Expansion and command substitution

The essential idea

Expansion and command substitution is easiest when you first identify its job. Name the input, the change, and the result before memorising syntax. Small explicit steps make programs easier to repair later.

Run a tiny experiment

This idea is one part of the path from a source file to useful output.

#!/usr/bin/env bash
# Explore expansion one small step at a time.
value=0
echo "$value"

Output:

0

A common trap

Do not trust code just because it looks familiar. For Expansion and command substitution, change one value, run the smallest example, and compare the result with your prediction.

Quick check

Quick check: What is the best first practice for Expansion and command substitution?

• Run one small, focused example.
• Memorize syntax without testing it.
• Ignore error messages.