Use `declare -n` for indirect variable references

Mastering Indirect References in Bash with `declare -n`

Q&A on Using `declare -n` in Bash

Q1: What is the `declare -n` command in Bash?

A1: The declare -n command in Bash creates a nameref, or a name reference, to another variable. This means that the nameref variable points to the original variable, allowing you to access or modify its value indirectly.

Q2: How can `declare -n` be practically used in scripts?

A2: declare -n is very useful in scenarios where you need to dynamically reference variables based on runtime conditions. For example, it can be used in functions to modify variables that are passed as arguments without having to know their names in advance.

Q3: Can you provide a simple example of `declare -n`?

A3: Sure! Let's say you have two variables, var1 and var2, and based on some condition, you want to update one of them via a function:

var1=10
var2=20

updateValue() {
    local name=$1
    declare -n ref=$name
    ref=$((ref + 10))
}

updateValue var1
echo "var1 is now $var1"  # Output will be: var1 is now 20

In this example, declare -n ref=$name creates a nameref ref that points to var1, and changes to ref will affect var1.

Background: Understanding `declare -n` with Simple Examples

Indirect references can sometimes be confusing. Here's a bit more background on the topic with simpler examples:

Example 1: Swapping Variables

a="Apple"
b="Banana"

swap() {
    declare -n first=$1
    declare -n second=$2
    local temp=$first
    first=$second
    second=$temp
}

swap a b
echo "a: $a, b: $b"  # Output will be: a: Banana, b: Apple

This script swaps the values of a and b using namerefs.

Example 2: Configurable Function Output

result1=0
result2=0

performCalculation() {
    local returnType=$1
    declare -n resultVar=$returnType
    resultVar=42  # Simulating some calculation result assignment.
}

performCalculation result1
echo "Result1: $result1"  # Output will be: Result1: 42

By using declare -n, the function performCalculation can dynamically decide where to store its result.

Conclusion

The declare -n in Bash opens up a new dimension of flexible scripting by allowing indirect and dynamic references to variables. This can greatly simplify complex scripts where variable names need to vary at runtime. As with any powerful feature, it comes with the responsibility to ensure it's used wisely to keep scripts readable and maintainable.

Mastering Indirect References in Bash with declare -n

Q&A on Using declare -n in Bash

Q1: What is the declare -n command in Bash?

Q2: How can declare -n be practically used in scripts?

Q3: Can you provide a simple example of declare -n?

Background: Understanding declare -n with Simple Examples