new_tab.sh

#!/bin/sh

CWD=`pwd`
case $1 in
/*)
  DIR=$1
  ;;
*)
  DIR=$CWD/$1
  ;;
esac

osascript<<END
set thePath to "$DIR"
set thePath to (POSIX path of thePath as string)
tell application "Terminal"
  tell application "System Events" to tell process "Terminal" to keystroke "t" using command down
  do script with command "cd " & quoted form of thePath in front window
end tell
END

new_term.sh

#!/bin/sh

CWD=`pwd`
case $1 in
/*)
  DIR=$1
  ;;
*)
  DIR=$CWD/$1
  ;;
esac

osascript<<END
set thePath to "$DIR"
set thePath to (POSIX path of thePath as string)
tell application "Terminal"
  do script with command "cd " & quoted form of thePath
end tell
END