Agent Skills: [CODING_BASH]

[CODING_BASH]

All code follows these governing principles:

• [FUNCTIONAL] — immutable locals, pure functions, dispatch tables, and tightly bounded mutable shell state
• [POLYMORPHIC] — one parser, one dispatcher, one logger; extend via table entries not code branches
• [PRODUCTION_HARDENED] — ERR traps, atomic I/O, signal forwarding, cleanup registries, version gating
• [FORK_MINIMAL] — printf -v, $(<file), EPOCHSECONDS, fork-free ${ }, BASH_MONOSECONDS timing, mapfile over subshell patterns
• [ECOSYSTEM_FIRST] — rg/fd/jq/sd/choose/mlr over sed/grep/find/cut when available
• [EXECUTABLE_DOCTRINE] — examples and templates must pass syntax, ShellCheck, and their own self-tests

[01]-[ROUTING]

[FOUNDATION]: every task loads bash-scripting-guide.md — primitives, strict mode, expansion, arrays.

[TASK_ROUTED]: load only when the task matches.

• [01]-VERSION_FEATURES: runtime features, fork-free substitution, version gating
• [02]-VARIABLE_FEATURES: call stacks, namerefs, traps, process lifecycle, runtime variables
• [03]-ARRAY_OPERATIONS: set algebra, structural transforms, higher-order traversal
• [04]-STRING_OPERATIONS: transform pipelines, regex extraction, codecs, templates
• [05]-FILE_OPERATIONS: atomic writes, FD multiplexing, directory traversal
• [06]-SCRIPT_PATTERNS: arg parsing, help, ERR traps, parallel, retry
• [07]-BASH_LOGGING: structured logging, CI integration, tracing
• [08]-BASH_TESTING: bats-core suites, coverage, hypothesis PBT
• [09]-BASH_PORTABILITY: cross-shell compat, containers, POSIX
• [10]-TEXT_PROCESSING_GUIDE: rg/awk/sd/jq/yq/mlr tool selection
• [11]-VALIDATION: ShellCheck codes, static analysis, CI

[EXAMPLES]: read the one matching the target archetype before writing.

• [01]-CLI_TOOL: two-dimensional verb:resource dispatch CLI
• [02]-DATA_PIPELINE: file processing with jq pipelines, accumulation
• [03]-SERVICE_WRAPPER: container entrypoint, signal dispatch, coproc

[TEMPLATE]: scaffold a new script from standard.template.md — strict-mode header, logging, cleanup stack, traps, flag parsing, self-test.

[02]-[PARADIGM]

• [IMMUTABILITY]: local -r for every non-mutating function local, readonly for module-level constants
• [FROZEN_ARGS]: Mutable state only for argument parsing — frozen via readonly in _main before core logic
• [DISPATCH_TABLES]: declare -Ar for O(1) command routing and two-dimensional verb:resource dispatch
• [DATA_TABLES]: declare -Ar also tables option metadata, validation rules, log-level gating, and env contracts (regex per var)
• [CASE_RESERVED]: case/esac reserved exclusively for glob/regex pattern matching — never conditional routing
• [PURE_FUNCTIONS]: Input via positional parameters, output via stdout or nameref (local -n); no global reads except readonly constants
• [SIDE_EFFECTS]: Side effects isolated to _main, trap handlers, cleanup registries, and explicitly marked shell boundary loops
• [METADATA_DRIVEN_HELP]: _OPT_META with short|long|desc|VALUE_NAME|default entries generates _usage. One entry + one case branch per option
• [MIDDLEWARE_COMPOSITION]: _use() registers middleware into _MIDDLEWARE; _run_with_middleware() executes the chain before handler dispatch
• [PARSE_PHASES]: Argument parsing composes subcommand dispatch (O(1) table lookup), flag parsing (case/esac), positional collection
• [EXPRESSION_OVER_STATEMENT]: ${var:-default} over if-empty, ${var:?message} over assert-nonempty, (( expr )) over test, expansion over forks
• [FORK_ELIMINATION]: printf -v var '%(%F %T)T' -1 over $(date), $(<file) over $(cat file), EPOCHSECONDS/EPOCHREALTIME over $(date +%s), mapfile over while read, BASH_REMATCH over grep -oP
• [ATOMIC_IO]: Every file write goes mktemp + write + mv — rename is atomic on the same filesystem; umask 077 before mktemp for sensitive data
• [DYNAMIC_FD]: Dynamic FDs via exec {fd}>file

[03]-[CONVENTIONS]

