Moving around

Every file on a Linux machine lives somewhere in one big tree of directories. Before you can read a log or change a config file, you have to get to it, and on a server there is no window to click through. You move around by typing.

By the end you can say where you are, describe any location as an absolute or a relative path, and get from one directory to another with cd.

The directory tree

Linux has a single tree of directories. It starts at the root directory, written as a single /. Everything else sits below it: other directories, files, and even other disks, which are attached somewhere inside the tree rather than getting a letter of their own.

A directory is what other systems call a folder. A directory inside another one is a subdirectory. Run ls / to see what sits at the top of the tree:

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

You do not need to know all of these. The ones you will visit most:

The layout is much the same on every distribution, so once you know where logs and config files live on one server, you know where they live on the next.

Where you are

The shell always has a working directory: the directory you are "in". Commands that work on files look there unless you tell them otherwise. pwd prints it:

$ pwd
/root

When you log in, you start in your home directory. For an ordinary user called sam that is /home/sam. For root it is /root. The Docker container from the first topic is an exception: it starts you in /, and cd on its own takes you home. The prompt shows your home directory as ~, and most of the time it 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 the variable HOME:

$ echo $HOME
/root

Absolute and relative paths

A path names a place in the tree. There are two kinds.

An absolute path starts with / and spells out the full route from the root. /var/log/apt means: start at the root, go into var, then log, then apt. It means the same thing wherever you are.

A relative path does not start with /. The shell reads it starting from your working directory. If you are in /var, then log/apt means /var/log/apt. If you are somewhere else, it means something else, or nothing at all.

Three short names help you build relative paths:

You can chain them. From /usr/bin, the path ../lib means "up to /usr, then into lib", which is /usr/lib. ../../etc means "up two levels to /, then into etc".

The name ~ is turned into your home directory by the shell before the command runs:

$ echo ~
/root

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

Moving 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. The prompt changes to show where you are now.

Some shortcuts worth knowing:

cd - prints the directory it takes you to, and running it twice brings you back. It is handy when you are switching between two places, such as a config directory and a log directory:

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

When cd fails

Paths are case-sensitive, like commands. /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

The second error means the path exists, but it is a file, not a directory. You read files; you do not move into them.

The most common mistake is a relative path used from the wrong place. cd root only works when you are in /, because it means "the root directory inside where I am". From anywhere else it fails:

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

Spaces in names also trip people up, because the shell splits a command on spaces. 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 avoids most of these problems. Type the start of a directory name and press Tab; the shell completes it if there is only one match, adding the backslashes for you. Press Tab twice to list the matches when there are several. If nothing happens, the name you started does not exist 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 and moving 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?