如何“设置”与subshel​​ls一起工作？ [英] How does "set -e" work with subshells?

问题描述

我想知道是否 set -e 通过subshel​​ls传播（即，subshel​​l是否继承 -e 它的父母），所以我做了一些实验。我发现一些奇怪的结果，我无法解释。

I was wondering whether set -e propagates through subshells (i.e. does a subshell inherit the -e setting of its parent), so I made some experiments. I found some strange results that I can't explain.

首先，这里有一些基本的测试。他们返回我的期望。

First, here are some basic tests. They return what I expect.

( true; false )         # 1
( false; true )         # 0
( set -e; false; true ) # 1

现在我尝试了如果我把在我的子壳内的一个子壳。此表达式返回1，这表示它传播。

Now I tried what happens if I put a subshell within my subshell. This expression returns 1, which suggests that it propagates.

( set -e; ( false; true ) )

然后我尝试了这些表达式。我希望他们返回1，但是我发现他们返回0。

Then I tried these expressions. I expected them to return 1, but I found that they return 0.

( set -e; ( true; false ); true )
( set -e; ( set -e; false; true ); true )

为什么呢？在这两种情况下，内部子shell返回1，无论 set -e 是否传播（正如我在开始检查）。外部子shell具有 set -e ，这意味着内部子shell退出后应该失败，但不会。有人可以解释一下吗？

Why? In both cases, the inner subshell returns 1, whether set -e propagates or not (as I checked in the beginning). The outer subshell has set -e, which means that it should fail after the inner subshell exits, but it does not. Can someone explain this?

推荐答案

在 bash之前 4， set -e 似乎只会导致shell退出，如果一个简单的命令有一个非零退出（强调我的）：

Prior to bash 4, set -e appears to only cause the shell to exit if a simple command has a non-zero exit (emphasis mine):

-e如果简单命令（参见上面的SHELL GRAMMAR）退出，则立即退出。

-e Exit immediately if a simple command (see SHELL GRAMMAR above) exits with a non-zero status.

在 bash 4（可能4.1，我没有4.0检查）， -e 被扩展到更复杂的命令：

In bash 4 (possibly 4.1, I don't have a 4.0 to check), the effect of -e was extended to more complicated commands:

-e如果管道（可能包含单个简单命令），一个包含在括号中的subshel​​l
命令，或者作为命令列表的一部分执行的命令列表中的一个命令列表（通过上面的SHELL GRAMMAR）（包括上面的SHELL GRAMMAR）退出，零状态。

-e Exit immediately if a pipeline (which may consist of a single simple command), a subshell com- mand enclosed in parentheses, or one of the commands executed as part of a command list enclosed by braces (see SHELL GRAMMAR above) exits with a non-zero status.

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