Introduction to Linux in HPC/Environment variables

HPC.NRW
Other HPC Courses
1.	Gprof Tutorial
2.	OpenMP in Small Bites
Introduction to Linux in HPC
1.	Background and History
2.	The Command Line
3.	Directory structure
4.	Files
5.	Text display and search
6.	Users and permissions
7.	Processes
8.	The vim text editor
9.	Shell scripting
10.	Environment variables
11.	System configuration
12.	SSH Connections
13.	SSH: Graphics and File Transfer
14.	Various tips

Quiz

Which bash command below assigns "value" to variable var?

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

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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