How do I open a file from inside a bash script?

I think your script should work. But you might add something to it to get a little more information:

#!/bin/bash
T=`xdg-mime query filetype $1`
echo "opening file "  $1  " of type " $T "with " `xdg-mime query default $T`
xdg-open $1
echo "finished script"

when running this script (named my_open.sh) in a terminal like this:

my_open.sh path/to/somefile.txt

I get the following output:

opening file  path/to/somefile.txt  of type  text/plain with  gedit.desktop
finished script

which tells me that the path to the file is ok, the mimetype is recognized and the desktopfile which is used to open the file is ok as well. And gedit opens with the file in question.

Now when run on another file:

my_open.sh path/to/README

I get the following output:

opening file  path/to/README  of type  text/x-readme with
finished script

Note the different mimetype and the missing desktop file. Nevertheless, xdg-open opens the default for all text files (gedit).

So, you might want to add something like this to your script and see if you get unexpected output (which you can then add to your question...).

Make a bash script test.sh as:

#!/bin/bash
gedit myfile.txt

Then, make the script executable as:

chmod +x test.sh

Finally, run the script as:

./test.sh

Maybe gnome-open instead of xdg-open