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) What is documented is that when that variable is set, then bash enters the POSIX mode. Now, that variable is understood as "enter POSIX mode" for many GNU utilities and several non-GNU utilities (like those using the GNU getopt API) not just bash. When bash enters the POSIX mode while the allexport option is on, like in: - sh -a - sh -o allexport - bash -o posix -o allexport... - env SHELLOPTS=posix,allexport bash - set -o posix # within bash -a Then that POSIXLY_CORRECT ends up in the environment of all commands started by that bash thereafter, which means they start behaving differently. IMO, bash's posix mode should only affect bash and its builtin commands, not the other commands started within bash. That a #! /bin/sh -a script should make all commands within behave POSIXly is unexpected and means a script will behave differently if /bin/sh is bashed on bash or another shell. Maybe bash should not set POSIXLY_CORRECT, or not export it when it sets it by itself. If it sets it, the documentation should reflect it. Maybe more generally, I think with allexport, bash should not export the variables that it sets itself. One uses that option for the variables they set to be available in other scripts called within. It doesn't make sense to export things like BASH_VERSION, BASHPID, OPTIND. Per POSIX, "the export attribute shall be set for each variable to which an assignment is performed (see Variable Assignment)", though it goes on about getopts and read even though those are not listed under "Variable Assignment". In set -a getopts o VAR printenv VAR Some shells export VAR, few export OPTIND. few export VAR in set -a for VAR in 1; do printenv VAR; done (even though here I'd say it could be desirable). All export it in "read VAR". In any case, I think the POSIX specification should be clarified. -- Stephane
