If you don't mind using sed then,
$ cat test this is line 1 $ sed -i '$ a\this is line 2 without redirection' test $ cat test this is line 1 this is line 2 without redirection
As the documentation may be a bit long to go through, some explanations :
-i means an inplace transformation, so all changes will occur in the file you specify$ is used to specify the last linea means append a line after \ is simply used as a delimiter