How do you declare and use variables in bash?

Assign with name=value, with no spaces around the =, and read the value back with $name or ${name}. The spaces matter, because name = value makes Bash run a command called name with = and value as its arguments.

# Declaration
name="John"
age=25

# Usage
echo "Name: $name, Age: $age"
echo "Name: ${name}, Age: ${age}"  # Preferred syntax

The braces mark where the name ends, so ${name}_backup works where $name_backup would look for a variable called name_backup. Wrap each use in double quotes, as in "$name", so a value with spaces stays one word.

A variable you create stays in the current shell, and programs the script starts do not see it. export passes it to them through the environment. local limits a variable to the function that declares it and the functions that function calls. readonly stops later changes anywhere in the script.