Environment Variables

In addition to command options, Linux servers offer another means of sharing information values that is used extensively by the MDI, called environment variables.

An environment variable is a value present in the shell where a command is executed.

Some common environment variables

Here are a few environment variables to know - these are all set by the system and ready to use. There will certainly be others set also.

Variable Name	Description
USER	the system name of the current user
HOME	the full path to the user’s home directory
SHELL	the kind of command shell in use, e.g., bash
HOSTNAME	the name of the computer, e.g. server, running the shell
PATH	a list of directories where the system will look for executable files, i.e. programs
PWD	the full path to the current working directory (the output of pwd command)

Using environment variables

You can set and access your own environment variables, or modify existing variables, as follows:

MY_VAR=abc
echo $MY_VAR
echo ${MY_VAR}

Notice that:

it is common practice to use uppercase variable names with ‘_’ separators for environment variables
you set a variable without the leading $ symbol
you read a variable with the leading $ symbol, or as ${MY_VAR}

Environment variable scope

When you run a program on a Linux server, the program is started in it’s own instance of the command shell. That “subshell” may or may not have access to the environment variables in the shell that called the program.

By default, a variable set in a shell is not passed to its subshells. Thus, in our example above, MY_VAR is not set in the shell that runs a child program.

We often want our variables to pass into the programs we call. We sometimes do that by using the environment variable to set a program’s options:

MY_VAR=abc
command --option $MY_VAR

But for the program to read the environment variable itself, we must export the variable into the subshell, as follows:

export MY_VAR=abc
command