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 as well as all programs or shell sessions that are started from this session.

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

	var="value"
	var = "value"
	var=="value"

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. a. Write a script that
   b. Prints an environment variable
   c. Saves the output of the date command to a variable
   d. Sleeps briefly
   e. Prints the new and old date and time

Answer:

     One possible way to write the script is

      #!/bin/bash

      echo "I am $USER, my home directory is $HOME"

      olddate=$(date)
      sleep 10s
      echo "old date: ${olddate}"
      echo "new date: $(date)"

     Explanation:
        #!/bin/bash: the shebang line for bash script
        echo "I am $USER, my home directory is $HOME": the variables $USER and $HOME are your username and home directory, respectively. This line prints a sentence embedded with your username and home directory.
        olddate=$(date) assigns the output of the date command to the variable olddate as a string.
        sleep 10s puts the terminal into idle for 10 seconds.
        echo "old date: ${olddate}" prints the previously saved date in the olddate variable. The form of ${olddate} is for using (or referencing) this variable.
        echo "new date: $(date)" prints the current date, in which the output of the command date will take the place of $(date).

 2. What do different types of quotes (single ' vs. double ") do?

Answer:

     single quote ' gives literal string, the variable will not be interpreted, e.g.

       var=abc
       echo '$var'

     This script prints the literal string $var (instead of its value abc) to terminal.
     double quote " allows the variable to be interpreted, e.g.

       var=abc
       echo "$var"

     This script prints the value of var, which is abc (instead of the literal string $var) to terminal. 

3. create an shell variable MYIDENTITY and export it as below: 
   $ export MYIDENTITY=whoami
   How will you list the shell variable MYIDENTITY?
   Execute the shell variable MYIDENTITY, what is the output? 

Answer:

     you can list the variable by using the echo command as follows

        $ echo $MYIDENTITY
        whoami

     The arguments passed to echo are printed to the standard output.

        $ $MYIDENTITY     
        username

     The shell variable upon execution runs the command whoami, which is assigned to it. whoami command prints the user name of the effective user ID

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