EXIT trap in implicit vs. explicit pipeline subshell

[Miroslav Koskar]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='x86_64' 
-DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='x86_64-unknown-linux-gnu' 
-DCONF_VENDOR='unknown' -DLOCALEDIR='/usr/share/locale' -DPACKAGE='bash' 
-DSHELL -DHAVE_CONFIG_H   -I.  -I. -I./include -I./lib  -D_FORTIFY_SOURCE=2 
-march=x86-64 -mtune=generic -O2 -pipe -fstack-protector-strong 
--param=ssp-buffer-size=4 
-DDEFAULT_PATH_VALUE='/usr/local/sbin:/usr/local/bin:/usr/bin' 
-DSTANDARD_UTILS_PATH='/usr/bin' -DSYS_BASHRC='/etc/bash.bashrc' 
-DSYS_BASH_LOGOUT='/etc/bash.bash_logout'
uname output: Linux mirci 4.0.4-2-ARCH #1 SMP PREEMPT Fri May 22 03:05:23 UTC 
2015 x86_64 GNU/Linux
Machine Type: x86_64-unknown-linux-gnu

Bash Version: 4.3
Patch Level: 39
Release Status: release

Description:

    For lack of better terminology I call subshell introduced by pipeline
    as implicit when written like this:

        echo 'This will run in subshell' | cat

    and explicit when written like this:

        ( echo 'This will run in subhshell' ) | cat

    One would expect above snippets of code to be equivalent in function,
    in second one parentheses are unnecessary, because pipeline will run
    every part in subshell (unless "lastpipe" is set, which is of no interest
    for us here).

    I've observed that it's actually not true for traps, namely EXIT trap.
    If EXIT trap is set in parent (main) shell, then this is inherited in
    subshell in implicit case. It can be observed by running `trap` or `trap -p`
    in subshell. This inherited trap is actually not executed on exit from
    subshell. In explicit case, EXIT trap is rightly not inherited.

    Going further, if subshell in implicit case will set it's own EXIT trap,
    that one will not be executed, as oppose to explicit case where it executes.

    As of writing this I've encountered also difference between the implicit 
case
    for simple command and a group command. See 2) in next section. I find that
    strange also for the reason that $BASH_SUBSHELL evaluates to 0 when clearly
    it is running inside subshell since $$ is different from $BASHPID.

Repeat-By:

    1) implicit case with simple command

    #!/bin/bash
    trap 'echo EXIT-$$.$BASHPID.$BASH_SUBSHELL' EXIT
    trap 'echo EXIT-$$.$BASHPID.$BASH_SUBSHELL' EXIT | cat

    Output:
    EXIT-8843.8843.0

    2) implicit case with group command

    #!/bin/bash
    trap 'echo EXIT-$$.$BASHPID.$BASH_SUBSHELL' EXIT
    { trap 'echo EXIT-$$.$BASHPID.$BASH_SUBSHELL' EXIT; } | cat

    Output:
    EXIT-9136.9137.0
    EXIT-9136.9136.0

    3) explicit case

    #!/bin/bash
    trap 'echo EXIT-$$.$BASHPID.$BASH_SUBSHELL' EXIT
    ( trap 'echo EXIT-$$.$BASHPID.$BASH_SUBSHELL' EXIT ) | cat

    Output:
    EXIT-9189.9190.1
    EXIT-9189.9189.0

Fix:

    I haven't had a time to look at source but I expect that the fact that
    EXIT trap from parent (main) shell is inherited in subshell (as listed
    by `trap` — not shown in examples above) for 1) and 2) but rightly not
    for 3) is indicative and probably right place to start looking. Maybe
    fixing that would fix the rest. But that wouldn't explain difference
    between 1) and 2) though.

I don't consider myself a Bash expert but, when I've raised this in #bash
on freenode we've concluded that it should be reported so I gather there
is something to it.

Best Regards,
Miro

-- 
Miroslav Koskar
http://mkoskar.com/

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