# Expansion and command substitution — Bash / Shell Scripting

Source: https://www.geekswithgeeks.com/en/bash/bash-expansion

> Explain Expansion and command substitution, run a small example, and check the result.

## 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.

```bash
#!/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

**Quiz:** What is the best first practice for Expansion and command substitution?

- [x] Run one small, focused example.
- [ ] Memorize syntax without testing it.
- [ ] Ignore error messages.

*Answer:* Run one small, focused example.. A small experiment gives direct feedback about cause and effect.