The tree and how to move in it

One tree, one root

Linux has a single directory tree. Its top is /, called the root. Every file on the system is somewhere below it, including files on other disks, which are attached to the tree at a directory (lesson 6). There are no drive letters.

You can always ask where you are:

pwd

/home/alex

pwd prints the working directory: the place relative paths start from. Every shell has one.

What lives where

You do not need to memorise the tree, but you do need to know which top-level directories you will touch.

Directory	What it holds
/home	One directory per user. Your files and settings.
/etc	System-wide configuration, as text files.
/var	Data that changes while the system runs: logs, caches, queues.
/usr	Installed programs, libraries and shared data. Mostly read-only.
/tmp	Scratch space. Cleared on reboot, or sooner.
/opt	Self-contained third-party software.
/srv	Data served by this machine, such as web or file shares.
/mnt, /media	Places to attach extra disks by hand or automatically.
/dev	Device files. Disks, terminals and /dev/null appear here.
/proc, /sys	Live views of the kernel. Not stored on disk.
/root	The home directory of the root user.

List the top level to see them:

ls /

bin   boot  dev  etc  home  lib  lib64  lost+found  media  mnt  opt  proc  root  run  sbin  srv  sys  tmp  usr  var

Absolute and relative paths

A path names a file by walking the tree. There are two ways to write one.

An absolute path starts with / and means the same thing wherever you are:

cd /usr/share/doc
pwd

/usr/share/doc

A relative path does not start with / and starts from the working directory. Three short names make relative paths work:

Name	Meaning
.	The current directory
..	The parent directory
~	Your home directory

From /usr/share/doc, the parent is /usr/share. Going up twice and into bin reaches /usr/bin:

cd ../../bin
pwd

/usr/bin

cd - returns to the previous directory, which is how you flip between two places:

cd -

/usr/share/doc

With no argument, cd goes home. These four are equivalent when you are alone in your home directory tree: cd, cd ~, cd $HOME and cd /home/alex.

Listing a directory

ls lists the working directory, or whatever you name:

ls /etc/ssh

moduli  ssh_config  ssh_config.d  ssh_host_ed25519_key  ssh_host_ed25519_key.pub  sshd_config  sshd_config.d

The options you will use daily:

Option	Effect
-l	Long format: type, permissions, owner, size, date
-a	Include hidden entries (names starting with .)
-h	Sizes like 4.0K and 12M (with -l)
-t	Newest first
-S	Largest first
-r	Reverse the order
-d	Show a directory itself, not its contents
-R	Recurse into subdirectories

Combine them: ls -lAht is “long, almost all, human sizes, newest first”.

Reading a long listing

ls -l /etc/hostname /etc/ssh

-rw-r--r-- 1 root root    4 Mar  3  2024 /etc/hostname

/etc/ssh:
total 580
-rw-r--r-- 1 root root 581843 Aug 12  2024 moduli
-rw-r--r-- 1 root root   1649 Aug 12  2024 ssh_config
drwxr-xr-x 2 root root   4096 Aug 12  2024 ssh_config.d

Take the first line apart, left to right:

Field	Example	Meaning
Type	-	- file, d directory, l symbolic link
Permissions	rw-r--r--	Owner, group, others (lesson 7)
Links	1	Number of names for this file (lesson 4)
Owner	root	The user who owns it
Group	root	The group that owns it
Size	4	Bytes, or the unit with -h
Modified	Mar 3 2024	Time of the last content change
Name	/etc/hostname	And for a link, -> target

Hidden files

A leading dot hides a name. It is only a convention, and it is how programs keep settings out of the way:

ls -a ~

.  ..  .bash_history  .bash_logout  .bashrc  .config  .profile  lab

. and .. are in every directory. ls -A shows hidden entries without those two.

Finding out what a command is

type tells you where a command comes from:

type ls cd pwd

ls is aliased to `ls --color=auto'
cd is a shell builtin
pwd is a shell builtin

For a program on disk, command -v prints its path:

command -v ls

/usr/bin/ls

That is the answer to “where is this installed?”, and it is why PATH matters: the shell searches the directories listed in it, in order.

Practice setup

Exercises in this skill use a scratch directory, ~/lab, so nothing touches your real files. If you have not made it yet:

mkdir -p ~/lab