Bash script that buffers standard output of given input command and enables operations upon it

buf

#!/bin/sh
# buf: Interactive facility for buffer operations
USAGE="$0 (<cmd> [<arg>]*) | -h(elp)
Summary:
  Executes input command and stashes standard output
  in buffer. Interactive prompt (#) supports shell 
  interaction with buffer with supporting builtin's
Interaction:
  :!     Re-run input command and overwrite buffer
  ::     Echo buffer contents to standard out
  :q     Exit
  :<cmd> Run command with no standard input. Useful
         for non-input commands
  <cmd>  Run command with buffer as standard input.
         Output overwrites buffer if exit-code is 
         zero. Otherwise prompted to overwrite
"

# argument check
case $# in
0) echo "${USAGE}" >&2; exit 1 ;;
1) if [ "$1" == "-h" ]; then
     echo "${USAGE}" >&2; exit 0
   fi ;;
esac

# evalulate
stdout=`"$@"`
echo "$stdout"
while :; do
  printf "# " >&2
  read opt
  case $opt in
  :!) stdout=`"$@"`; echo "$@" "$?" >&2 ;;
  ::) echo "$stdout" ;;
  :q) break ;;
  :*) cmd=`echo "$opt" | sed 's/^://g'`;
      $cmd 1>/dev/null 2>&2
      echo "$?" >&2 ;;
  *)  cmd=$opt
      tmp=`eval "echo \"$stdout\" | $cmd"` 2>/dev/tty
      ext=$?
      if [ "$ext" -ne "0" ]; then
        printf "bad exit code ($ext). Overwrite? (y/n): " >&2
        read opt
        case opt in
        y) stdout=$tmp
        esac
      else
        stdout=$tmp
      fi
      echo "$stdout" >&2 ;; 
  esac
done
echo "$stdout"
exit 0