shell-scripting-fundamentals

# Shell Scripting Fundamentals

Core patterns and best practices for writing robust, maintainable shell scripts.

## Script Structure

Always start scripts with a proper shebang and safety options:

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

# Script description here
```

### Safety Options Explained

- `set -e`: Exit on any command failure
- `set -u`: Error on undefined variables
- `set -o pipefail`: Pipeline fails if any command fails

## Variables

### Declaration and Assignment

```bash
# No spaces around =
name="value"

# readonly for constants
readonly CONFIG_DIR="/etc/myapp"

# local in functions
my_function() {
    local result="computed"
    echo "$result"
}
```

### Always Quote Variables

```bash
# Good - prevents word splitting and glob expansion
echo "$variable"
cp "$source" "$destination"

# Bad - can break on spaces or special characters
echo $variable
cp $source $destination
```

### Default Values

```bash
# Use default if unset
name="${NAME:-default}"

# Use default if unset or empty
name="${NAME:-}"

# Assign default if unset
: "${NAME:=default}"

# Error if unset
: "${REQUIRED_VAR:?Error: REQUIRED_VAR must be set}"
```

## Conditionals

### Test Syntax

```bash
# Modern syntax - preferred
if [[ -f "$file" ]]; then
    echo "File exists"
fi

# String comparison
if [[ "$string" == "value" ]]; then
    echo "Match"
fi

# Numeric comparison
if (( count > 10 )); then
    echo "Greater than 10"
fi

# Regex matching
if [[ "$input" =~ ^[0-9]+$ ]]; then
    echo "Numeric input"
fi
```

### Common Test Operators

| Operator | Description |
|----------|-------------|
| `-f` | File exists and is regular file |
| `-d` | Directory exists |
| `-e` | Path exists |
| `-r` | Readable |
| `-w` | Writable |
| `-x` | Executable |
| `-z` | String is empty |
| `-n` | String is not empty |

## Loops

### For Loops

```bash
# Iterate over list
for item in one two three; do
    echo "$item"
done

# Iterate over files (use glob, not ls)
for file in *.txt; do
    [[ -e