On 2020-06-05 at 10:21:41, Mikael Magnusson wrote: > On 6/5/20, brian m. carlson <sandals@xxxxxxxxxxxxxxxxxxxx> wrote: > > zsh typically runs the final command in a pipeline in the main shell > > instead of a subshell. However, POSIX requires that all commands in a > > pipeline run in a subshell, but permits zsh's behavior as an extension. > > What POSIX actually says is: > "each command of a multi-command pipeline is in a subshell > environment; as an extension, however, any or all commands in a > pipeline may be executed in the current environment" > Ie, it does not say "shall", so it doesn't require a subshell all, in > fact it explicitly does permit not using one as you also say. The > patch is possibly useful (seems unlikely to me), but to say it is > required by POSIX is not true. If someone depends on every command in > a pipeline being a subshell, they should fix their code, for example > by adding ( ) around it (the command(s) or the whole pipeline). POSIX makes a declarative statement about the behavior of a pipeline. It is true that it doesn't explicitly use the word "shall" in this case, since such a statement would explicitly prohibit the inclusion of an extension at all and make it explicitly non-conforming. What POSIX does say is that one “shall define an environment in which an application can be run with the behavior specified by POSIX.1-2017.” I'm proposing that "zsh --emulate sh" implement the POSIX behavior for that reason. I will tell you that as a practical matter, nobody writing code for sh expects the last command not to be run in a subshell and consequently lots of code is practically broken in this case with zsh as /bin/sh. The Git Project is very fastidious about writing portable shell, as is Debian, and I can tell you from experience that both are broken with zsh as sh with the current behavior, even if they should not have made that assumption. zsh is a very popular interactive shell, and allowing it to be used as a portable sh on systems where the system sh is less capable would be really beneficial. I would also like to see macOS users who decide to use zsh as /bin/sh have a good experience with existing code that overwhelmingly does make this assumption. If your objection is to the wording, I'm happy to revise it to remove the word "requires", but I do think this provides a lot of benefits for the sh scripting case while not impacting users who are expecting different behavior for the zsh case. -- brian m. carlson: Houston, Texas, US OpenPGP: https://keybase.io/bk2204
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