shell - Subshell created using (...) behaves differently from bash -c '...' -


i use set -e inside (...) on command line doesn't exit subshell when false command executed.

code 1:

( set -e; echo "$bashpid: start"; false; echo "foobar"; date; ) &&    echo "$bashpid: ok" || echo "$bashpid: nope" 9136: start foobar wed, apr 29, 2015  7:14:24 pm 7292: ok

however if use bash -c subshell creation behaves expect.

code 2:

bash -c 'set -e; echo "$bashpid: start"; false; echo "foobar"; date;' &&   echo "$bashpid: ok" || echo "$bashpid: nope" 7880: start 7292: nope

interestingly if remove && , || part after subshell (...) behaves fine.

code 3:

( set -e; echo "$bashpid: start"; false; echo "foobar"; date; ) 5940: start

so conclusions getting are:

  1. (...) behaves differently bash -c
  2. (...) && false behaves differently (...) , changes behavior of subshell well.

am making obvious mistake in interpreting strange (at least me) behavior?

the behavior of set -e && intentional.

it explicitly doesn't trigger when command part of && sequence.

from posix spec:

-e

when option on, when command fails (for of reasons listed in consequences of shell errors or returning exit status greater zero), shell shall exit following exceptions: failure of individual command in multi-command pipeline shall not cause shell exit. failure of pipeline shall considered.

the -e setting shall ignored when executing compound list following while, until, if, or elif reserved word, pipeline beginning ! reserved word, or command of and-or list other last.

if exit status of compound command other subshell command result of failure while -e being ignored, -e shall not apply command.

this requirement applies shell environment , each subshell environment separately. example, in:

set -e; (false; echo one) | cat; echo 2

the false command causes subshell exit without executing echo one; however, echo 2 executed because exit status of pipeline (false; echo one) | cat zero.

i assume difference here whether shell knows fact.

(...) presumably because current shell executes , bash -c presumably doesn't because that's external process (or something).

that last bit speculation , mind sub-shell behavior here entirely unhelpful , makes set -e unreliable (which why many people suggest avoiding it).

though seems might provide sort of "escape hatch" reliability problem if explicitly run shell doesn't have issue composition of scripts.


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