actually necessary to run apt? Let's check by going to... what directory?

Check: how can we find the access permissions for apt? What do you see? Who is the owner?

Check: on my machine, the permissions are rwxr-xr-x. Which users can read to it? What about writing? What about executing?

Check: why does apt install nginx fail? Which files is missing access permissions? How does sudo resolve this? What is special about the permission bits for the sudo command to allow this?

Closing thoughts

You may be feeling a bit frustrated at all the work and learning so far. This is natural. After all, you didn't actually learn how to do that much here. You can go around your filesystem, change permissions, install packages, and use sudo. But that's still a very small subset of what you can do on your computer.

I want to reiterate the importance of learning deeply. I could give you a series of groups of words to type into your shell. But everybody's machine is slightly different. It's a waste of time to help every single person debug every single edge case by giving a new command to type in; instead, it is much more efficient to make clear what is actually going on when you use the shell and the underlying design that forms it. (Incidentally, "just try it again and hope it works" seems to be the general theme behind using LLMs these days. It doesn't work well.)

It's perfectly okay and normal to copy paste things on StackOverflow or things outputted by an LLM. But please, keep in mind:

1. Your machine is not the same. Be aware of the things that could be different. Make appropriate modifications to the command. If you don't understand the command enough to make these modifications, that is a problem; read more about what it does and how to use it. Never copy paste something without thinking.
2. It won't always work first try. Read the error message. Almost all the time, it will be something helpful! A good understanding of Linux will help you interpret and fix the error very well.
3. Subtle differences matter. (see: the question posed earlier this reading with 17 different file paths).