How can I display text from a file automatically after powering up my computer, in text editor or terminal?

Your first step is to find out the name of your notepad / GUI editor / writer program as described in this Q&A: How does one find out the command line corresponding to GUI app (eg, LibreOffice Writer)? In my case I use LibreOffice Write and the command is libreoffice --writer.

The next step is to select Dash which is the first option on the Launcher. In the search field type in Startup and select the icon Startup Applications. The following window pops up:

Select the Add button. And this window appears:

Fill in your writer / program name and pass it the name of your to-do file. This is probably the hardest part to get right and often requires a little experimentation.

Edit 1

Proving that experimentation is needed when using the above example tonight an error was displayed:

Checking the actual file name ends in .odt edit the Startup Application profile and change the file name by adding the extension .odt and then voila on every boot you get:

To display it using a shell script copy & paste this:

#!/bin/bash
# my to do list
cat filename

into a new text file & save it with a .sh extension, then add it to startup as explained above. If you want to display the contents of a longer file replace "cat" with "less".