How can I get the contens of applications by programming ?

[a qi]

HI,ALL

I am new to  GUI Programming under linux,  so forgive me if there is
an obvious answer to this question.

My question is:

How can I get the contens of applications by programming ?

for example,

scene 1:  I want to select some text( line 10,  for simplicity ) by
press one hotkey  when   I edit a file by vi ,  so that I can paste it
in other applications later.

scene 2:  I want to save some text  ( line 10 ~ 20, for simplicity)
directly into ' /home/guy/useful-text.db ' by press one hotkey when  I
visit some webpages by firefox .


Any suggestions or help will be appreciated. Thanks.