Computational Techniques for Life Sciences

Environment Variables

On a Linux system, environment variables are variables that are defined in the current shell, and can be passed into processes that the shell spawns. They have many practical purposes, including but not limited to:

• Point to the locations of executables
• Point to the locations of libraries
• Point to other special locations in the shell
• Provide information about the user to the programs
• Provide other special information to programs

To print all of the environment variables currently defined in your shell, use the env command:

$ env

There are quite a few, many of which you will never need to use. There are some important ones, however. To filter the output, you can print the contents of a specific variable. Use the echo command to print the contents of an environment variable called PATH or the env | grep combination::

$ echo $PATH
$ env | grep PATH

Note: The name of the variable is PATH; use $PATH to access the contents of the variable. You can also manually edit the content of environment variables using the export command:

$ echo BLAH                     # this is the variable itself
$ echo $BLAH                    # use $ to access the contents
$ export BLAH="some text here"
$ echo $BLAH

You can edit existing variables as well. The PATH variable, which defines which executables are available to you in your current shell, is often edited by users. We should be careful editing it because it stores the location of our very common commands like ls and cd. If we lose those paths, then it will be difficult to execute commands. Edit the beginning or the ending of the PATH variable as follows (contents at the beginning supersedes contents at the end):

$ export PATH=$PATH:/new/path/to/add
$ export PATH=/new/path/to/add:$PATH

Environment variables can be reset by logging out and in:

$ logout

Exercise

1. Print the contents of the PATH environment variable.
2. What files exist in some of the directories found in the PATH environment variable?
3. Find an environment variable that stores your username.
4. Store Webster’s dictionary in an environment variable called DICTIONARY.

Click here for solution

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