# Shell shell-basics

## Variables

**Variables** store values. Assign with `VAR=value` (no spaces around `=`). Access with `$VAR` or `${VAR}`. Variables are case-sensitive and untyped—everything is a string unless used in arithmetic.

```bash
$ NAME="Alice"
$ echo $NAME                     # prints: Alice
$ echo ${NAME}_suffix           # prints: Alice_suffix
$ X=5; Y=$((X + 3)); echo $Y   # arithmetic: prints 8
```

Unset variables are empty strings. Quote variable references to preserve spaces: `"$VAR"` vs `$VAR`.

## Quoting and escaping

**Quoting** controls how the shell interprets special characters. Double quotes `"..."` preserve most meaning but allow expansion of `$VAR` and backticks. Single quotes `'...'` prevent all expansion. Backslash `\` escapes a single character.

```bash
$ echo "Hello $USER"            # expands $USER
$ echo 'Hello $USER'            # literal: Hello $USER
$ echo "Cost: \$5"              # literal: Cost: $5
$ echo $'Hello\nWorld'          # ANSI-C quoting: interprets \n
```

Use single quotes for literals, double quotes when you need expansion.

## Expansion

**Expansion** is how the shell transforms text before running commands. Types include:

- **Parameter expansion**: `$VAR`, `${VAR}`, `${VAR:-default}`
- **Arithmetic expansion**: `$((2 + 2))`
- **Command substitution**: `$(command)` or `` `command` ``
- **Brace expansion**: `{a,b,c}` expands to `a b c`
- **Tilde expansion**: `~` becomes `$HOME`
- **Pathname expansion (globbing)**: `*.txt` matches files ending in `.txt`

```bash
$ echo {1,2,3}                  # expands to: 1 2 3
$ echo /home/*/Desktop          # expands to matching directories
$ echo "Today is $(date +%Y)"   # command substitution
```

Expansions happen after quoting, so `"$VAR"` expands but `'$VAR'` does not.