Introduction to Variables

Assigning and Reading a Variable

You assign a variable with name=value, with no spaces around the =. To read the value back you prefix the name with a dollar sign, $name, which Bash expands (substitutes) with the stored text before running the command.

Warning: name = "Ada" (with spaces) fails because Bash tries to execute a command called name with arguments = and "Ada".

Example: Assigning and Reading a Variable

#!/bin/bash
name="Ada Lovelace"
echo "Hello, $name!"

Using Braces: ${name}

Wrapping a variable name in braces, ${name}, makes the boundary of the variable name explicit, which matters when you want to immediately follow it with other text with no separator. Without braces, Bash would try to include that following text as part of the variable name.

Example: Using Braces: ${name}

#!/bin/bash
fruit="apple"
echo "I have one ${fruit}s"
# echo "I have one $fruits" would look for a variable named 'fruits' instead

Reassigning and Building on Variables

Variables can be reassigned at any point in a script, and a variable's current value can be used while constructing its new value. This is common for building up strings or counters across several steps.

Example: Reassigning and Building on Variables

#!/bin/bash
greeting="Hello"
greeting="$greeting, world"
echo "$greeting"
count=1
count=$((count + 1))
echo "Count is now $count"

Everything Is a String by Default

Bash does not have distinct numeric or boolean variable types out of the box -- every variable holds text, and Bash reinterprets that text as a number only in numeric contexts like arithmetic expressions. This is why age="5" and age=5 behave identically when stored.

Example: Everything Is a String by Default

#!/bin/bash
age="5"
echo "Stored value: $age"
echo "Type as text length: ${#age} character(s)"