[ECOSYSTEM_TOOL_SELECTION]: richer ecosystem tools own each available operation:

| [INDEX] | [TASK] | [PREFERRED] | [FALLBACK] | [NEVER] | | :-----: | :--------------- | :---------- | :----------------- | :-------------------- | | [01] | File search | fd | find | ls -R | | [02] | Content search | rg | grep -rn | find -exec grep | | [03] | JSON | jq | python3 -c | sed/awk on JSON | | [04] | YAML | yq eval | python3 -c | sed on YAML | | [05] | CSV/TSV | mlr | awk -F | cut for multi-field | | [06] | Stream edit | sd | sed | awk for simple sub | | [07] | Column select | choose | awk '{print $N}' | cut -d | | [08] | Interactive JSON | jnv | jq | -- |

[SELECTION_RULES]:

• Probe availability via command -v before use; fall back gracefully
• rg and fd own repo-aware searches through default .gitignore integration
• jq is mandatory for JSON — never parse JSON with sed/awk/grep
• mlr handles format conversion (CSV to JSON, TSV to JSON) natively
• Pipeline preference: single awk program over chained grep | sed | cut

[04]-[CONTRACTS]

[VARIABLE_DISCIPLINE]:

• local -r for all non-mutating function locals. readonly for all module-level constants.
• Mutable state (parsed args, log level) declared at module level, frozen via readonly in _main before core logic.
• declare -Ar for all dispatch tables, option metadata, and lookup maps.
• local -n (nameref) for passing arrays to functions — never eval or indirect expansion.
• Nameref return channels: a scalar-returning function takes the result var last (_ext "$item" key), writing via printf -v "$2" or local -n.
• Callers pass a name, never $(). Multi-return via multiple namerefs (_project_meta "$slug" name created).
• Naming: UPPER_SNAKE for constants/env, lower_snake for locals/functions, _ prefix for internal functions.

[CONTROL_FLOW]:

• case/esac for pattern matching (globs, regexes) only — never for if/elif-style routing.
• declare -Ar dispatch tables for command routing: "${_DISPATCH[${cmd}]}" "${args[@]}". Nest for subdomains: _CONFIG_SUBCMDS, _INIT_SUBCMDS.
• [[ ]] over [ ]. (( )) for arithmetic. &&/|| for short-circuit.
• mapfile -t / readarray -d '' over while read loops for collection.
• Streaming consumers may use while IFS= read -r with a comment naming the stream boundary.
• Ternary via arithmetic: (( condition )) && action1 || action2 or ${var:+if_set}${var:-if_unset}.
• Bounded concurrency: wait -n -p finished_pid with job-count gate (( ${#jobs[@]} >= MAX_JOBS )) — see _run_pool pattern in examples.
• Shell reality exceptions are explicit: option parsing uses case; cleanup stacks may use a static eval template over shell-quoted commands.
• Bounded counters and polling loops may mutate when the mutation is the resource protocol.

[ERROR_HANDLING]:

• set -Eeuo pipefail + shopt -s inherit_errexit in every script. No exceptions.
• ERR trap with BASH_COMMAND, BASH_LINENO, FUNCNAME context. Stack trace for multi-level call chains.
• _CLEANUP_STACK LIFO registry invoked by EXIT trap. _CLEANING guard prevents re-entrant execution on cascading signals.
• Exit codes: 0=success, 1=general error, 2=usage error. Custom codes in EX_* constants.
• _die() for fatal errors (log + exit). _die_usage() for argument errors (log + hint + exit 2).
• Timing: BASH_MONOSECONDS for elapsed-time durations — monotonic, immune to NTP drift, zero forks.
• EPOCHREALTIME only for absolute timestamps and microsecond benchmarks (_bench() shape: (end_s - start_s) * 1000000 + 10#end_us - 10#start_us).
• Version-safe BASH_MONOSECONDS / EPOCHREALTIME fallback dispatch: references/version-features.md.

[LOGGING_ARCHITECTURE]:

• declare -Ar _LOG_EMIT=([json]=_log_json [text]=_log_text_emit) tables the emitters.
• Format resolves once at startup: readonly _LOG_EMITTER="${_LOG_EMIT[${LOG_FORMAT:-text}]}".
• _log() gates on _LOG_LEVELS numeric threshold, then dispatches via "${_LOG_EMITTER}" — zero branching per call.
• JSON emitter: jq -nc --arg for injection-safe serialization with EPOCHREALTIME microsecond timestamps and optional W3C trace context fields.
• FUNCNAME offset accounts for _info -> _log -> _LOG_EMITTER call chain depth (typically FUNCNAME[3], BASH_LINENO[2]).

