#!/bin/sh
set -euo pipefail

usage() {
	cat <<-'EOF'
		Usage:
		  ifok [-o OUTFILE] COMMAND...
	EOF
}

OUTFILE='/dev/stdout'
while getopts 'o:' flag; do
	case "$flag" in
		(o)
			OUTFILE="$OPTARG"
			;;
		(*)
			usage >&2
			exit 2
			;;
	esac
done
shift $((OPTIND - 1))


if [ -w "$(dirname -- "$OUTFILE")" ]; then
	TEMPFILE="$OUTFILE.$(uuidx).tmp"
else
	TEMPFILE="$(mkstemp)"
fi
trap 'rm -f "$TEMPFILE"' EXIT

STATUS=0
"$@" > "$TEMPFILE" || STATUS=$?

if [ "$STATUS" = 0 ]; then
	mv "$TEMPFILE" "$OUTFILE" 2>/dev/null || cp "$TEMPFILE" "$OUTFILE"
fi

exit "$STATUS"