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zsh typically runs the final command in a pipeline in the main shell
instead of a subshell. However, POSIX specifies that all commands in a
pipeline run in a subshell, but permits zsh's behavior as an extension.
The default /bin/sh implementations on various Linux distros and the
BSDs always use a subshell for all components of a pipeline.
Since zsh may be used as /bin/sh in some cases (such as macOS Catalina),
it makes sense to have the common sh behavior when emulating sh, so do
that by checking for being the final item of a multi-item pipeline and
creating a subshell in that case.
From the comment above execpline(), we know the following:
last1 is a flag that this command is the last command in a shell that
is about to exit, so we can exec instead of forking. It gets passed
all the way down to execcmd() which actually makes the decision. A 0
is always passed if the command is not the last in the pipeline. […]
If last1 is zero but the command is at the end of a pipeline, we pass
2 down to execcmd().
So there are three cases to consider in this code:
• last1 is 0, which means we are not at the end of a pipeline, in which
case we should not change behavior.
• last1 is 1, which means we are effectively running in a subshell,
because nothing that happens due to the exec is going to affect the
actual shell, since it will have been replaced. So there is nothing
to do here.
• last1 is 2, which means our command is at the end of the pipeline, so
in sh mode we should create a subshell by forking.
input is nonzero if the input to this process is a pipe that we've
opened. At the end of a multi-stage pipeline, it will necessarily be
nonzero.
Note that several of the tests may appear bizarre, since most developers
do not place useless variable assignments directly at the end of a
pipeline. However, as the function tests demonstrate, there are cases
where assignments may occur when a shell function is used at the end of
a command. The remaining assignment tests simply test additional cases,
such as the use of local, that would otherwise be untested.
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