sponge as a shell script for non-interactive use, i.e., where we do not need to check the second file

   #!/bin/sh
   test $# = 1||exec echo usage: $0 file;
   test -f $1||exec echo $1 does not exist;
   test ! -f $1.tmp||exec echo $1.tmp exists;
   cat > $1.tmp;
   mv $1.tmp $1;