5. env (Environment)

Run the following command:

$ echo $HOME

You should see the path to your home directory, mine looks like /home/pete.

What about this command?

$ echo $USER 

You should see your username!

Where is this information coming from? It's coming from your environment variables. You can view these by typing

$ env 

This outputs a whole lot of information about the environment variables you currently have set. These variables contain useful information that the shell and other processes can use.

Here is a short example:

PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/bin

PWD=/home/user

USER=pete

One particularly important variable is the PATH Variable. You can access these variables by sticking a $ infront of the variable name like so:

$ echo $PATH

/usr/local/sbin:/usr/local/bin:/usr/sbin:/bin

This returns a list of paths separated by a colon that your system searches when it runs a command. Let's say you manually download and install a package from the internet and put it in to a non standard directory and want to run that command, you type $ coolcommand and the prompt says command not found. Well that's silly you are looking at the binary in a folder and know it exists. What is happening is that $PATH variable doesn't check that directory for this binary so it's throwing an error.

Let's say you had tons of binaries you wanted to run out of that directory, you can just modify the PATH variable to include that directory in your PATH environment variable.