Introduction to Linux in HPC/Environment variables

This part of the Linux tutorials introduces environment variables and explains the difference to shell variables that have been introduced in Shell Scripting. A few important use cases for environment variables are discussed such as the OATH variable that determines where the shell searches for executable programs. Environment variables are also used by the so-called environment modules that are the main way to access software installed on an HPC cluster. Environment modules are explained shortly in this tutorial.

Video

( Slides as pdf)

Quiz

How do you define an environment variable VARTEST and assign the value 42 to it?

	VARTEST=42
	export VARTEST=42
	env VARTEST=42

What is the naming convention for environment variables?

	Use uppercase letters
	Use lowercase letters
	There is no naming convention.

Which statement is correct for environment variables?

	Environment variables are stored in a central database on a computer.
	An environment variable that was defined is valid and accessible in every shell session on this computer.
	An environment variable is valid in the shell session in which it was defined and is inherited to all programs or shell sessions that are started from this session.

What is the expected output of the following commands? VARTEST="bla"; export VARTEST; VARTEST="blub"; env

	VARTEST=""
	VARTEST="bla"
	VARTEST="blub"

Assume that the environment variable PATH has the following content PATH="/usr/bin:/usr/local/bin". There is a program in each directory, i.e., /usr/bin/program and /usr/local/bin/program. Which will be executed if you run program in the shell?

	/usr/bin/program
	/usr/local/bin/program
	The shell will ask you which one to execute.

How do you add the directory /opt/bin/ to the PATH environment variable and make sure that it is searched last?

	export PATH=/opt/bin/:$PATH
	export PATH=$PATH:/opt/bin/
	export PATH=/opt/bin/

Exercises in Terminal

1. Run the following step in a shell session
   a. Define an environment variable named TEST_VAR1="test1".
   b. Define a shell variable names TEST_VAR2="test2".
   b. Make sure that TEST_VAR1 it is in the list of environment variables.
   c. Start a new shell session within the current session.
   d. Output the content of the two variables TEST_VAR1 and TEST_VAR2.
   e. Change content of the environment variable to TEST_VAR1="test_new".
   f. Exit the shell session and output the content of TEST_VAR1.

Answer:

     The commands to enter are:

      export TEST_VAR1="test1"
      TEST_VAR2="test2"
      env | grep TEST_VAR1
      bash
      echo $TEST_VAR1
      echo $TEST_VAR2
      export TEST_VAR1="test_new"
      exit
      echo $TEST_VAR1

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