On Sun, Aug 28, 2022 at 12:46:17AM +1000, David wrote: > I think there might be one remaining aspect still mysterious, so there > might be yet another factor beyond the FIVE you identified ... > > The fact that this statement, first shown by Tim, does NOT error: > > $ ( bash -uc : ; : )
(inside an ssh session) This is related to how bash optimizes subshell commands when it can. unicorn:~$ (bash -uc :) /etc/bash.bashrc: line 7: PS1: unbound variable unicorn:~$ (bash -uc : ; :) unicorn:~$ In the first one, there's just one command inside the subshell, so bash can replace one of the fork() calls, and treat it like (exec bash -uc :) In the second one, there's a second command, so bash can't perform that same optimization. (exec bash -uc : ; :) would not work the same as (bash -uc : ; :) This optimization affects whether the shell_level is reset: unicorn:~$ (bash -c 'declare -p SHLVL') declare -x SHLVL="1" unicorn:~$ (bash -c 'declare -p SHLVL'; :) declare -x SHLVL="2" Under normal conditions, this would have no visible effect. But because of the other factors involved, we can see a difference.
