mv: Move file only if destination does not exist

mv -vn file1 file2. This command will do what you want. You can skip -v if you want.

-v makes it verbose - mv will tell you that it moved file if it moves it(useful, since there is possibility that file will not be moved)

-n moves only if file2 does not exist.

Please note however, that this is not POSIX as mentioned by ThomasDickey.

mv -n

From man mv on a GNU system:

-n, --no-clobber
do not overwrite an existing file

On a FreeBSD system:

-n Do not overwrite an existing file. (The -n option overrides any previous -f or -i options.)

if [ ! -e file2 ] && [ ! -L file2 ]
then
    mv file1 file2
# else echo >&2 there is already a file2 file.
fi

Or:

if ! ls -d file2 > /dev/null 2>&1
then
    mv file1 file2
fi

Would only run mv if file2 doesn't exist. Note that it does not guarantee that a file2 won't be overridden because a file2 could have been created between the test and the mv, but note that at least current versions of GNU mv with -i or -n don't give that guarantee either (though the race condition is narrower there since the check is done within mv).

On the other end, it is portable, allows you to discriminate between the cases, and works regardless of the type of the file2 file (regular, pipe, even directory).