It's easy to overwrite (clobber) existing files when you are redirecting output to a file. To prevent this from happening you need to set the noclobber shell variable by adding a line to your shell start-up file.

Depending on which shell you are using this is:

Shell Line to add

C shell set noclobber

TC shell set noclobber

Korn shell set -o noclobber

BASH set noclobber