Less dimwitted shell required

Question

On my Ubuntu system, /usr/bin/ssh-copy-id contains a curious piece of code at the top. It appears to check that /bin/sh is a "sane shell". If it's not, it tries to re-run the script with ksh. If that fails, it throws up its hands and displays an error message.

What exactly is it checking for? In particular, what does if false ^ printf do, and why is it only triggered only in old shells? Did ancient shells use to have an XOR operator, or what?

#!/bin/sh
# ...
# check that we have something mildly sane as our shell, or try to find something better
if false ^ printf "%s: WARNING: ancient shell, hunting for a more modern one... " "$0"
then
  SANE_SH=${SANE_SH:-/usr/bin/ksh}
  if printf 'true ^ false\n' | "$SANE_SH"
  then
    printf "'%s' seems viable.\n" "$SANE_SH"
    exec "$SANE_SH" "$0" "$@"
  else
    cat <<-EOF
        oh dear.

          If you have a more recent shell available, that supports \$(...) etc.
          please try setting the environment variable SANE_SH to the path of that
          shell, and then retry running this script. If that works, please report
          a bug describing your setup, and the shell you used to make it work.

        EOF
    printf "%s: ERROR: Less dimwitted shell required.\n" "$0"
    exit 1
  fi
fi

Answer 1

Original Bourne supported ^ as the pipe operator. This was dropped in the Korn shell (from which the POSIX sh spec derived), and is thus a feature available in Bourne but not in POSIX sh.

Thus, this code tests for pre-POSIX Bourne shells.

Answer 2

This part is there for the script to work on Solaris 10 and older where /bin/sh is not POSIX compliant. Note that the latter is not a bug as POSIX doesn't specify what sh path should be.

/bin/sh (and /sbin/sh) on Solaris 10 are probably the only remaining shells in current OSes still supporting this form of pipe which appeared in the original Bourne shell.