trap ERR for Custom Error Handling

#!/bin/bash
(
    trap 'echo "an error was trapped"' ERR
    set -e
    false
    echo "unreached"
) || echo "subshell exited after the ERR trap ran"

#!/bin/bash
(
    trap 'echo "error occurred near line $LINENO"' ERR
    set -e
    true
    false
) || echo "handled"

#!/bin/bash
(
    set -E -e
    trap 'echo "caught failure inside a function"' ERR
    fail_inside() {
        false
    }
    fail_inside
    echo "unreached"
) || echo "subshell stopped after the function's failure was trapped"

1. Assuming trap ERR fires for commands exempted from set -e too (like an if condition); it shares the same exemptions.
2. Forgetting trap ERR only fires reliably when set -e semantics are in effect for that command; behavior can otherwise be inconsistent across contexts like functions unless set -E (errtrace) is also used.
3. Writing an ERR handler that itself might fail, causing confusing recursive or missed error handling.

• trap handler ERR runs handler whenever a command fails, in the same contexts where set -e would trigger.
• set -o errtrace (set -E) makes the ERR trap also fire inside functions and subshells, not just top-level commands.
• An ERR trap is commonly used to log diagnostic information (like the failing line number via $LINENO) before the script exits.
• Combining trap ERR with an EXIT trap lets you separate error-specific logging from general cleanup that should always run.