Defining and Referencing Variables

Define a variable (no spaces around =).

name=value

Reference a variable.

echo $name
echo ${name}

Delete a variable.

unset name

Define a read-only variable.

readonly name=value

Syntax	Description
name=value	Assigns a value to a variable. No spaces are allowed around the =.
$name / ${name}	References the value of a variable. Curly braces clarify the boundary of the variable name within a string.
unset name	Deletes a variable. Any subsequent reference returns an empty string.
readonly name=value	Defines a read-only variable. Any subsequent assignment causes an error.
declare -i name	Declares a variable as an integer. Arithmetic evaluation is performed on assignment.
declare -r name	Declares a read-only variable (equivalent to readonly).
declare -a name	Declares an array variable.
name=(a b c)	Defines an array. Indices start at 0.
${name[0]}	References the first element (index 0) of the array.
${name[@]}	References all elements of the array.

Defining and referencing variables. The rule is to never place spaces around the = sign.

variable.sh

name="Alice"
age=30
echo "Name: $name, Age: $age"

bash variable.sh
Name: Alice, Age: 30

Use curly braces ${} to clarify the boundary of a variable name. $fruitsa and ${fruit}s reference different variables.

brace.sh

fruit="apple"
echo "${fruit}s are delicious"

bash brace.sh
apples are delicious

Use readonly to define a read-only variable. Reassigning it causes an error.

readonly.sh

readonly PI=3.14159
echo "Pi: $PI"
PI=3

bash readonly.sh
Pi: 3.14159
bash: PI: readonly variable

Use unset to delete a variable. After deletion, referencing it returns an empty string.

unset.sh

tmp="temporary data"
echo "Before deletion: $tmp"
unset tmp
echo "After deletion: $tmp"

bash unset.sh
Before deletion: temporary data
After deletion:

Defining and referencing an array. Indices start at 0.

array.sh

colors=("red" "green" "blue")
echo "First: ${colors[0]}"
echo "All: ${colors[@]}"
echo "Count: ${#colors[@]}"

bash array.sh
First: red
All: red green blue
Count: 3

Use += to append an element to an array.

array_add.sh

colors=("red" "green" "blue")
colors+=("yellow")
echo "After append: ${colors[@]}"

bash array_add.sh
After append: red green blue yellow

When defining shell variables, it is critical that there are no spaces around the = sign. A space causes Bash to interpret the statement as a command (e.g., name = Alice means "run the command name with arguments = and Alice", which results in an error).

When the boundary of a variable name is ambiguous, use curly braces. For example, $fruits and ${fruit}s reference different variables.

To pass a variable to child processes as an environment variable, use export. For advanced variable expansion such as default values and string manipulation, see Parameter Expansion.