Install Ruby using rbenv in a bash script
Not a lot can be improved on a simple installer script like this.
I have a few tips but nothing really significant.

The script doesn't check if it has already been executed.
If you run it again by mistake, it will append to ~/.bashrc lines that were already there.

As a simple workaround, you could add -e flag to the shebang:

#!/bin/bash -e


This will cause the script to exit on the first error.
Which will happen on the first git clone command,
which will fail because the local clone already exists.
The script will stop there and not reach the lines that append again to .bashrc.

Speaking of shebang, you have a second #!/bin/bash in the middle of the script, which was probably a copy-paste mistake and has no purpose there, so should be removed.