ia-linux-bash-scripting

# Linux Bash Scripting Produce bash scripts that pass `shellcheck --enable=all` and `shfmt -d` with zero warnings. Target: GNU Bash 4.4+ on Linux. No macOS/BSD workarounds, no Windows paths, no POSIX-only restrictions. ## Script Foundation ```bash #!/usr/bin/env bash set -Eeuo pipefail shopt -s inherit_errexit readonly SCRIPT_DIR="$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd -P)" trap 'printf "Error at %s:%d\n" "${BASH_SOURCE[0]}" "$LINENO" >&2' ERR trap 'rm -rf -- "${_tmpdir:-}"' EXIT ``` - `-E` propagates ERR traps into functions - `inherit_errexit` propagates errexit into `$()` command substitutions - Resolve the script's own data files against `SCRIPT_DIR`, never the caller's cwd or `git rev-parse --show-toplevel`. A shared linter invoked from another project's git hook, a cron job, or a wrapper runs with someone else's cwd, so a caller-relative rules path resolves to a file that does not exist: the rule set loads empty, zero violations are found, exit 0. It is a silent no-op, not an error, and running it from inside its own repo passes for the wrong reason. Exercise it once from a scratch directory that is not the script's own tree - Always create temp dirs under the EXIT trap: `_tmpdir=$(mktemp -d)` - Wrap body in `main() { ... }` with source guard: `[[ "${BASH_SOURCE[0]}" == "$0" ]] && main "$@"`; enables sourcing for testing ## Core rules - Quote expansions, use arrays for commands, and never evaluate external data as shell code. - Validate numeric syntax, sign, and application bounds before arithmetic. Convert unsigned digits with `10#` before applying the sign; `10#-08` is invalid. - Keep secrets out of process arguments and tracing. Feed them through stdin and use a JSON-aware encoder. - Check exact exit statuses where “absent” differs from “failed to inspect.” Separate `local` declarations from command substitutions. - Use NUL-delimited file iteration, validate required flag values, and reject conflicting output/target combinations. - For atomic replacement, stage beside the destination; for multi-file activation, switch a single staged release reference. - Preserve unrelated files and report the actual signal or command status after cleanup. - Do not assume Bash options work in `sh`, GNU utility modes behave like syscall modes, or a configured fallback path is usable. ## Task-specific references Read the relevant reference before implementing the matching behavior: - For command execution, external input, numeric conversion, argument parsing, iteration, or subprocess status handling: [input-and-process-safety.md](./references/input-and-process-safety.md). - For file activation, secrets, locking, retries, cleanup, permissions, logging, or restartable automation: [production-patterns.md](./references/production-patterns.md). ## Performance - Parameter expansion over externals: `${path%/*}` not `dirname`, `${path##*/}` not `basename`, `${var//old/new}` not `sed` - `(( ))` over `expr`; `[[ =~ ]]` over `echo | grep` - Cache results: `val=$(cmd)` once, reuse `$val` - `xargs -0 -P "$(nproc)"` for parallel work - `declare -A map` for lookups instead of repeated grep ## Bash 4.4+ / 5.x - `${var@Q}` shell-quoted, `${var@U}` uppercase, `${var@L}` lowercase - `declare -n ref=varname` nameref for indirect access - `wait -n` wait for any background job - `$EPOCHSECONDS`, `$EPOCHREALTIME`: timestamps without forking `date` ## Linux-Specific - GNU coreutils differ from macOS: `sed -i` (no `''` suffix), `grep -P` (PCRE support), `readlink -f` (canonical path) - `timeout 30s cmd` to prevent automation hangs - `sed -i` replaces a symlink with a regular file: it writes a temp file and renames it over the path, so the two copies diverge later with no error. Use `sed -i --follow-symlinks` or edit the target by name, and check `git status --short` for `T` (typechange) after scripted edits ## ShellCheck Run `shellcheck --enable=all script.sh`. Key rules: - **SC2155**: Separate declaration from assignment - **SC2086**: Double-quote variables - **SC2046**: Quote command substitutions - **SC2164**: `cd dir || exit` - **SC2327/SC2328**: Use `${BASH_REMATCH[n]}` not `$n` for regex captures Pre-commit: `shellcheck *.sh && shfmt -i 2 -ci -d *.sh` ## Verify Run `shellcheck --enable=all` and `shfmt -d` with zero warnings before declaring done. Test edge cases: empty input, missing files, spaces in paths. If `shellcheck` or `shfmt` is not installed (`command -v shellcheck` fails), the check was skipped, not passed: report "static analysis not run: shellcheck unavailable" and fall back to `bash -n` for syntax only. A skipped linter is the same silent no-op as the empty rules file above.