Detecting interactive shell within ksh ENV script

Question

What's the preferred way to determine if a given ksh invocation is running an interactive shell?

I have some commands in an ENV file that I would like to skip for non-interactive ksh invocations (e.g. when executing a shell script).

I've seen suggesting ranging from:

if [[ $- = *i* ]]; then
    # do interactive stuff
fi

...to not even sourcing .kshrc unless the shell is determined to be interactive using this cryptic incantation:

ENVIRON=$HOME/.kshrc                                    export ENVIRON
ENV='${ENVIRON[(_$-=1)+(_=0)-(_$-!=_${-%%*i*})]}'       export ENV

Answer 1

In bash, these two methods are often used inside ~/.bashrc:

• Check if stdin is a tty:

  [ -t 0 ] || return
  

  or

  if [ -t 0 ]; then
      # do interactive stuff
  fi
  

• Check if the prompt ($PS1) is set:

  [ -z "$PS1" ] || return
  

But I don't know how to do that in ksh.