From f7a417388c73e7cfefb8e93fa8beba193fb1dd1f Mon Sep 17 00:00:00 2001 From: "brian m. carlson" Date: Sun, 3 Jan 2021 18:23:00 -0600 Subject: 47794: exec: run final pipeline command in a subshell in sh mode MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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. --- README | 4 ++++ 1 file changed, 4 insertions(+) (limited to 'README') diff --git a/README b/README index f47adfe7c..5e3b8cc59 100644 --- a/README +++ b/README @@ -95,6 +95,10 @@ right thing, but this should be viewed as an unsupported hack.) The XTRACE option is now disabled while running user-defined completion widgets. See NEWS. +emulate sh: When zsh emulates sh, the final command in a pipeline is now run in +a subshell. This differs from the behavior in the native (zsh) mode, but is +consistent with most other sh implementations. + Incompatibilities between 5.7.1 and 5.8 --------------------------------------- -- cgit 1.4.1