Copy all the lines to clipboard

Use:

:%y+

to yank all lines.

Explanation:

• % to refer the next command to work on all the lines
• y to yank those lines
• + to copy to the system clipboard

NB: In Windows, + and * are equivalent see this answer.

You should yank the text to the * or + registers:

gg"*yG

Explanation:

• gg to get the cursor to the first character of the file
• "*y to start a yank command to the register * from the first line, until...
• G to go the end of the file