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

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

sample_variable.sh

name="Ayanami Rei"
age=14
echo "Name: $name, Age: $age"

bash variable.sh
Name: Ayanami Rei, Age: 14

Use curly braces ${} to clarify the boundary of a variable name. $series_name and ${series}name reference different variables.

sample_brace.sh

series="eva"
echo "${series}_nerv"

bash brace.sh
eva_nerv

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

sample_readonly.sh

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

bash readonly.sh
Pi: 3.14159
sample_readonly.sh: line 3: PI: readonly variable

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

sample_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.

sample_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.

sample_array_add.sh

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

bash array_add.sh
After append: red green blue yellow

Common Mistake 1: Spaces around = cause a "command not found" error

In Bash, there must be no spaces around = when assigning a variable. A space makes Bash treat the name as a command to execute.

name = "Ayanami Rei"
bash: name: command not found

Remove all spaces around the = sign.

name="Ayanami Rei"
echo "$name"
Ayanami Rei

Common Mistake 2: Confusing $var and ${var} when appending strings

Without curly braces, Bash may not know where the variable name ends. This causes it to look up the wrong variable name entirely.

fruit="apple"
echo "$fruits"
（empty — Bash looked for a variable named "fruits", not "fruit" + "s"）

Use ${var} to clearly mark the variable name boundaries.

echo "${fruit}s"
apples

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 = Ayanami Rei means "run the command name with arguments = and Ayanami Rei", 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.