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3. Absolute vs Relative Paths

The cd command takes an argument which is a directory name. Directories can be specified using either a relative path or a full absolute path. The directories on the computer are arranged into a hierarchy. The full path tells you where a directory is in that hierarchy. Navigate to the home directory, then enter the pwd command.

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$ cd
$ pwd

You will see:

/users/group/user

This is the full name of your home directory. This tells you that you are in a directory called training, which sits inside a directory called home which sits inside the very top directory in the hierarchy. The very top of the hierarchy is a directory called / which is usually referred to as the root directory. So, to summarize: training is a directory in home which is a directory in /. More on root and home in the next section.

Now enter the following command:

$ cd /users/group/user/shell_data/.hidden

This jumps forward multiple levels to the .hidden directory. Now go back to the home directory.

$ cd

You can also navigate to the .hidden directory using:

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$ cd ~/shell_data/.hidden

These two commands have the same effect, they both take us to the .hidden directory. The first uses the absolute path, giving the full address from the home directory. The second uses a relative path, giving only the address from the working directory. A full path always starts with a /. A relative path does not.

A relative path is like getting directions from someone on the street. They tell you to "go right at the stop sign, and then turn left on Main Street". That works great if you're standing there together, but not so well if you're trying to tell someone how to get there from another country. A full path is like GPS coordinates. It tells you exactly where something is no matter where you are right now.

You can usually use either a full path or a relative path depending on what is most convenient. If we are in the home directory, it is more convenient to enter the full path. If we are in the working directory, it is more convenient to enter the relative path since it involves less typing.

Over time, it will become easier for you to keep a mental note of the structure of the directories that you are using and how to quickly navigate amongst them.

Relative path resolution

Using the filesystem diagram below, if pwd displays /Users/thing," what will ls ../backup display?

  1. ../backup: No such file or directory
  2. 2012-12-01 2013-01-08 2013-01-27
  3. 2012-12-01/ 2013-01-08/ 2013-01-27/
  4. original pnas_final pnas_sub

File System for Challenge Questions

Solution
  1. No: there is a directory backup in /Users.
  2. No: this is the content of Users/thing/backup, but with .. we asked for one level further up.
  3. No: see previous explanation. Also, we did not specify -F to display / at the end of the directory names.
  4. Yes: ../backup refers to /Users/backup.

The root directory is the highest level directory in your file system and contains files that are important for your computer to perform its daily work. While you will be using the root (/) at the beginning of your absolute paths, it is important that you avoid working with data in these higher-level directories, as your commands can permanently alter files that the operating system needs to function. In many cases, trying to run commands in root directories will require special permissions which are not discussed here, so it's best to avoid them and work within your home directory. Dealing with the home directory is very common. The tilde character, ~, is a shortcut for your home directory. In our case, the root directory is two levels above our home directory, so cd or cd ~ will take you to /home/<username> and cd / will take you to /. Navigate to the shell_data directory:

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$ cd
$ cd ~/shell_data

Then enter the command:

$ ls ~

shell_data

This prints the contents of your home directory, without you needing to type the full path.

The commands cd, and cd ~ are very useful for quickly navigating back to your home directory. We will be using the ~ character in later lessons to specify our home directory.

Summary

  • The /, ~, and .. characters represent important navigational shortcuts.
  • Hidden files and directories start with . and can be viewed using ls -a.
  • Relative paths specify a location starting from the current location, while absolute paths specify a location from the root of the file system.