Remember to make the script executable, like
$ chmod u+x dumpback
Also, remember that the current directory is not normally part of the command search path (environment variable $PATH), so you might need to run it as
$ ./dumpback
or move to a directory that is part of the PATH (the home directory is, but if you make a lot of scripts, make some other directory, or move them to /usr/local/bin).
DO NOT make current directory (.) part of the PATH. Even if we forget crackers etc., think about this: what is the most common name of the script (or compiled program for that matter) ?
"test" (like I am just testing the compiler, a new library etc.).
So what, you ask. One of the most common command used by several scripts is test. It is used for example to test if a file exists etc. Think how some installation script works, if test prints only "Hello world"...