From man bash
:
set -u
- Treat unset variables and parameters other than the special parameters
"@"
and "*"
as an error when performing parameter expansion. If expansion is attempted on an unset variable or parameter, the shell prints an error message, and, if not -i
nteractive, exits with a nonzero status.
POSIX states that, in the event of an expansion error, a non-interactive shell shall exit when the expansion is associated with either a shell special builtin (which is a distinction bash
regularly ignores anyway, and so maybe is irrelevant) or any other utility besides.
- Consequences of Shell Errors:
- An expansion error is one that occurs when the shell expansions defined in Word Expansions are carried out (for example,
"${x!y}"
, because !
is not a valid operator); an implementation may treat these as syntax errors if it is able to detect them during tokenization, rather than during expansion.
- [A]n interactive shell shall write a diagnostic message to standard error without exiting.
Also from man bash
:
trap ... ERR
- If a sigspec is ERR, the command arg is executed whenever a pipeline (which may consist of a single simple command), a list, or a compound command returns a non-zero exit status, subject to the following conditions:
- The ERR trap is not executed if the failed command is part of the command list immediately following a
while
or until
keyword...
- ...part of the test in an
if
statement...
- ...part of a command executed in a
&&
or ||
list except the command following the final &&
or ||
...
- ...any command in a pipeline but the last...
- ...or if the command's return value is being inverted using
!
.
- These are the same conditions obeyed by the errexit
-e
option.
Note above that the ERR trap is all about the evaluation of some other command's return. But when an expansion error occurs, there is no command run to return anything. In your example, echo
never happens - because while the shell evaluates and expands its arguments it encounters an -u
nset variable, which has been specified by explicit shell option to cause an immediate exit from the current, scripted shell.
And so the EXIT trap, if any, is executed, and the shell exits with a diagnostic message and exit status other than 0 - exactly as it should do.
As for the rc: 0 thing, I expect that is a version specific bug of some kind - probably to do with the two triggers for the EXIT occurring at the same time and the one getting the other's exit code (which should not occur). And anyway, with an up-to-date bash
binary as installed by pacman
:
bash <<\IN
printf "shell options:\t$-\n"
trap 'echo "EXIT (rc: $?)"' EXIT
set -eu
echo ${UNSET_VAR}
IN
I added the first line so you can see that the shell's conditions are those of a scripted shell - it is not interactive. The output is:
shell options: hB
bash: line 4: UNSET_VAR: unbound variable
EXIT (rc: 1)
Here are some relevant notes from recent changelogs:
- Fixed a bug that caused asynchronous commands to not set
$?
correctly.
- Fixed a bug that caused error messages generated by expansion errors in
for
commands to have the wrong line number.
- Fixed a bug that caused SIGINT and SIGQUIT to not be
trap
pable in asynchronous subshell commands.
- Fixed a problem with interrupt handling that caused a second and subsequent SIGINT to be ignored by interactive shells.
- The shell no longer blocks receipt of signals while running
trap
handlers for those signals, and allows most trap
handlers to be run recursively
(running trap
handlers while a trap
handler is executing).
I think it is either the last or the first that is most relevant - or possibly a combination of the two. A trap
handler is by its very nature asynchronous because its whole job is to wait for and handle asynchronous signals. And you trigger two simultaneously with -eu
and $UNSET_VAR
.
And so maybe you should just update, but if you like yourself, you'll do it with a different shell altogether.
bash
broke with the standard and started putting traps in subshells. The trap is supposed to be executed in the same environment from whence came the return, butbash
hasn't done that for quite awhile.set -e
andset -u
are both designed specifically to kill a scripted shell. Using them in conditions which might trigger their application will kill a scripted shell. There's no getting around that, except to not use them, and instead to test for those conditions when they apply in a code sequence. So, basically, you can write good shell-code, or you can useset -eu
.-u
would not trigger the ERR trap (it's an error, so shouldn't it trigger the trap) or the error code is 0 instead of 1. The latter seems to be a bug what has already been fixed in later version, so that's that. But the first part is quite hard to understand if you haven't realized that errors in shell evaluation (parameter expansion) and actual errors in commands seem to be two different things. For the solution, well, as you suggested, I'm now trying to avoid-eu
and check manually when it's necessary.(set -u; : $UNSET_VAR)
and similar. This kind of stuff can be good too - you can drop a lot of&&
's occasionally:(set -e; mkdir dir; cd dir; touch dirfile)
, if you get my drift. It's just that those are controlled contexts - when you set them as global options, you lose control and become controlled. There are usually more efficient solutions, though.