Always Quote Variables and Substitutions

This rule applies to files matching the patterns above.

# Always Quote Variables and Substitutions

## Rule
ALL variable expansions MUST be double-quoted: "${var}", "$(command)", "$@". The only exceptions are inside [[ ]] tests and intentional word splitting with explicit IFS.

## Format
```bash
# Always quote
echo "${variable}"
result="$(some_command)"
command "${array[@]}"
```

## Good Examples
```bash
#!/usr/bin/env bash
set -euo pipefail

# Quoted variables
file_path="${1:?Missing argument}"
echo "Processing: ${file_path}"

# Quoted command substitution
current_date="$(date +%Y-%m-%d)"
file_count="$(find . -name '*.log' | wc -l)"

# Quoted array expansion
args=("--verbose" "--output" "/tmp/result.txt")
my_command "${args[@]}"

# Quoted in conditionals
if [[ "${status}" == "ready" ]]; then
  echo "System is ready"
fi

# Quoted in loops
while IFS= read -r line; do
  echo "Line: ${line}"
done < "${input_file}"
```

## Bad Examples
```bash
# BAD: Unquoted variable — breaks on spaces
file=/tmp/my file.txt
cat $file         # Becomes: cat /tmp/my file.txt (two arguments!)

# BAD: Unquoted glob expansion
dir="*.txt"
echo $dir         # Expands to matching filenames!

# BAD: Unquoted $@ in function
my_func() {
  other_func $@   # Breaks arguments with spaces
}

# BAD: Unquoted command substitution
files=$(ls /tmp)
for f in $files; do  # Word splits on spaces in filenames
  rm $f
done
```

## The One Exception
```bash
# Intentional word splitting with controlled IFS
IFS=',' read -ra parts <<< "${csv_line}"
```

## Enforcement
- ShellCheck SC2086 catches all unquoted variable expansions
- Enable ShellCheck in CI and pre-commit hooks
- Use `${var}` brace syntax consistently for clarity