#!/bin/sh
##########################
## Resume/create screen sessions intelligently.  Never creates a second session.
## Usage: resume-screen [OPTIONS] [HOST...]
##  -h      This help information
##  -i      Installation help
##  --debug Debug this script
##  HOST    A host to ALWAYS use screen for, including from multiple sources.
##          HOST arguments are regular expressions.  You can also space-delimit.
##          The $screen_host variable can be used in addition to HOST arguments.
## $RESUME_HOSTS  acts just like HOST but hides the list from process lists (ps)
## resume-screen v0.6 Copyright 2008+ by Adam Katz <scriptsATkhopiscom>, GPL 2+
##########################
### INSTALLATION HELP:
###   Add this to your ~/.screenrc file:
###     setenv inscreen 0
###   Call this from the very top of your rc file (~/.bashrc, etc).
###   The rest of this is only needed if you have multiple "screen" binaries.
### 
###   Add this to the very bottom of your rc file (after the $PATH is resolved):
###     which -a screen |grep ^/ |head -n1 >~/.screen.`uname -n`
###   (Note that this might fail if the binary needs a different library path)
###   If you don't have GNU or Debian which, you'll need these lines instead:
###     OLDIFS="$IFS"  IFS=":"
###     for dir in $PATH; do
###       if [ -f "$dir/screen" ] && [ -x "$dir/screen" ]; then
###         echo "$dir/screen" > ~/.screen.`uname -n`
###         break
###       fi
###     done
###     IFS="$OLDIFS"
### 
### For general help, try 'resume-screen -h'
##########################
# Half of this code is for debug, but it remains because I suspect it would be
# useful, e.g. the next time 'who' changes, etc.  Another note on this script:
# Solaris uses jsh which cannot handle ${foo#bar} style substitution.
##########################

gethelp() { sed -e "/$1/!d" -e s/// `which $0 2>/dev/null || echo $0`; exit $2;}

while [ -n "$1" ]; do
  case $1 in
    --debug  ) DEBUGME=true; shift ;;
    -i*|--i* ) gethelp '^### ' ;;
    -h|--he* ) gethelp '^## ' ;;
    -* ) echo "`basename $0`: invalid argument \`$1'" >&2; gethelp '^## ' 1 ;;
    *  ) break ;;
  esac
done

if [ -z "$DEBUGME" ]
  then debug() { true; }
  else debug() { echo "`date +%M%S`: $*"; }
       debug "  Debug enabled for `basename $0`."
fi

# Saved location lets this run before $PATH is set. See INSTALLATION HELP above.
HOST="`uname -n`"
myscreen="`cat ~/.screen.$HOST 2>/dev/null || which screen 2>&1 |grep /screen$`"
debug "  screen='$myscreen'"

# NOTE: 'inscreen' is a custom variable.  See INSTALLATION HELP above.
if [ ! -x "$myscreen" ] || [ -z "$SSH_TTY" ] || [ -n "$inscreen" ]; then
  debug "'$myscreen' isn't executable, not an SSH TTY, or already in screen"
  exit 1
fi

debug "STEP ONE:  try to re-attach a detached screen session"
$myscreen -r >/dev/null 2>&1 && exit 0
# EXIT after we successfully re-attached to a screen session


debug "  No un-attached screen found."

# smoosh all hosts into one regular expression
for arg in $RESUME_HOSTS "$@"
  do screen_host="$screen_host${screen_host:+|}$arg"
done
debug "  HOST regexp='$screen_host'"

if [ -n "$RESUME_HOSTS$1" ] && echo "$HOST" |egrep "$screen_host" >/dev/null
then # if host{
  debug "STEP TWO:  Attach to an in-use screen or create new one"
  if $myscreen -ls 2>/dev/null |grep '(Attached)' >/dev/null # if attached {

    then
      debug "Attached screen found.  Trying to attach..."
      mytty=`echo $SSH_TTY |sed -e 's:^/dev/::' -e 's:/:.:g'`
      me=`id -un`
      screen_client=`who |sed -e "/^$me.*$mytty[^0-9]/!d" -e 's/.*(//'`
      debug "  tty='$mytty' client='$screen_client' user='$me'"
      if [ `who |grep -c "$me.*$screen_client"` = 1 ]; then
        debug "Success, you are not logged on from this SSH client IP."
        echo "Connecting to already-attached session, press CTRL+C to cancel..."
        sleep 3 && $myscreen -x
      else
        debug "Found more than one SSH connection from your IP.  No action."
      fi

    else
      debug "No screens (attached or not) found.  Creating new session..."
      $myscreen

  fi # } end if attached
fi # } end if host