Bash Coding Guidance

This skill adds portable Bash implementation, refactoring, and review guidance.

Adjacent Skills

This skill provides portable Bash engineering principles. Compose with:

• Workflow: thinking (planning), recursive-thinking (stress-testing),

security (threat modeling)

• Domain overlays: project-core-dev (repo-specific build/test commands),

project-platform-diagnose (environment-sensitive diagnosis)

When Not to Lean on This Skill

• non-shell work
• POSIX sh portability work where Bash-only features are not allowed

(adapt the guidance to the repo's POSIX-shell contract instead of importing Bash-only defaults)

• large data processing jobs that should realistically move to Python, awk, or

another language with stronger structure

Implementation Workflow

1. Read the touched scripts, entrypoints, call sites, and nearby docs before

editing.

1. Infer the intended contract from current usage text, flags, tests, and

environment assumptions. Ask only when multiple plausible script behaviors would change semantics.

1. Keep the contract narrow: inputs, outputs, exit codes, environment

dependencies, filesystem effects, and external tool requirements should be explicit.

1. Implement with defensive mode, safe quoting, arrays for argv construction,

functions for meaningful substeps, and cleanup traps for temporary state.

1. Add or update shell tests when the repo has them; otherwise add the smallest

reproducible validation path you can run directly.

1. Run the narrowest relevant formatter, linter, and script tests the repo

supports.

Refactoring Workflow

Use this instead of the default implementation workflow when the task is primarily cleanup or restructuring:

1. Capture current behavior, flags, environment assumptions, platform

dependencies, and failure modes.

1. Break the refactor into small slices that preserve behavior.
2. Replace copy-pasted command assembly, hidden globals, unsafe loops, and

tangled control flow one step at a time.

1. Keep tests or runnable validation passing after each slice; add

characterization coverage first when behavior is unclear.

1. Stop when the script is easier to read, safer to invoke, and easier to

debug.

Review Workflow

When reviewing (not implementing), skip the implementation workflow and use this instead:

1. Read the change in full before commenting.
2. Identify findings, ordered by severity: Critical > Important >

Suggestion.

1. Prioritize quoting and word-splitting bugs, globbing hazards, accidental

masking of failing commands, trap and cleanup bugs, unsafe temp-file handling, destructive command risks, portability mismatches, environment assumptions, and missing tests.

1. State findings with concrete evidence and the likely consequence.

Bash Rules

First tier - causes bugs

• Start executable Bash scripts with #!/usr/bin/env bash unless the repo has

a stricter shebang convention

• Use set -Eeuo pipefail unless the script has a documented reason to manage

failures differently; scope exceptions narrowly

• Quote expansions by default: "$var", "${arr[@]}", and "$(cmd)"
• Use arrays for argument vectors; do not build command lines with string

concatenation

• Distinguish stdout data from stderr diagnostics so callers can compose the

script safely

• Check exit statuses deliberately; do not rely on pipelines, subshells, or

command substitutions without understanding how failures propagate

• Clean up temp files and directories with trap when the script allocates

them

• Treat $IFS, globs, current working directory, and environment variables as

boundary conditions, not stable ambient assumptions

Second tier - prevents mistakes

• Prefer functions for meaningful units of behavior; keep top-level script flow

readable

• Use local inside functions unless a variable is intentionally shared
• Prefer [[ ... ]] for Bash conditionals and case for multi-branch string

matching

• Prefer printf over echo when escaping, flags, or portability ambiguity

matter

• Use command substitution $() instead of backticks
• Name flags, env vars, and functions for what they do; shell scripts become

unreadable quickly when names get vague

• Keep shellcheck findings at zero in repo-owned code unless the script has a

documented exception

Defensive patterns

• Consider shopt -s inherit_errexit when the Bash version and repo contract

allow it and command-substitution failures must propagate cleanly

• Validate required env vars explicitly with : "${VAR:?message}"
• Detect missing external tools up front with command -v tool >/dev/null 2>&1
• Use mktemp for temp files and directories; never hand-roll temp paths in

shared locations

• Prefer dry-run modes and idempotent behavior before destructive or expensive

operations

• Use trap for cleanup and, when helpful, targeted ERR reporting with line

or function context

Command and process discipline

• Pass user-controlled values as individual argv elements, not through eval

or re-parsed strings

• Avoid eval unless the script is explicitly a shell metaprogramming tool and

the risk is justified

• Prefer explicit path resolution and file existence checks before destructive

operations

• When calling external tools, preserve argument boundaries and treat tool exit

codes as part of the contract

• Be deliberate about cd; either scope it to a subshell or restore the prior

directory clearly

• End option parsing with -- when forwarding user arguments to external

commands

• If the script spawns background jobs, make ownership explicit: track PIDs,

aggregate wait results deliberately, and forward or handle termination signals rather than abandoning children

