Lesson 21: The shell environment

The Linux environment is a collection of environment variables. An environment variable is a user-specific or system-wide object that contains data used by one or more applications. In simple terms, it is a variable with a name and a value.

Note

By convention, environment variables are capitalised.

Some frequently used environment variables include:

Environment variable   Purpose
---------------------- ------------------------------------------------------------------------
`HOME`                 The home directory for the current logged in user
`USER`                 The username of the currently logged in user
`PWD`                  The current working directory, retrieved also with `pwd`
`PATH`                 A colon separated list of directories Linux looks for executable files
`LD_LIBRARY_PATH`      A colon separated list of directories Linux looks for library files

Retrieving an environment variable

To retrieve a single variable, either one of the below commands will yield the same result using printenv or echo followed by the variable, for example:

[learner planets]$ printenv PATH
/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin

[learner planets]$ echo $PATH
/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin

Notice how the $ character is included in the second command. This tells the shell to interpret the argument (PATH) as a variable and not a literal. Try executing the command without the $ character!

To retrieve all environment variables, just type printenv.

Setting an environment variable

To create (or change) an environment variable, we assign a value to the variable using the equals operator. As an example, to update the PATH variable to include directory /home/learner, the following command will be executed:

[learner planets]$ export PATH=$PATH:~
[learner planets]$ echo $PATH
/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin:/home/learner

We included $PATH in our new PATH variable as we want to append the directory ~ to the existing path. If $PATH was not included, PATH would only contain ~. Knowing how to change your $PATH to include custom directories can be necessary sometimes (e.g. if you install some new software in a non-standard location).

Take care when updating $PATH because removing system paths will cause the shell to start displaying 'No such file or directory' errors as it cannot find the location where programs such as cd and ls are installed. If this occurs, restart your shell to fix the issue.

If the environment variable does not already exist on the system (you can check with printenv), you will need to export your variable to the environment, for example:

[learner planets]$ export PLANETSDIR=~/learning_linux/planets

Note

Changing environment variables will only work in the current shell. To permanently make changes, add the export commands to your ~/.bashrc file.

The bashrc file is a shell script that Bash runs when it is started interactively. It can contain commands to initialise the environment (export commands), or assign command aliases.