Variables and Quoting

Making and using variables

A variable gives a name to a piece of text. Set it with name=value and no spaces around the =. Read it back by putting $ in front:

ship="Black Pearl"
echo "$ship"
echo "The $ship sails at dawn"

In the terminal

student@vulntech:~$ ship="Black Pearl"
student@vulntech:~$ echo "$ship"
Black Pearl
student@vulntech:~$ echo "The $ship sails at dawn"
The Black Pearl sails at dawn

Common mistake: Spaces break an assignment. ship = Pearl tries to run a command called ship; it must be ship=Pearl, with no spaces.

Curly braces: where the name ends

Use ${name} when the variable is touching other text, so bash knows where the name stops:

word=sail
echo "$words"
echo "${word}s"

In the terminal

student@vulntech:~$ word=sail
student@vulntech:~$ echo "$words"
student@vulntech:~$ echo "${word}s"
sails

$words was a different, empty variable, so the first line is blank. ${word}s is sail followed by s.

Capturing output: $( )

From the pipes module: $(command) runs a command and gives you its output, perfect for putting into a variable:

today=$(date +%F)
files=$(ls /etc | wc -l)
echo "On $today, /etc had $files entries"

In the terminal

student@vulntech:~$ today=$(date +%F)
student@vulntech:~$ files=$(ls /etc | wc -l)
student@vulntech:~$ echo "On $today, /etc had $files entries"
On 2026-10-17, /etc had 65 entries

Why double quotes matter

Put variables in double quotes. Without them, a value with spaces is split into several words, which causes some of the most common script bugs:

name="Jolly Roger"
touch $name
ls
rm *.txt 2>/dev/null; touch "$name"
ls

In the terminal

student@vulntech:~$ name="Jolly Roger"
student@vulntech:~$ touch $name
student@vulntech:~$ ls
Jolly  Roger
student@vulntech:~$ rm *.txt 2>/dev/null; touch "$name"
student@vulntech:~$ ls
 Jolly  'Jolly Roger'   Roger

Without quotes, touch $name made two files, Jolly and Roger. With quotes, it made the one file you meant.

fruit=apple
echo "I like $fruit"
echo 'I like $fruit'

In the terminal

student@vulntech:~$ fruit=apple
student@vulntech:~$ echo "I like $fruit"
I like apple
student@vulntech:~$ echo 'I like $fruit'
I like $fruit

Remember: Set with name=value (no spaces), read with "$name". Double quotes keep a value in one piece; single quotes turn off $.