Basic variables in Bash

Introduction to Bash Scripting

Alex Scriven

Data Scientist

Assigning variables

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

var1="Moon"

Then reference with $ notation.

echo $var1

Moon

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

firstname='Cynthia'
lastname='Liu'

echo "Hi there" $firstname $lastname

Hi there Cynthia Liu

Both variables were returned - nice!

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

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

Hi there firstname lastname

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

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 (`sometext`) = Shell runs the command and captures STDOUT back into a variable

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

now_var='NOW'

now_var_singlequote='$now_var'
echo $now_var_singlequote

$now_var

now_var_doublequote="$now_var"
echo $now_var_doublequote

NOW

The Date program will be useful for demonstrating backticks

Normal output of this program:

date

Mon  2 Dec 2019 14:07:10 AEDT

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

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

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!

There is an equivalent to backtick notation:

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

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 backticks is older. Parentheses is used more in modern applications. (See http://mywiki.wooledge.org/BashFAQ/082)

Introduction to Bash Scripting