[SURFACE]:

• _ prefix for all internal functions. Public surface = _main entry point only.
• One dispatch table per concern — extend by adding entries, not code branches.
• No utility/helper files — colocate all logic in the script. source only for test frameworks.
• --self-test flag runs embedded smoke tests and exits — validates dispatch tables, config parsing, and key pure functions.
• ~350 LOC scrutiny threshold — investigate compression via dispatch tables and awk programs, not file splitting.

[RESOURCES]:

• Temporary files: mktemp + _register_cleanup "rm -f -- $(printf '%q' "${tmp}")" or an equivalent static quoted cleanup template.
• Work directories: mktemp -d with SRANDOM in path for uniqueness.
• Signal forwarding for PID 1: trap TERM/INT, kill -"${sig}" "${_CHILD_PID}", exit with signal code (143/130). Guard on (( _CHILD_PID > 0 )).
• BASH_TRAPSIG carries the signal number, so one unified handler routes every signal through a dispatch table.
• GLOBSORT controls glob ordering (e.g., -mtime for newest-first file discovery).
• Retry: _retry_exec max delay max_delay cmd... — backoff delay=$(( delay * 2 > max_delay ? max_delay : delay * 2 )) with SRANDOM jitter.
• Env contracts: declare -Ar _ENV_CONTRACT=([VAR]='^regex$') validated at startup — dispatch table over env vars, regex per key.
• Health endpoint: socat TCP-LISTEN:${port},reuseaddr,fork SYSTEM:"printf 'HTTP/1.1 200 OK\r\n...'" backgrounded with cleanup registration.
• W3C tracing: parse TRACEPARENT via BASH_REMATCH, generate via printf -v TRACE_ID '%08x%08x%08x%08x' "${SRANDOM}"..., export for children.

[05]-[ANTI_PATTERNS]

[STATE_VIOLATIONS]:

• MUTABLE STATE: let/global mutation outside declare -g config loading. Use local -r/readonly; freeze parsed args in _main.
• FORK IN HOT PATH: $(date), $(cat file), $(wc -l < file) in loops. Use printf -v, $(<file), EPOCHSECONDS, mapfile.

[CONTROL_FLOW_VIOLATIONS]:

• IMPERATIVE DISPATCH: if/elif/else chain for command routing. Use declare -Ar dispatch table + O(1) lookup.
• WHILE-READ COLLECTION: while IFS= read -r line loop to build arrays. Use mapfile -t arr < <(cmd).
• UNMARKED STREAM LOOP: while read without a streaming-boundary comment. Streaming consumers are valid; collection loops are not.
• NAKED WRITE: Direct > or >> for output files. Use mktemp + mv atomic pattern.

[SAFETY_VIOLATIONS]:

• HARDCODED FD: exec 3>file with literal FD numbers. Use exec {fd}>file for safe dynamic allocation.
• UNQUOTED EXPANSION: $var without quotes. Always "${var}" — exceptions only in (( )) arithmetic.
• EVAL INJECTION: eval "$user_string" with untrusted input. Use declare -Ar dispatch or case/esac pattern match.
• ECHO OVER PRINTF: echo -e/echo -n for formatted output. Use printf — portable, no ambiguity, format strings.

[ORGANIZATION_VIOLATIONS]:

• UTILITY EXTRACTION: lib/utils.sh, common.sh helper files. Colocate all logic in the script.
• RANDOM OVER SRANDOM: $RANDOM for security-relevant randomness (temp names, jitter, tokens). Use $SRANDOM (cryptographic entropy).

[06]-[VALIDATION_GATE]

• Required: bash -n script.sh (syntax check), ShellCheck clean (static analysis).
• Required for executable examples: run --self-test when present.
• Reject completion when strict mode, readonly discipline, ShellCheck compliance, or example self-tests are not satisfied.

[07]-[FIRST_CLASS_TOOLS]

| [INDEX] | [TOOL] | [PROVIDES] | | :-----: | :----------- | :----------------------------------------- | | [01] | bash | Shell runtime, builtins, ${ } | | [02] | shellcheck | Static analysis, SC codes | | [03] | bats-core | Test framework, TAP output | | [04] | kcov | Coverage instrumentation | | [05] | rg | Content search, .gitignore-aware | | [06] | fd | File search, .gitignore-aware | | [07] | jq | JSON processing, streaming, trim, skip | | [08] | yq | YAML processing | | [09] | mlr | CSV/TSV/JSON format transforms | | [10] | sd | Stream editing (sed replacement) | | [11] | choose | Column selection (cut replacement) |