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+# Bash Code Rules
+
+Applies to: `**/*.sh`, `**/*.bash`, and extensionless files with a `sh`/`bash` shebang
+
+Shell-specific style and structure. Pairs with `bash-testing.md` for tests and
+the generic `verification.md` / `commits.md` rules. When in doubt, defer to
+[ShellCheck](https://www.shellcheck.net/wiki/) (every SCxxxx code has a wiki
+page explaining the fix) and Google's
+[Shell Style Guide](https://google.github.io/styleguide/shellguide.html).
+
+## ShellCheck Is the Gate, Not a Suggestion
+
+The bundle's PostToolUse hook runs `shellcheck` on every edited shell file and
+blocks on a violation; the pre-commit hook re-checks staged files. ShellCheck
+catches the bugs that define shell: unquoted expansions that word-split, unset
+variables, `[ ]` pitfalls, masked exit codes. Fix the finding rather than
+silence it. When a warning is a genuine false positive, disable it narrowly with
+a `# shellcheck disable=SCxxxx` directive on the line above and a comment saying
+why, never a file-wide blanket disable.
+
+## The Header: Strict Mode
+
+Every script starts with `#!/usr/bin/env bash` and `set -euo pipefail`:
+
+- `-e` exits on an unhandled non-zero command. Handle the expected-failure cases
+  explicitly (`cmd || true`, an `if`, a `case`) so the exit is a real error.
+- `-u` treats an unset variable as an error. Use `"${VAR:-default}"` for the
+  ones that are legitimately optional.
+- `-o pipefail` makes a pipeline fail if any stage fails, not just the last.
+
+A script meant to be *sourced* (a library) skips `set -e` — it would change the
+caller's shell. Libraries guard their own commands instead.
+
+## Quote Everything
+
+- Double-quote every expansion: `"$var"`, `"$@"`, `"${arr[@]}"`,
+  `"$(command)"`. Unquoted is the single largest source of shell bugs — a path
+  with a space becomes two arguments.
+- `"$@"` (quoted) passes arguments through untouched; `$*` and unquoted `$@`
+  word-split. Use `"$@"` unless you specifically want the joined string.
+- Loop over arrays and `find -print0 | while IFS= read -r -d ''`, never over
+  unquoted command substitution or `ls` output.
+
+## Test, Compare, Branch
+
+- Use `[[ ]]` for tests, not `[ ]` / `test`. `[[ ]]` doesn't word-split its
+  operands, supports `&&`/`||`/`=~`, and has fewer quoting traps.
+- Arithmetic goes in `(( ))` or `$(( ))`, not `[ ]` with `-eq`.
+- `$(command)`, never backticks — nests cleanly and reads better.
+- Prefer `printf` over `echo` for anything but a fixed literal string;
+  `echo` mangles values that start with `-` or contain backslashes.
+
+## Functions and Scope
+
+- Declare function-local variables with `local`. A bare assignment in a
+  function writes a global and leaks across calls.
+- `local var; var="$(cmd)"` on two lines when you need the command's exit
+  status: `local var="$(cmd)"` masks `cmd`'s exit code behind `local`'s.
+- Keep functions focused. A function that fetches, parses, and writes is three
+  functions; the test difficulty in `bash-testing.md` is the tell.
+- Put `main "$@"` at the bottom for a script with more than a couple of
+  functions, so definition order doesn't dictate execution order.
+
+## Robustness
+
+- `trap 'rm -rf "$tmpdir"' EXIT` right after creating a temp resource, so
+  cleanup runs on every exit path including errors.
+- Make a temp file or dir with `mktemp` / `mktemp -d`, never a fixed
+  `/tmp/name` (race + collision).
+- Check that a required command exists before the work: `command -v jq
+  >/dev/null || { echo "jq required" >&2; exit 1; }`.
+- Never parse `ls` output and don't `cat` a file into a pipe you could read
+  directly. Glob, or use `find`, or read the file in place.
+
+## What Not to Do
+
+- Don't leave an expansion unquoted to "save a quote" — quote it.
+- Don't use `[ ]` when `[[ ]]` is available, or backticks when `$()` is.
+- Don't silence a ShellCheck warning file-wide to clear it; fix it or disable
+  the one code with a reason.
+- Don't refactor surrounding code while fixing a bug — keep the diff scoped.
+- Don't commit credentials or API keys — the pre-commit hook catches common
+  patterns but isn't a substitute for care.