Foundations Updated 2026-09 View as Markdown

Variables

Shell variables are created once they are assigned a value. A variable can contain a number, a character or a string of characters.

Shell variables are created once they are assigned a value. A variable can contain a number, a character or a string of characters. Variable name is case sensitive and can consist of a combination of letters and the underscore “_”. Value assignment is done using the “=” sign. Note that no space permitted on either side of = sign when initializing variables.

PRICE_PER_APPLE=5
MyFirstLetters=ABC
greeting='Hello        world!'

Referencing the variables

A backslash “\” is used to escape special character meaning

PRICE_PER_APPLE=5
echo "The price of an Apple today is: \$HK $PRICE_PER_APPLE"

Encapsulating the variable name with ${} is used to avoid ambiguity

MyFirstLetters=ABC
echo "The first 10 letters in the alphabet are: ${MyFirstLetters}DEFGHIJ"

Encapsulating the variable name with “” will preserve any white space values

greeting='Hello        world!'
echo $greeting" now with spaces: $greeting"

Variables can be assigned with the value of a command output. This is referred to as substitution. Substitution can be done by encapsulating the command with `` (known as back-ticks) or with $()

FILELIST=`ls`
FileWithTimeStamp=/tmp/my-dir/file_$(/bin/date +%Y-%m-%d).txt

Note that when the script runs, it will run the command inside the $() parenthesis and capture its output.

BIRTHDATE is correct, it is Jan 1, 2000
I have received 10 presents
I was born on a Saturday

#!/bin/bash
# Change this code
BIRTHDATE="Jan 1, 2000"
Presents=10
BIRTHDAY=`date -d "$BIRTHDATE" +%A`

# Testing code - do not change it

if [ "$BIRTHDATE" == "Jan 1, 2000" ] ; then
echo "BIRTHDATE is correct, it is $BIRTHDATE"
else
    echo "BIRTHDATE is incorrect - please retry"
fi
if [ $Presents == 10 ] ; then
    echo "I have received $Presents presents"
else
    echo "Presents is incorrect - please retry"
fi
if [ "$BIRTHDAY" == "Saturday" ] ; then
    echo "I was born on a $BIRTHDAY"
else
    echo "BIRTHDAY is incorrect - please retry"
fi