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

#!/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

#!/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

#!/bin/bash
set -euo pipefail

TEMP_FILE=$(mktemp)

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