Some times I copy something to the clipboard, and it doesn't paste with Shift+Insert, but with Ctrl+V or right-click paste. The behavior seems confusing and a bit unpredictable. Is there some logic behind the variation?
There are two types of clipboards in Unix/Linux: PRIMARY (often used with Ctrl-X/C/V) and SELECTION (mouse selected text, inserted with Shift-Insert or clicking the mouse middle button).
See https://wiki.archlinux.org/index.php/clipboard for more details.
Here's what I used to do:
Ctrl+Insert : Copy to the clipboard
Shift+Insert : PASTE from the clipboard
Shift+Delete : Cut TO the clipboard
Now I have an Apple Keyboard, I do: (The apple keyboard does not have an insert key)
Ctrl+C to Copy to the clipboard
Ctrl+V to PASTE FROM the clipboard
Shift+Delete to cut TO THE clipboard.
In a terminal using Putty:
Select the text with the mouse copies directly to the clipboard
Right-Clicking anywhere in the terminal window does the paste
In a regular shell session:
I select with the mouse, and use Ctrl+Shift+C to COPY to clipboard
I use Ctrl+Shift+V to paste.
This question seems to be related to the behaviour of keys combinations, but you don't give an application in which you perform these keys combo. So, taking in consideration what Javier Rivera says, the reason of why it is needed to use SHIFT+CTRL in a terminal, is because of some of these combinations are used in terminal command's control.
In example: when you wish to "cancel" the current command, you pŕess CTRL+C, which is used in the majority of the systems to perform the "Copy" task. This way, if you press CTRL+C in a terminal in order to "Copy" a portion of text and an application or process is running, you will "cancel" the process or "close" the running application.
Depending on the GUI that you use, or the software application itself, SHIFT+CTRL is useful for formatting or performing the "special-paste" task, which allow you to choose the format on which the copied text will be dropped into the application.
In example: if you use OpenOffice Word Processor or Spreadsheet, when SHIFT+CTRL+V you will be prompted to choose what format will you use for the clipboard to be dropped into the document.
The next image illustrates how the CTRL-V and SHIFT+CTRL+V keys drops the clipboard's content into a document:
The next image illustrates how the CTRL-V and SHIFT+CTRL+V keys drops the clipboard's content into a document, AFTER CHOOSING THE FORMAT in the right side documen:
You should check into the application's "Edit" menu in order to see how the "Paste special" task is handled.
The next image illustrates how OpenOffice Word Processor handles the "Paste Special" feature:
As far as I know I use always CRTL+V to paste but when I'm in the terminal. For historical reasons CTRL+letter can't be used there, so I use SHIFT+CTRL+V in terminal.
I always believe that Shitf+Insert is an heritage from older Unix keep them to get old timers comfortable, but that modern GUIs use CTRL+V as default.