#!/bin/bash
# owrite v0.1
# Purpose: safely use a file for output redirection and command input
# simultaneously
# Written and Copyright 2001 by Bill Jonas, bill@billjonas.com
# Credit is given to Brian W. Kernighan and Robert Pike for _The UNIX
# Programming Environment_.  This program is based on an example I remember
# from that book.
# This program is licensed under the GNU GPL.
# Please see http://www.gnu.org/copyleft/gpl.html for details.
# Please note:
# Because the program is licensed free of charge, there is NO WARRANTY for
# the program, to the extent permitted by applicable law. Except when
# otherwise stated in writing the copyright holders and/or other parties
# provide the program "AS IS" WITHOUT WARRANTY OF ANY KIND, either
# expressed or implied including, but not limited to, the implied
# warranties of merchantability and fitness for a particular purpose. The
# entire risk as to the quality and performance of the program is with you.
# Should the program prove defective, you assume the cost of all necessary
# servicing, repair or correction.

print_help()
{
  echo "Usage: $0 <file> <command and args>"
  exit 1
}

case "$1" in
  -h|--help)
    print_help
    exit 1
    ;;
esac

if [ -z "$2" ] # Must have at least two args, file and command
then
  print_help
  exit 1
fi

# Try to use the tempfile command if available,
# otherwise, fall back to an inferior method.
if `type tempfile >/dev/null 2>&1`
then
  TEMPFILE=`tempfile`
else
  TEMPFILE=/tmp/owrite.$$
fi
FILE="$1"
shift

# Clean up if we're interrupted
trap "rm $TEMPFILE" HUP INT QUIT ABRT ALRM TERM STOP TSTP PWR
eval "$@ $FILE >$TEMPFILE"
RETVAL=$?  # How did the command exit?
if [ $RETVAL -gt 0 ]
then # Something bad happened.
  echo "Operation aborted.  Return value from command was $RETVAL"
  rm $TEMPFILE
  exit 1
fi

# Go for broke.  We don't want to be interrupted, if at
# all possible, and only a SIGKILL should do that now.
trap '' HUP INT QUIT ABRT ALRM TERM STOP TSTP PWR
mv "$TEMPFILE" "$FILE"