shell-scripting

# Shell Scripting

## Purpose

Provides patterns and best practices for writing maintainable shell scripts with error handling, argument parsing, and portability considerations. Covers POSIX sh vs Bash decision-making, parameter expansion, integration with common utilities (jq, yq, awk), and testing with ShellCheck and Bats.

## When to Use This Skill

Use shell scripting when:
- Orchestrating existing command-line tools and system utilities
- Writing CI/CD pipeline scripts (GitHub Actions, GitLab CI)
- Creating container entrypoints and initialization scripts
- Automating system administration tasks (backups, log rotation)
- Building development tooling (build scripts, test runners)

Consider Python/Go instead when:
- Complex business logic or data structures required
- Cross-platform GUI needed
- Heavy API integration (REST, gRPC)
- Script exceeds 200 lines with significant logic complexity

## POSIX sh vs Bash

**Use POSIX sh (#!/bin/sh) when:**
- Maximum portability required (Linux, macOS, BSD, Alpine)
- Minimal container images needed
- Embedded systems or unknown target environments

**Use Bash (#!/bin/bash) when:**
- Controlled environment (specific OS, container)
- Arrays or associative arrays needed
- Advanced parameter expansion beneficial
- Process substitution `<(cmd)` useful

For detailed comparison and testing strategies, see `references/portability-guide.md`.

## Essential Error Handling

### Fail-Fast Pattern

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

# -e: Exit on error
# -u: Exit on undefined variable
# -o pipefail: Pipeline fails if any command fails
```

Use for production automation, CI/CD scripts, and critical operations.

### Explicit Exit Code Checking

```bash
#!/bin/bash

if ! command_that_might_fail; then
    echo "Error: Command failed" >&2
    exit 1
fi
```

Use for custom error messages and interactive scripts.

### Trap Handlers for Cleanup

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

TEMP_FILE=$(mktemp)

cleanup() {
    rm -f "$TEMP_FILE"
}