Exit status 127 in pipelines with set -e and pipefail

Direct answer

pipefail never overrides the set -e exception for && (or ||) lists. A failing command anywhere in such a list is exempt from triggering set -e regardless of whether pipefail is enabled.

Does ((i++)) from zero cause a script exit under set -euo pipefail?

Yes. The arithmetic command (( )) returns exit status 1 when the evaluated expression yields 0. When i starts at 0, ((i++)) evaluates to 0, so (( )) returns 1. Because this command is not placed inside an if, while, until, &&, ||, or prefixed with !, it is not exempt from set -e. Hence the script aborts before any following statements run. Adding pipefail does not change this outcome.

Interaction of a non‑zero return from command_not_found_handle with set -e and pipefail

When Bash cannot find a command, it invokes command_not_found_handle (if defined). The handler’s return value becomes the exit status of the missing command (commonly 127). If that command appears outside set -e’s exception contexts, the non‑zero status triggers an abort exactly as any other failing command would. pipefail only affects the status of pipelines; it does not alter the handling of simple commands or the set -e exception list.

Verification steps (safe, scoped)

1. Test that pipefail does not override the && exception:

   bash -c 'set -e; set -o pipefail; false | true && echo ok; echo survived'
   

   The script prints both ok and survived, showing the pipeline failure did not abort.
2. Confirm that ((i++)) aborts under strict mode:

   bash -c 'set -euo pipefail; i=0; ((i++)); echo survived'
   

   No output appears; the script exits before the echo.
3. Show that a missing command’s handler return triggers abort when not exempt:

   bash -c 'set -e; command_not_found_handle() { return 127; }; nosuchcmd; echo survived'
   

   Again, no survived is printed.

Recommendation

For reliable error detection in Bash 4.4+, combine set -euo pipefail with explicit checks at decision points (e.g., cmd || { echo "failed"; exit 1; }). Relying solely on set -e’s implicit aborts is fragile due to its many context‑sensitive exceptions.