Adding -E made a silent ERR trap name the failing mkdir on line 5

Errexit's own exemptions are why this bites inside functions and not at the top level. Bash counts a command as checked when it is the condition of an `if`, when it sits left of `&&` or `||`, or under `!`. The exemption also covers everything inside a function called from one of those positions ^[10]. Wrap a careful helper in `if helper; then` and every line inside it loses errexit, and the output will not tell you ^[11].

An ERR trap looks like the place to recover the diagnostic, because it fires on the same conditions that make `-e` exit ^[8]. By default it is not inherited by functions, command substitutions or subshells ^[6]. In the nine-line test ^[23], the `mkdir` sat one level down inside the function. Errexit killed the script. The trap, registered in the top-level shell, missed the failure inside the function's scope ^[9]. `-E`, errtrace, is the option that makes the trap follow into functions and subshells ^[7].

The handler the author now runs takes the exit status first: `local code=$?` on the first line, before anything else overwrites it. Then one echo carries `BASH_COMMAND`, `BASH_LINENO[0]` and that code to stderr ^[14]. The dev.to post calls `-E` the flag missing from nearly every strict-mode line on the internet ^[22], and the author wrote: "If you take one thing from this, take the E." ^[15]

The other two letters each prevent a specific failure. `-u` turns an unset variable into an error instead of an empty string. When one place spells it `BUILDDIR`, that is the difference between `rm -rf "$BUILD_DIR/"` and `rm -rf /` ^[12]. `pipefail` is what stops `curl ... | jq ...` from reporting success when curl dies and jq parses the empty result ^[13].

The second silent spot is `local` itself. `local` is a command in its own right. The status bash sees is `local` succeeding at declaring the variable, so the command substitution's failure is discarded before anything looks at it ^[17]. Declare on one line, assign on the next, and the failure is visible again ^[18]. This is the bug behind ShellCheck's masked-return-value warning ^[19].

Both results come from one machine and one build ^[3]. For the `-E` change to alter your logs, your failures have to happen inside a function, a command substitution or a subshell ^[6]. The author says that is where most real failures happen ^[25]. A script whose commands all run at top level already gets the command and line from its trap, with or without the letter ^[6].

One behaviour to expect before you wire alerts to the handler: with `-E` set, a failure inside an explicit `( subshell )` fires the trap twice, once in the subshell and once in the parent as the non-zero status propagates. The author calls that expected and suggests handling it with a guard flag or by moving the work out of the subshell ^[20]. The EXIT trap is the separate piece. It runs on every termination, and that is what stops a script leaving a half-written file for the next stage to load as if it were complete ^[21].