Yes. You can use xsel tool (a command-line tool to access X clipboard and selection buffers). To install it from terminal, use the following command:

sudo apt-get install xsel

Then, using the following:

<command> | xsel -b

will copy the output of <command> to the clipboard which can be pasted after with Ctrl + V.

For example:

echo -n "string" | xsel -b

or, simple:

xsel -b <<< "string"

will copy to the clipboard the string string (I used -n argument for echo to suppress the trailing newline).

If you want to copy the text from a file named file_name from the current working directory:

cat file_name | xsel -b

or, simple:

xsel -b < file_name