Basic variables in Bash

• Assigning Variables

  Similar to other languages, you can assign variables with the equal notation.

    var1='Moon;
  

  Then reference with $ notation.

    echo $var
  

    Result: 
    Moon
  

• Assigning String Variables

  Name your variable as you like (something sensible!):

        firstname='Cynthia'
        lastname='Liu'
        echo 'Hi there' $firstname $lastname
  

    Result:
    Hi there Cynthia Liu
  

• Missing the $ notation

  If you miss the $ notation - it isn’t a variable

    firstname='Cynthia'
    lastname='Liu'
    echo 'Hi there ' firstname lastname
  

    Result:
    Hi there firstname lastname
  

• (Not) assigning variables

  Bash is not very forgiving about spaces in variable creation. Beware of adding spaces!

    var1 = "Moon"
    echo $var1
  

  script.sh: line 3: var1: command not found

  Don’t add spaces before and after the = sign.

• Single, double, backticks

  In Bash, using different quotation marks can mean different things. Both when creating variables and printing.

  • Single quotes ('sometext')=Shell interprets what is between literally
  • Double quotes ("sometext")=Shell interprets literally except using $ and backticks The last way creates a ‘shell-within-a-shell’, outlined below. Useful for calling command-line programs. This is done with backticks.
  • Backticks(``) = Shell runs the command and captures STDOUT back into a variable.

• Different variable creation

  Let’s see the effect of different types of variable creation

    now_var='NOW'
    now_var_singlequote='$now_var'
    echo $now_var_singlequote
  

  Returns: $now_var

    now_var_doublequote="$now_var"
    echo $now_var_doublequote
  

  Returns: NOW

• The date program

  The Date program will be useful for demonstrating backticks Normal output of this program:

    date
  

  Mon 2 Dec 2019 14:07:10 AEDT

• Shell within a shell

  Let’s use the shell-within-a-shell now:

    rightnow_doublequote="The date is `date`."
    echo $rightnow_doublequote
  

  Returns: The date is Mon 2 Dec 2019 14:13:35 AEDT.

  The date program was called, output captured and combined in-line with the echo call. We used a shell within a shell

• Parentheses vs backticks

  There is an equivalent to backtick notation:

    rightnow_doublequotes="The date is `date`."
    rightnow_parentheses="The date is $(date)."
    echo $rightnow_doublequotes
    echo $rightnow_parentheses
  

    Returns:
    The date is Mon 2 Dec 2019 14:54:34 AEDT.
    The date is Mon 2 Dec 2019 14:54:34 AEDT.
  

  Both work the same though using blackticks is older. Parentheses is used more in modern applications.

---

Numeric variables in Bash

• Numbers in other languages

  Numbers are not built in natively to the shell like most REPLs (console) such as R and Python

  In Python or R you may do:

    >>> 1 + 4
  

  Returns: 5

• Numbers in the shell

  Numbers are nto natively supported: (In the terminal)

    1 + 4
  

  bash: 1: command not found

• Introducing expr

  expr is a useful utility program (just like cat or grep) This will now work (in the terminal):

    expr 1 + 4
  

  Returns: 5

• expr limitations

  expr cannot natively handle decimal places; (In terminal)

    expr 1 + 2.5
  

  expr: not a decimal number: ‘2.5’ We can get past this limiation using bc

• Getting numbers to bc

  Using bc without opening the calculator is possible by piping:

    echo "5 + 7.5" | bc
  

  12.5

• bc scale argument

  bc also has a scale argument for how many decimal places.

    echo "10/3" | bc
  

  3

    echo "scale=3; 10/3" | bc
  

  Note the use of ; to separate ‘lines’ in terminal 3.333

• Numbers in Bash Scripts

  We can assign numeric variables just like string variables:

    dog_name='Roger'
    dog_age=6
    echo "My dog's name is $dog_name and he is $dog_age years old"
  

  Beware that dog_age="6" will work, but makes it a string!

  My dog’s name is Roger and he is 6 years old

• Double bracket notation

  A variant on single bracket variable notation for numeric variables:

    expr 5 + 7
    echo $((5+7))
  

  Returns:
  12
  12

  Beawre this method uses expr, not bc

• Shell within a shell revisited

  Very useful for numeric variables:

    model1=87.65
    model2=89.20
    echo "The total score is $(echo "$model1 + $modle2" | bc)"
    echo "The average score is $(echo "($model1 + $modle2) / 2" | bc)"
  

  Returns
  The total score is 176.85
  The average score is 88

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Arrays in Bash

• What is an array?

  Two types of arrays in Bash:

  • An array
    • ‘Normal’ numerical-indexed structure.
    • Called a ‘list’ in Python or ‘vector’ in R. In Python: my_list=[1,3,2,4] In R: my_vector <- c(1,3,2,4)`