This specific behavior is still found in "GNU bash, Version
4.4.12(1)-release" from Debian 9 Stretch, but a new shell option
enables the more consistent behavior of other shells.
Subshells for command substitutions do not automatically inherit
the option "errexit". This must be enabled by another option
"inherit_errexit". The manual page for the bash explains it like:
inherit_errexit
If set, command substitution inherits the value of the
errexit option, instead of unsetting it in the subshell
environment. This option is enabled when posix mode is
enabled.
Using this option gives the expected result:
hb1@debian:~$ bash -c 'set -e ; z=$(false;echo foo) ; echo $z'
foo
hb1@debian:~$ echo $?
0
hb1@debian:~$ bash -c 'set -e ; shopt -s inherit_errexit ; z=$(false;echo foo)
; echo $z'
hb1@debian:~$ echo $?
1
As mentioned in the manual, the POSIX mode also gives the expected
result:
hb1@debian:~$ bash --posix -c 'set -e ; z=$(false;echo foo) ; echo $z'
hb1@debian:~$ echo $?
1
The Bash Reference Manual explains the difference:
42. Enabling POSIX mode has the effect of setting the inherit_errexit
option, so subshells spawned to execute command substitutions
inherit the value of the -e option from the parent shell. When
the inherit_errexit option is not enabled, Bash clears the -e
option in such subshells.
The Bash Reference Manual can be installed with the Debian package
"bash-doc". Then see:
file:///usr/share/doc/bash/bashref.html#Bash-POSIX-Mode
The option inherit_errexit is new in bash-4.4. The file
/usr/share/doc/bash/NEWS.gz lists it under the first topic:
This is a terse description of the new features added to bash-4.4
since the release of bash-4.3. [...]
ii. inherit_errexit: a new `shopt' option that, when set, causes
command substitutions to inherit the -e option. By default,
those subshells disable -e. It's enabled as part of turning on
posix mode.
--
Regards,
Hartmut Buhrmester
