Relative and absolute paths in Linux

Paths is a topic that causes a lot of confusion for people that want to learn how to make use of the command line in Linux. In this post I will explain what paths are, and the difference between absolute and relative paths. By the end of this post you should be able to understand the diagram below.

The file system on Unix-like systems is built up like a tree starting from the root directory (/). One can view the content of the root directory by typing in the command ls /.

$ ls
bin  boot  dev  etc  home  lib  lib64  media  mnt  opt  proc  root
run  sbin  srv  sys  tmp  usr  var

The term “path” refers to the name of a file or directory that can be used to uniquely identify it in the file system. For example, the command pwd prints the name of the current working directory, in this case my home directory.

$ pwd
/home/olssont

In the above /home/olssont is the path to the directory that I am currently in. To illustrate this further we can create a file in this directory. The command below creates a file named raw_data.txt.

$ echo "Raw data isn't baked data" > raw_data.txt

We can see the files in the working directory using the command ls.

$ ls
raw_data.txt

Depending on how many files you have in your working directory you may see more output from the command above.

To print the content of the file one can use the command cat.

$ cat raw_data.txt
Raw data isn't baked data

In the above the text raw_data.txt could be used to find the file that we just created. This was because the file was present in the working directory.

So how can we refer to a file if it is not present in the working directory? This is where the use of absolute and relative paths comes into play.

To illustrate this we will use the command mkdir to create a new directory called raw_data.

$ mkdir raw_data

Then we will use the command mv to move the file into that directory.

$ mv raw_data.txt raw_data/

Before illustrating the correct way to refer to the file let us see what happens if we use the same command as previously.

$ cat raw_data.txt
cat: raw_data.txt: No such file or directory

The cat command is no longer able to find the file (or rather the file is no longer there, because we moved it into the raw_data directory). There are two methods that you could use to refer to the file in the raw_data directory. The first is to use the absolute path, in this case /home/olssont/raw_data/raw_data.txt. Note that the absolute path to your file will be different if your username is different to mine and/or if you are not working in your home directory.

$ cat /home/olssont/raw_data/raw_data.txt
Raw data isn't baked data

The second method is to make use of a relative path, that means a path that is relative to your current working directory, in this case raw_data/raw_data.txt. The forward slash (/) is the symbol used to separate directories and files.

$ cat raw_data/raw_data.txt
Raw data isn't baked data

A different way to represent this relative path is to prepend it with ./, where the dot is a symbol that is used to represent the current working directory. Some people prefer this way because it makes it easer to see that it is a relative path.

$ cat ./raw_data/raw_data.txt
Raw data isn't baked data

To illustrate the concept of relative paths further we will create another directory called scripts.

$ mkdir scripts

Now we will change our working directory to be scripts using the cd (change directory) command.

$ cd scripts

Let us see what happens if we run the previous cat command again (you should be able to get back to this by using the up and down arrows on your keyboard to navigate through the command line history).

$ cat ./raw_data/raw_data.txt
cat: ./raw_data/raw_data.txt: No such file or directory

The command above fails because it expects to be able to find a directory named raw_data in the current working directory and because we have moved into the scripts directory there is no such directory.

In order to make this work we need to be able to specify that we want to go up one level in the directory tree. This can be achieved using double dots, i.e. using the prefix ../. To refer to something two levels up in the directory tree one would use the prefix ../../, and so forth.

$ cat ../raw_data/raw_data.txt
Raw data isn't baked data

To summarise:

• Paths are used to refer to unique files
• An absolute path starts at the top of the directory tree and includes all parent directories separated by slashes as well as the file or directory of interest. Examples of absolute paths include /home/olssont (a directory), /home/olssont/raw_data/raw_data.txt (a file)
• Relative paths are used to refer to files and directories with respect to the current working directory
• In a relative path the prefix ./ means the current working directory, the prefix ../ means the parent directory, and the prefix ../../ means the parent’s parent directory, and so forth

This should be enough to get you started with paths in the Linux command line. In the next post I will show how relative paths can be used to make scripts more portable, and how they can be used to improve your data management.