cpifdiff

#!/bin/sh
#=======================================================================
#	$Id: cpifdiff,v 1.4 2017/03/01 14:17:12 chah Exp $
#	Copy ifile (or standard input if omitted) to ofile if they differ
#=======================================================================
progname=`basename $0`
usage() {
    echo "Usage: $progname [-l logmsg] [ifile] ofile" >&2
    exit 2
}

tmp=`mktemp -t cpifdiff.XXXXX`

trap 'trap 0; rm -f $tmp; exit 1' 1 2 15

fail () {
    echo $progname: "$@" >&2
    rm -f $tmp
    exit 1
}

opt_log=n
while getopts "l:" opt; do
    case $opt in
    l)
	opt_log=y
	logmsg=$OPTARG
	;;
    *)	
	usage
    	;;
    esac
done
shift `expr $OPTIND - 1`
test $# -ge 1 -a $# -le 2  ||  usage

#---	'-' as first argument means standard input
test $# -eq 2 -a "$1" = "-"  &&  shift

#---	if standard input, copy to temporary file
if [ $# -eq 1 ]; then
    cat > $tmp  ||  fail "Failed to copy standard input"
    ifile=$tmp
    ofile=$1
else
    ifile=$1
    ofile=$2
fi

#---	Test input file access
test -f "$ifile"  ||  fail "$ifile: No such file"
test -r "$ifile"  ||  fail "$ifile: Permission denied"

#---	If the output file does not exist, create it
if [ ! -f "$ofile" ]; then
    touch "$ofile"  ||  fail "$ofile: Permission denied"
    cp $ifile $ofile  ||  fail "Failed writing $ofile"
    if [ "$opt_log" = "y" ]; then
	echo "$logmsg"
    fi
#---	Otherwise, compare with existing file
else
    test -r "$ofile" -a -w "$ofile"  ||  fail "$ofile: Permission denied"
    if not cmp -s "$ifile" "$ofile"; then
	cp "$ifile" "$ofile"  ||  fail "Failed writing $ofile"
	if [ "$opt_log" = "y" ]; then
	    echo "$logmsg"
	fi
    fi
fi

rm -f $tmp

exit 0