Introduction to Linux in HPC/Environment variables

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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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