login shell check is not working in .profile

Ubuntu 20.04, 5.8.0-63-generic, gnome-shell, gdm3

My login shell is set to /bin/sh

and I have this line in my $HOME/.profile file:

shopt -q login_shell > /dev/null 2>&1 || export SHELL=/bin/zsh

Now from my understanding export SHELL=/bin/zsh should not be executed on login right? because it is a login shell so shopt -q login_shell > /dev/null 2>&1 is True.

But I am getting $SHELL as /bin/zsh after login.

Actually setting $SHELL is not my concern, I want to use shopt -q login_shell > /dev/null 2>&1 in .zshrc logic. I need to get that working to selectively run things in non-login shell.

Can't figure out what's going wrong. Need Help!

Edit:

Shopt is not a /bin/sh command. But even changing login shell to /bin/bash doesn't help.

shopt is not a valid command in the POSIX sh shell (nor zsh, which uses setopt/unsetopt), so will error out - making your test return non-zero unconditionally:

$ bash -lc 'shopt -q login_shell; echo $?'

0

but

$ sh -lc 'shopt -q login_shell; echo $?'
sh: 1: shopt: not found
127

and

$ zsh -lc 'shopt -q login_shell; echo $?'
zsh:1: command not found: shopt
127

Since ~/.profile (as well as /etc/profile, plus the files in /etc/profile.d that it sources) may be read by other shells, best practice is to keep it POSIX complient. AFAIK the POSIX way to check for a login shell is to test whether $0 begins with a - character ex.

case $0 in 
  -*) echo "login shell"
   ;; 
   *) echo "non-login shell"
   ;;
esac