The Linux file system

The Linux file system

A file is a named piece of data, such as a document, a program or a log.

A directory is a container that holds files and other directories. Windows and macOS call it a folder. A directory inside another one is a subdirectory.

A file system is the way the files and directories on a machine are organised and stored.

Linux keeps everything in a single tree of directories. The tree starts at the root directory, written as /, and every file and directory on the machine sits somewhere below it.

Windows gives each disk its own letter, such as C: and D:. Linux has no drive letters. A second disk is attached to a directory inside the tree, such as /data, and you open it like any other directory. The Disks and disk space topic covers this.

Run ls / to see the top of the tree:

$ ls /
bin   dev  home  media  opt   root  sbin  sys  usr
boot  etc  lib   mnt    proc  run   srv   tmp  var

What each directory holds

The layout follows the Filesystem Hierarchy Standard (FHS), so it is much the same on every distribution. Once you know where logs and config files live on one server, you know where they live on the next.

The directories you will use most:

The rest are used by the system, and you will open them less often:

/etc/hostname is a file that holds the machine's name. Print it with cat:

$ cat /etc/hostname
lab

The working directory and pwd

The working directory is the directory you are in right now. Commands that work on files look there unless you give them a different location.

pwd (print working directory) prints it:

$ pwd
/root

Your home directory is the directory that belongs to you, where you keep your own files. When you log in, you start there.

• For an ordinary user called sam, it is /home/sam.
• For root, it is /root.

The Docker container from the first topic is an exception: it starts you in /.

The prompt shows your home directory as ~, and usually shows where you are, so you rarely need pwd. It is still useful when the prompt is short or when a script needs to know.

The shell keeps the path of your home directory in a variable called HOME. A variable is a name that holds a value. echo prints it:

$ echo $HOME
/root

Absolute and relative paths

A path is the location of a file or directory in the tree, written as directory names separated by /. There are two kinds.

An absolute path starts with / and gives the full route from the root directory. /var/log/apt means: start at the root, go into var, then log, then apt. It points to the same place wherever you are.

A relative path does not start with /. It starts from your working directory. If you are in /var, then log/apt means /var/log/apt. From anywhere else it means something else, or nothing at all.

Three short names help you write relative paths:

You can chain them:

• From /usr/bin, ../lib means up to /usr, then into lib, which is /usr/lib.
• From /usr/bin, ../../etc means up two levels to /, then into etc, which is /etc.

The shell turns ~ into your home directory before the command runs:

$ echo ~
/root

Use an absolute path when you want to be sure, such as in scripts and in instructions for other people. Use a relative path when you are working nearby and want to type less.

Changing directory with cd

cd (change directory) moves you to another directory. Give it a path, absolute or relative:

$ cd /var/log
$ pwd
/var/log
$ cd ..
$ pwd
/var
$ cd log
$ pwd
/var/log

cd prints nothing when it works, and the prompt changes to show where you are now.

Shortcuts you will use often:

cd - prints the directory it takes you to, and running it twice brings you back. Use it to switch between two places, such as a config directory and a log directory:

$ cd /etc
$ cd /var/log
$ cd -
/etc
$ cd -
/var/log

Common cd errors

Paths are case-sensitive: /var exists, /Var does not. The shell tells you when a path is wrong:

$ cd /Var
bash: cd: /Var: No such file or directory
$ cd /etc/hostname
bash: cd: /etc/hostname: Not a directory

• No such file or directory means nothing exists at that path.
• Not a directory means the path exists, but it is a file. You can read a file, but you cannot move into it.

The most common mistake is using a relative path from the wrong place. cd root means "the root directory inside where I am", so it only works from /:

$ cd /tmp
$ cd root
bash: cd: root: No such file or directory
$ cd /root
$ pwd
/root

Names with spaces also cause errors, because the shell splits a command into parts at each space. Put the path in quotes, or put a backslash (\) before each space:

$ cd my files
bash: cd: too many arguments
$ cd "my files"
$ cd ../my\ files

The Tab key prevents most of these mistakes:

• Type the start of a name and press Tab. If only one name matches, the shell completes it and adds any backslashes for you.
• Press Tab twice to list the matches when there are several.
• If nothing happens, no name starting that way exists where you are looking.

Tasks

Do these in your Linux shell.

Task 1: Walk a small tree

Build a practice tree in your home directory. mkdir -p creates directories along with any parents they need; the Creating, copying and deleting files topic covers it properly.

$ cd ~
$ mkdir -p projects/shop/api projects/shop/web projects/notes

Now, using only relative paths and one cd at a time:

1. Go from your home directory into projects/shop/api.
2. Go from there into projects/shop/web.
3. Go from there into projects/notes.

Run pwd after each step.

$ cd projects/shop/api
$ pwd
/root/projects/shop/api
$ cd ../web
$ pwd
/root/projects/shop/web
$ cd ../../notes
$ pwd
/root/projects/notes

Task 2: Absolute paths and going back

From projects/notes, do the following, running pwd after each step:

1. Go to /var/log with an absolute path.
2. Go back to projects/notes without typing its name.
3. Go to your home directory with the shortest command you know.

$ cd /var/log
$ pwd
/var/log
$ cd -
/root/projects/notes
$ pwd
/root/projects/notes
$ cd
$ pwd
/root

Quiz

1. You are in /home/sam/app/config. Where does cd ../../logs take you?

2. What is the difference between cd etc and cd /etc?

3. You are on a server, reading logs in /var/log/nginx. You move to /etc/nginx to check a setting. How do you get back to the logs with the fewest keys?

4. cd /etc/hosts fails with Not a directory. What does that tell you?