2018-08-15 11:05:06 -0400, Chet Ramey: > On 8/14/18 11:50 AM, Stephane Chazelas wrote: > > Hi, > > > > This is from > > https://unix.stackexchange.com/questions/462333/why-does-a-in-bin-sh-a-affect-sed-and-set-a-doesnt > > (original investigation by Mark Plotnick) > > > > Though not documented, enabling the POSIX mode in bash whether > > with > > > > - bash -o posix > > - sh > > - env SHELLOPTS=posix bash > > - set -o posix # within bash > > > > causes it to set the value of the $POSIXLY_CORRECT shell > > variable to "y" (if it was not already set) > > Yes. This behavior dates from early 1997. It was put in on request so users > could get a posix environment from the shell, since GNU utilities > understand the POSIXLY_CORRECT variable. I could improve the documentation > there, but a 20-plus-year-old feature isn't going to change. [...]
Maybe there was a misunderstanding. It's fine that bash enters POSIX mode when $POSIXLY_CORRECT is set. IOW, it's fine that bash enters POSIX mode when the users request it. The problem I'm trying to raise is about the reverse behaviour: that bash takes upon itself to request POSIX mode of all other utilities when it itself enters POSIX mode, that it sets $POSIXLY_CORRECT when it enters POSIX mode. The problem would show up mostly with #! /bin/sh -a scripts on systems where sh is bash, and where the script relies on non-POSIX behaviours of some GNU utilities. I can't see how that could be seen as a feature, I can't imagine anyone wanting that. If one wants to get a POSIX environment on a GNU system, they would do: export POSIXLY_CORRECT=y (and yes, it's good that bash does honour it) (and yes, it's not a very good interface as that means it can break scripts called within that environment and that rely on non-POSIX behaviour of some utilities, but that's beside the point being made here). If one wants a POSIX shell, they can use #! /bin/sh - Or: #! /usr/bin/env bash set -o posix if they can't rely on /bin/sh being a POSIX shell. But that should not affect the behaviour of all other utilities called within the script. Without "-a", it's OK as the $POSIXLY_CORRECT variable is not exported (it's not very useful that bash sets it (especially considering it's not documented), but at least it's harmless). -- Steohane
