ERR trap not called when setting readonly variable

Question

Script test.sh:

set -euo pipefail

function _trap_ext
{
    echo '_trap_ext'
}
function _trap_error
{
    echo '_trap_error'
}
trap "_trap_ext" EXIT
trap "_trap_error" ERR

readonly foo='bar'
foo='bar'

echo 'foobar'

Output:

./test.sh: line 14: foo: readonly variable
_trap_ext

The script terminates at line 14 because of the error (-e option) but the _trap_error function is not called. Why not?

GNU bash, version 4.1.2(1)-release (x86_64-unknown-linux-gnu), 4.2.45(1)-release (i586-suse-linux-gnu)

Answer 1

This sounds like it might be a bug. From the man pages:

-e
Exit immediately if a pipeline (which may consist of a single simple command), a list, or a compound command (see SHELL above), exits with a non-zero status.
...
A trap on ERR, if set, is executed before the shell exits. This option applies to the shell environment and each subshell environment separately (see COMMAND ENVIRONMENT above), and may cause subshells to exit before executing all the commands in the subshell.

From what the man pages say, it should execute the ERR trap. You can test that it works as expected in other situations by inserting a false before the foo='bar' statement.

It also appears that bash doesn't call the ERR trap on syntax errors either, so it may be that attempting to overwrite a readonly variable falls into a similar category of errors which skip the ERR trap. However, that explanation is pure speculation.