Bash set -e and subshells: error propagation during system upgrades

Goal

Develop an upgrade script that aborts immediately when any command fails, preventing a half‑upgraded system.

Constraints

The script must rely on Bash’s documented error‑handling options: set -e (errexit), set -o pipefail for pipelines, and set -o errtrace to propagate traps into subshells. It should also use trap on EXIT for cleanup.

Unresolved behavior

While POSIX defines set -e for simple commands, its effect on commands executed inside subshells invoked by functions or command substitutions is unspecified. Additionally, the interaction between trap handlers and subshell failures is unclear: does an EXIT trap run if a subshell exits with a non‑zero status before the parent script terminates?

Questions

• Does set -e automatically propagate to subshells spawned by functions or command substitutions in Bash 5.1?
• When a subshell fails, is the parent’s EXIT trap guaranteed to execute before the script exits?
• What minimal combination of set options ensures consistent error propagation and cleanup across Bash 4.x and 5.x?