On macOS, one can use pbcopy
echo "this is what I want to copy to the system clipboard" | pbcopy
This copies the piped content into the system clipboard, so that it can be pasted with ctrl+V shortcut.
On Ubuntu, this can be accomplished with xclip but with an extra option:
echo "this is what I want to copy to the system clipboard" | xclip -sel clip
I find it a bit annoying to specify the option every time I use it, so I added an alias into my .bashrc file
echo 'alias xclip="xclip -sel clip"' >> ~/.bashrc