.switch_shell

#!/bin/sh
#
# Adam's .switch_shell
#
# Try switch shell if we're interactive, aiming for safety, but
# not so much that we end up hogging memory.
#
# $Id$
#
# Usage:
#
# . /path/to/.switch_shell [-d] [ /path/to/new_shell [ <new shell options> ]
#    -d turns on debugging

# FIXME: move shell chooser code into .ns

# Give hook a .local suffix for consistency in case we need to exclude local stuff
PREFERRED_SH_HOOK_FILE=$HOME/.preferred_shell.local

# only switch if we're interactive
case "$-" in
  *i*)
    : ;; # OK
  *)
    echo "Warning: tried to switch a non-interactive shell"
    return 1
    ;;
esac

switch_shell_debug () {
  [ -n "$debug" ] || return 0
  if [ -n "$_have_sh_load_status" ]; then
    sh_load_status "$*\n"
  else
    echo "$*"
  fi
}

switch_shell_safely () {
  # we do this rather than exec() just in case $myshell fails to run.
  switch_shell_debug "Switching to $myshell safely ..."

  # if "$myshell" $myshell_args; then
  #   switch_shell_debug "$myshell exited OK; exiting parent shell."
  #   exit
  # else
  #   switch_shell_debug "$myshell had exit code $?, back to pid $$"
  # fi

  # Very cute trick from Bart Schaefer which checks in advance whether
  # we can switch shell.  This is better than the above because it
  # is not dependent on the subshell exiting with 0 status.
  eval `$myshell -f -c "echo exec $myshell" || echo :` '$myshell_args'
}

switch_shell_dangerously () {
  switch_shell_debug "Switching to $myshell dangerously ..."
  exec "$myshell" $myshell_args
}

switch_shell () {
  if [ ! -x $myshell ]; then
    switch_shell_debug "$myshell not executable; aborting switch."
    return
  fi

  if [ -n "$NO_SWITCH" ]; then
    switch_shell_debug 'Shell switching disabled by $NO_SWITCH; aborting.'
    return
  fi

  export SHELL_ARGS="$myshell_args" # no other way of retrieving these?

  switch_shell_debug "Switching to $myshell, args: $myshell_args"

  case "$SHLVL" in
   "") # unknown, be careful
       switch_shell_safely $myshell_args
       ;;
    1) # login shell, be careful
       switch_shell_safely $myshell_args
       ;;
    force_danger|*) # other shell, be risky and save memory
       switch_shell_dangerously $myshell_args
#       switch_shell_safely $myshell_args
       ;;
  esac
}

debug=
if [ "$1" = '-d' ]; then
  debug=yes
  shift
fi

# Sensible default shell to switch to.
myshell=`which zsh` 2>/dev/null
#myshell=
myshell_args=

if [ -n "$1" ]; then
  # shell to switch to specified via ARGV
  myshell="$1"
  shift
  myshell_args="$@"
else
  # get preferred shell 
  if [ -e $PREFERRED_SH_HOOK_FILE ]; then
    if grep '=' $PREFERRED_SH_HOOK_FILE >/dev/null 2>&1; then
      . $PREFERRED_SH_HOOK_FILE
    else
      echo "$PREFERRED_SH_HOOK_FILE doesn't look like a shell script; aborting shell switch" >&2
      return 0
    fi
  fi
fi

if [ -n "$ZDOTDIR" ]; then
  . "$ZDOTDIR/.shared_env"
fi

if sh_load_status "testing if we have sh_load_status" >/dev/null 2>&1; then
  _have_sh_load_status=1
else
  _have_sh_load_status=
  #echo "Warning: no sh_load_status" >&2
fi

if [ -z "$myshell" ]; then
  echo "No shell preference found; not switching shell." >&2
  return 0
fi

switch_shell $myshell_args