For this case I am going to imagine a scenery. First of all let's say I need a script to open a web browser in certain Youtube Video(s) and each time it opens on boot I need the videos to be played with a random different duration.

First of all let's create the file, I am calling this "youtuviewer.sh", inside of which I am going to set this portions of code:

#!/bin/bash

chromium-browser http://www.youtube.com/watch?v=7bLaLJ51rRk http://www.youtube.com/watch?v=OxYSaT_NfjQ &

n=$((RANDOM%90+30))

echo $n

sleep $n

killall chromium-browser

echo "all done!"

I explain it quickly:

1. Line 1: The executable will invoke bash to interpret the
   instructions after which


2. Line 2: chromium-browser will run (it can be replaced by firefox or
   any other web browser). The browser will open the youtube
   links in the list in separated tabs, the list should be separated by a space. The ampersand (&) will
   instruct that after executing that line, the rest of the script
   should be executed and the chromium-browser instance should be left
   running meanwhile.


3. Line 3: We are going to generate a random number, between 30 and 90
   and store it in a variable called "n". This will be the number of
   seconds that we are going to use in the next lines.


4. Line 4: We write on the terminal how many seconds will be used based
   on the random number.


5. Line 5: We make the script to "sleep" the "n" number of seconds.


6. Line 6: We kill chromium-browser in order to continue. This line
   will be executed only after Line 5 finishes waiting ("sleeping") the
   amount of seconds stored on the "n" variable.


7. Line 7: We celebrate saying "all done!" in the terminal.

A more complex example can be made with this 7 lines, indeed. You can make it in just one line, everything depends on what you wish to achieve.

We are going to set execution permissions to the file via your favorite file browser (nautilus comes to my mind) or via terminal. See this: How do I run .sh files?

Now we need to setup the file to be run on boot, we can achieve this via cron (see this: How do I set up a Cron job?) or by using the "Startup applications" dialog (see this: How to add application to startup application menu?).

This is the simplest way I find to do what the edits by @ImaginaryRobots let's see.

If you need further assistance don't hesitate to let us know.

Good luck!