• Avoid casual parallel destructive work; if concurrency matters, define the

locking, isolation, or idempotency rule up front

• If concurrent invocations can interfere with each other, use an explicit lock

strategy such as flock or a documented equivalent instead of hoping paths or timing stay unique

Input, output, and contract design

• Provide --help or usage text for non-trivial scripts
• Make required env vars, positional args, flags, and side effects explicit

near the top of the file

• Exit with non-zero status on real failure and reserve zero for success
• Print machine-consumable output in a stable format when other tools are meant

to parse it

• Avoid silent fallbacks on missing tools or files unless the script is

explicitly best-effort

• Use stable exit-code meanings when the script is likely to be called by other

automation

Safe iteration and file handling

• Prefer while IFS= read -r line; do ...; done over loops that split on

whitespace implicitly

• Use NUL-safe patterns for filenames from find, git, or similar tools:

-print0, xargs -0, or read -r -d ''

• Use readarray or mapfile when populating arrays from command output in

Bash-specific scripts

• Avoid for f in $(...) for filenames or untrusted data; it is usually a bug
• Prefer built-ins and parameter expansion over unnecessary subprocesses when

they make the script clearer and safer

Portability and platform fit

• Use Bash-specific features only when the shebang and repo contract allow them
• If the script must run on multiple platforms, check GNU vs. BSD tool

differences before adding flags with inconsistent behavior

• If platform behavior matters, detect it explicitly instead of assuming Linux
• Prefer the repo's existing helper scripts and path conventions over ad hoc

temp locations or duplicated wrappers

• Move non-trivial parsing, JSON handling, or data shaping to a better-suited

language when shell stops being the clearest tool

Style, testing, and tooling

• Prefer long, readable option names for user-facing interfaces when the repo

does not already prescribe a short-only style

• Keep functions small enough to understand locally; split when one function

starts owning parsing, validation, execution, and reporting all at once

• Run shellcheck and shfmt where the repo uses them
• Prefer Bats or the repo's existing shell-test framework for non-trivial

scripts

• Document required tools and minimum Bash features when the script depends on

them

Decision Heuristics

Use these when the right choice is not obvious:

• Language fit: if the task needs nested data structures, complex parsing,

or large in-memory transforms, shell may be the wrong tool.

• Quoting pressure: if command construction becomes hard to reason about,

switch to arrays or redesign the interface before adding more flags.

• Failure visibility: if set -e behavior is unclear in a construct, make

the check explicit rather than assuming the shell will fail the right way.

• Process ownership: if the script backgrounds work, define who waits,

cleans up, and reports failures before adding more parallelism.

• Portability pressure: if a script relies on GNU-only flags, a Bash 5.x

feature, or OS-specific tools, document and validate that boundary instead of hiding it.

• Repo conventions: if the repo has established patterns for shebangs,

strict mode, shellcheck, shfmt, or helper libraries, follow them unless they create a correctness or safety problem.

• Narrowness vs. quality: implement the narrowest change that solves the

problem. When narrowness conflicts with correctness or safety, prefer correctness. When it conflicts with style alone, prefer narrowness unless the task is explicitly a cleanup.

• Refactor boundary: outside explicit refactor work, fix at most one small

adjacent issue while you are in the file.

• Abstraction threshold: three similar command sequences or repeated flag

parsing pain is a pattern; before extracting, check whether a function, a helper script, or a small move to another language is the simpler move.

• External-tool boundary: if a script depends on jq, awk, sed, or

platform-specific tooling, treat that dependency as part of the user-facing contract.

Validation

A change is done when:

• shellcheck or the repo's equivalent linter reports no new findings
• shell formatting is run when the repo has a formatter
• changed --help, argument parsing, or top-level script startup paths are

smoke-tested before deeper functional validation

• existing shell tests pass
• new or changed behavior has test coverage, or the lack of coverage is called

out with a concrete reason

• non-trivial scripts have a direct smoke path such as --help or a minimal

fixture invocation

• destructive or environment-mutating paths were verified against a safe test

fixture rather than assumed correct

• portability-sensitive changes were tested on the affected platform or the

remaining platform risk was called out explicitly

• review findings at Critical and Important severity are addressed

Examples

• Review this Bash deploy script for quoting, traps, and rollback safety
• Refactor this repo automation script to use arrays, safe temp files, and better argument parsing