Navigate the filesystem

On Linux and macOS, every file lives in one tree of directories (folders) that starts at the root, written /. There are no drive letters: disks and USB sticks are attached somewhere inside the tree.

1/
2├── bin/         essential programs (ls, cat, ...)
3├── etc/         system configuration files
4├── home/
5│   └── ada/     your home directory, also written ~
6│       └── station/
7│           ├── logs/
8│           └── notes.txt
9├── tmp/         temporary files, cleared regularly
10└── var/log/     system logs

Your shell always has a current working directory - the place you are “standing”. pwd (print working directory) shows it, ls lists what’s there, and cd (change directory) moves you.

A path names a file or directory:

• An absolute path starts with / and is spelled out from the root: /home/ada/station/notes.txt. It means the same thing wherever you are.
• A relative path doesn’t start with /, so it’s followed from the current directory: from /home/ada, station/notes.txt is the same file.

Three shortcuts appear everywhere:

And cd has a few tricks: cd alone goes home, cd - jumps back to the previous directory, and cd .. climbs one level.

A few things to notice:

• mktemp -d makes a new empty directory with a random name, and cd "$(mktemp -d)" moves into it. Exercises use this so every run starts clean. (The $(...) part runs a command and uses its output - you’ll learn it properly later.)
• mkdir -p creates a whole path at once, including missing parents.
• Files whose names start with a dot are hidden from a plain ls. Configuration files like ~/.bashrc hide this way.
• $PWD holds the current directory. We print only its last part with basename, because the sandbox’s full path is random.

ls has dozens of options; these cover most days:

Options combine: ls -lah is the classic “show me everything here”.