Shell Scripts

Writing Shell Scripts

Overview

Write shell code with an explicit portability target first, then apply strict quoting and a bounded ShellCheck remediation loop. Default to Bash readability and safety; switch to POSIX-only mode when the user asks for strict portability.

When to Use

• Creating or refactoring Bash scripts.
• Reviewing shell snippets for correctness and safety.
• Standardizing shebangs, quoting, variable expansion style, and test syntax.
• Deciding between POSIX-compliant syntax and Bash-specific features.
• Improving compatibility with zsh environments.

When NOT to use:

• The task is strictly fish, powershell, or Windows batch.
• The user explicitly wants pure POSIX sh and no Bash features (use POSIX mode from references/compatibility-matrix.md).

Invocation Notice

• Tell the user when this skill is running: shell-scripts. Skip the notice when the user asked for the skill by name or slash command; spelling and spacing need not match. A phrase from this skill's own trigger list is not a name — naming the work is not naming the skill.

Workflow

1. Select the target mode from references/compatibility-matrix.md: POSIX strict, Bash-first, or Bash-with-zsh-compatibility.
2. Start from assets/script-template.sh or pull focused snippets from:
   • assets/usage-template.txt
   • assets/logging-template.sh
   • assets/getopts-template.sh
3. Apply style defaults:
   • Shebang per target mode.
   • Prefer ${VAR} expansion form for clarity.
   • Quote expansions unless intentionally relying on shell splitting/pattern behavior.
   • In Bash mode, prefer arrays for argument vectors and list handling; in POSIX mode, avoid arrays.
   • Use [[ ... ]] for Bash conditionals; use [ ... ] when POSIX compatibility is required.
   • For command execution, verify resolution with command -v / type -a when shadowing is possible.
4. Validate syntax and lint:
   • bash -n path/to/script.sh
   • shellcheck -x path/to/script.sh (if available)
5. ShellCheck remediation budget:
   • Do at most two fix rounds.
   • Round 1: fix correctness/safety and high-confidence issues.
   • Round 2: re-run and fix remaining practical issues.
   • Stop after round 2 and report remaining findings with rationale.
6. For advanced logic and portability traps, load:
   • references/advanced-patterns.md
   • references/command-resolution-and-os-portability.md
   • references/quoting-and-expansion.md
   • references/tests-and-conditionals.md
   • references/shellcheck-workflow.md

Output

• A script (or patch) with explicit target shell assumptions.
• Consistent quoting/expansion style and conditional style.
• ShellCheck findings reduced within the two-pass budget, with unresolved items documented.

References

• references/compatibility-matrix.md
• references/quoting-and-expansion.md
• references/tests-and-conditionals.md
• references/shellcheck-workflow.md
• references/advanced-patterns.md
• references/command-resolution-and-os-portability.md
• references/shellcheck-codes.md