- `set -e` is a tripwire, not error handling: it stays quiet inside `if`, inside `&&` chains, and behind `local x=$(cmd)`, so the failure that would corrupt state still needs its own guard.
- A required variable gets `${VAR:?why}` at its first use, not a validation block at the top: one expansion, names the variable, exits nonzero, nothing to keep in sync.
- Feed a `while read` loop by redirection, never by piping into it. The piped loop runs in a subshell, so every variable it set is gone by `done`.
- Pass arguments as `"$@"` or an array, never a space-joined string. The string reads fine until a path has a space, and every fix from there is quoting piled on quoting.
- Loop over a glob, never over `ls` output. An unmatched pattern comes through literally, so one existence check inside the loop is the whole guard.